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The Biology of Canadian Weeds. 143. Apocynumcannabinum L.

Antonio DiTommaso1, David R. Clements2, Stephen J. Darbyshire3, and Joseph T. Dauer4

1Department of Crop and Soil Sciences, Cornell University, Ithaca, NY, USA 14853 (e-mail: [email protected]);2Department of Biology, Trinity Western University, Langley, British Columbia, Canada V2Y 1Y1; 3Eastern Cerealand Oilseed Research Centre, Agriculture and Agri-Food Canada, Wm. Saunders Building #49, Ottawa, Ontario,

Canada K1A 0C6; and 4Department of Crop and Soil Sciences, Oregon State University, Corvallis, OR, USA97331. Received 28 May 2008, accepted 28 April 2009.

DiTommaso, A., Clements, D. R., Darbyshire, S. J. and Dauer, J. T. 2009. The Biology of Canadian Weeds. 143. Apocynumcannabinum L. Can. J. Plant Sci. 89: 977�992. Hemp dogbane, Apocynum cannabinum (Apocynaceae), is a perennial herbwith white to greenish flowers in terminal clusters that produces pencil-like pods 12�20 cm long. A highly variable plant, A.cannabinum may be distinguished from spreading dogbane (Apocynum androsaemifolium) by its shorter corolla (2�6 mmcompared with 5�10 mm), erect greenish-white petals (compared with recurved or spreading pinkish petals), seeds morethan 3 mm long (compared with seeds less than 3 mm), and more erect leaves (compared with spreading or droopingleaves), although frequent hybridization between the two species obscures the identity of some individuals. Hemp dogbaneis native to the United States and southern Canada, but most abundant in the upper Mississippi River Valley and east tothe Atlantic coast. It has been increasing in other areas, and becoming more of a problem where conservation tillage isadopted. It infests crops such as corn (Zea mays), soybeans (Glycine max), wheat (Triticum aestivum), sorghum (Sorghumbicolor) and forages, and may cause livestock poisoning due to cardiac glycosides within its milky sap (but livestockgenerally avoid it). Potential medicinal uses of these compounds have been investigated, and the roots are a source of fibre.Control of A. cannabinum with various herbicides is difficult due to a thick cuticle, and one solution may be to targetsusceptible stages, such as seedlings or early spring growth. Cultivation may also control A. cannabinum, but care must betaken not to promote the proliferation of the plant through regrowth from fragmented roots and rhizomes. Rotation withalfalfa also reduces populations of A. cannabinum.

Key words: Hemp dogbane, APCCA, Apocynum cannabinum, Apocynaceae, weed biology

DiTommaso, A., Clements, D. R., Darbyshire, S. J. et Dauer, J. T. 2009. La biologie des mauvaises herbes au Canada. 143.

Apocynum cannabinum L. Can. J. Plant Sci. 89: 977�992. L’apocyn chanvrin, Apocynum cannabinum (Apocynacees), estune herbacee vivace aux grappes terminales de fleurs blanches a verdatres qui donnent des gousses en forme de crayon de12 a 20 cm de longueur. L’espece connaıt d’importantes variations. A. cannabinum se distingue du gobe-mouches(Apocynum androsaemifolium) par une corolle plus petite (2 a 6 mm contre 5 a 10 mm), des petales blanc verdatre dresses(plutot que rosatres, recourbes ou etales), des graines de plus de 3 mm de longueur (comparativement a moins de 3 mm) etdes feuilles plus droites (et non etalees ou tombantes), bien que la frequente hybridation entre les deux especes rendel’identite de certains plants plus obscure. L’apocyn chanvrin est une plante indigene des Etats-Unis et du sud du Canadaqui abonde surtout dans le haut de la vallee du Mississipi et a l’est, jusqu’a la cote de l’Atlantique. L’espece prolifereneanmoins dans d’autres regions et s’avere plus problematique aux endroits ou l’on pratique la conservation du sol. Elleinfeste les cultures tels le maıs (Zea mays), le soja (Glycine max), le ble (Triticum aestivum), le sorgho (Sorghum bicolor) etles plantes fourrageres, au risque d’intoxiquer le betail en raison des glycosides cardiaques que renferme sa seve laiteuse (lesanimaux l’evitent generalement). On s’est penche sur les vertus medicinales potentielles de ces composes ainsi que sur lesracines de la plante en tant que source eventuelle de fibres. Les herbicides viennent difficilement a bout d’A. cannabinum acause de l’epaisse cuticule qui la protege. Une solution serait de s’attaquer aux stades plus fragiles, comme les plantules oula germination, au printemps. Le travail du sol pourrait aussi faciliter la lutte, mais il faut prendre soin de ne pas favoriserla proliferation de la plante a partir des fragments de racines et de rhizomes. L’assolement avec la luzerne diminue aussi lapopulation d’A. cannabinum.

Mots cles: Apocyn chanvrin, APCCA, Apocynum cannabinum, Apocynacees, biologie des mauvaises herbes

1. NamesApocynum cannabinum L. Common names: Hemp dog-bane, apocyn chanvrin, bitter root, black hemp, blindhemp, bowman root, Canadian hemp, choctaw root,clasping-leaved dogbane, dropsy root, honey bloom,

lechuguilla, Indian hemp dogbane, rheumatism weed,and westernwall; chanvre sauvage (Crockett 1977;Becker 1981; Darbyshire et al. 2000; Darbyshire 2003;USDA-NRCS 2004). European and Mediterranean

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Plant Protection Organization (Bayer) code: APCCA.Apocynaceae, dogbane family, Apocynacees.Apocynum is derived from Greek roots, with apo

meaning ‘‘off’’ or ‘‘away from’’, and kunon meaning‘‘dog’’ (Dalby 2004).

2. Description and Account of Variation(a) Species Description * The following description isbased on information taken from the literature (Fernald1950; Gleason and Cronquist 1991; Voss 1996), supple-mented by observations made by the authors. Measure-ments are given as the typical range with unusualextremes in parentheses.A herbaceous perennial (Fig. 1) producing a milky

latex sap. Plants tough and fibrous, usually glabrous butmay be variously pubescent on stems, leaves andinflorescences. Root system extensive with long verticaland horizontal roots and short rhizomes. Stems usuallyerect or sometimes prostrate, (1) 3�10 (15) dm tall, often

reddish at the base. Branches usually arise in the upperhalf with the lateral branches surpassing the central stemat flowering. Leaves opposite or rarely whorled oralternate, yellowish-green to dark green or sometimesglaucous, erect or slightly spreading (leaves tend to bemore spreading in prostrate forms), glabrous or sparselypubescent beneath, oval to lanceolate, rounded to acuteat the apex and usually abruptly mucronate (Fig. 2). Thelargest leaves arise near the middle of the stem, the sizediminishes toward the top and bottom, (1) 4�12 (14) cmlong, 0.5�5.5 cm wide. Leaves acute to rounded orcordate at the base (Fig. 2) and may be sessile or onshort petioles to about 1 cm long. Inflorescencescompact paniculate cymes, usually terminal on branchesand only rarely axillary. Flowers hermaphroditic,5-merous, (2) 2.5�4.5 (6) mm long, whitish to greenish-white and cylindric to urceolate (urn-shaped) (Fig. 1C).Calyx lobes lanceolate, about half as long as to nearlyequal the corolla. Corolla lobes (petals) more or lesslanceolate and acute to oblong-lanceolate and erect orslightly spreading. Pistil with two semi-inferior ovaries.Each flower produces two thin pods (follicles) (4) 8�20(22) cm long straight or slightly sickle-shaped (Fig. 1D).Seeds 4�6 mm long, 0.75 mm wide, with a coma (tuft ofwhite silky hairs) (1) 3�5.5 cm long (Fig. 1E, F).Cotyledons strap-like (Fig. 3), 4�11 mm long, about1.5 mm wide.Some authors have reported a chromosome numbers

of n�8 (Schurhoff and Muller 1937) and 2n�16 (Vander Laan and Arends 1985) for A. cannabinum ofunknown provenances; although these counts havebeen called into question. Counts of 2n�22 have beenreported by Breslavetz et al. (1934) from an unknown

Fig. 1. Apocynum cannabinum var. cannabinum. A. habit, scalebar�10 cm; B. roots, scale bar�10 cm; C. flower, scale bar�1 cm; D. follicles, scale bar�10 cm; E. seed with coma, scalebar�5 cm; F. seeds, scale bar�1 cm. Illustration by ReginaO. Hughes, United States Department of Agriculture.

Fig. 2. Variation in the shape of leaves from the middle of thestem in Apocynum cannabinum as drawn from herbariumspecimens. Upper row with short petioles and rounded orclasping base of the blade attributed to var. hypericifolium;lower row with longer petioles and tapering base of the bladeattributed to var. cannabinum. Scale bar�1 cm.

978 CANADIAN JOURNAL OF PLANT SCIENCE

provenance, Love and Love (1982) from Manitoba, andHill (1989) from Virginia. Studies by Balbach (1965) ona number of populations in the United States andCanada found the diploid number of 22. Inconsistenciesin reported numbers may also be due in part to the smallsize (0.8-1.0 mm) of the chromosomes, which hinderstheir preparation and counting (Balbach 1965).

(b) Distinguishing Features * Because of frequenthybridization between species in the genus (see Section9) and the extensive morphological variation in Apoc-ynum cannabinum, there is confusion about character-istics that accurately differentiate this species from theclosely related A. androsaemifolium L. (spreading dog-bane) and the extent to which species segregates shouldbe recognized (see Section 2c and Key at the end of thepaper). The corolla of A. cannabinum differs from thatof A. androsaemifolium, being shorter [(2) 2.5�4.5 (6)mm versus (5) 5.5�8 (10) mm], having the 5 erect orslightly spreading petals (versus strongly spreading orrecurved) and being white to greenish-white and withoutany rosy hue or interior red markings. The leaves ofhemp dogbane are usually erect or slightly spreadingfrom the stem, while those of spreading dogbane arestrongly spreading or drooping. Although highly vari-able in shape, the leaves of hemp dogbane tend to beelliptic to more or less lanceolate (Fig. 2), while those ofspreading dogbane tend to be broadly elliptic to some-what ovate (egg-shaped). Another species often con-fused with A. cannabinum is Asclepias syriaca L.(common milkweed), but the leaves of A. cannabinumare lighter green and more pointed than those ofA. syriaca, both the follicles and seeds of A. cannabinumare narrower and more elongate than those of A. syriaca(Doll 1994) and the flower structure of A. cannabinum isvery different, lacking the typical highly specialized

asclepioid features of pollinaria and anthoecial coronae(hoods and horns) in Asclepias.

(c) Intraspecific Variation * Highly variable in bothshape (Fig. 2) and pubescence of the leaves (Gleasonand Cronquist 1991), A. cannabinum is self-sterile (seeSection 8a) with out-crossing promoting variability andhybridization (Ransom et al. 1998b). This variability isevident from the number of papers claiming plants areglabrous (Anonymous 1970; Hartman 1986; Bradleyand Hagood 2001), tomentulose (Doll 1994) or both(Balbach 1965; Becker 1981; Gleason and Cronquist1991). It is not known what proportion of this varia-bility is due to phenotypic plasticity versus geneticdiversity. Within the hemp dogbane complex, variousspecies, subspecies or varieties have been distinguishedin different ways by different authors (cf., Woodson1930; Fernald 1950; Boivin 1966; Gleason andCronquist 1991) with taxa recognized and ranked basedprimarily on growth habit, pubescence, leaf shapeand petiole length. The bewildering array of intermedi-ate forms between the described taxa provides littleconfidence in intraspecific classifications (e.g., Hartman1986; Gleason and Cronquist 1991; Voss 1996). Fre-quently, two species are recognized within the complex,A. cannabinum and A. sibiricum Jacq. (e.g., Darbyshireet al. 2000), but here we prefer to recognize these twoforms as varieties, A. cannabinum var. cannabinum, withleaves tapering to a distinct petiole (3�10 mm long),which is more common in southern and eastern popula-tions, and var. hypericifolium A. Gray (�A. sibiricum),which usually has a short petiole (B3 mm long) and ismore common in northern and western regions of NorthAmerica. A semi-decumbent or prostrate type,A. cannabinum var. hypericifolium f. arenarium (F.C.Gates) B. Boivin, has been recognized from shores ofrivers and lakes in eastern Canada and the United States(Schaffner 1910). Extreme mat-forming plants, withsmall, glabrous and sessile or slightly clasping leaves,have been collected along the St. John River in NewBrunswick and other scattered localities. This growthform appears to be a phenotypic response to growingconditions (Schaffner 1910). Distinct ecotypes identifiedfrom different geographic areas may be recognized bygrowth or morphological characteristics, and ecotypesmay be characterized by unique leaf shape (Ransomet al. 1998a), but these forms probably do not warrantformal taxonomic distinction.Since the species commonly frequents disturbed

habitats, it is possible that the landscape alterationsfollowing European settlement have facilitated thespread and inter-mingling of forms that were previouslyseparate and more or less distinct. Ransom et al. (1998b)found that the genetic distance-based grouping ofcollections of this species were not highly correlatedwith the geographic location from which the plantsoriginated, or with differences in morphologicalcharacteristics. In Colorado, however, 11 populations of

Fig. 3. Seedling of Apocynum cannabinum var. hypericifoliumwith cotyledons and first true leaves. Scale bar�5 mm.

DITOMMASO ET AL. * APOCYNUM CANNABINUM L. 979

A. cannabinum exhibited low heterozygosity at 11 lociwith few polymorphisms at these loci; none containedmore than two alleles (Johnson et al. 1998). This lowheterozygosity is unusual for an obligate outcrossingspecies, but may be due to the limited range (i.e., onestate) from which the populations were sampled, relativeto the much wider North America range of this species.Johnson et al. (1998) speculated that this low hetero-zygosity indicated a long history of inbreedingor bottlenecking, but without random genetic drift.Ransom et al. (1998b) found genetic variation among16 populations from Michigan and Illinois was less thanwould be expected based on morphological differentia-tion. Relatively short genetic distances observedbetween A. cannabinum and other Apocynum specieswere seen to indicate some interbreeding among thesespecies (Ransom et al. 1998b). Outcrossing with closelyrelated species could have provided some of the geneticvariability observed among A. cannabinum populations.

(d) Illustrations * The plant and its principal parts areillustrated in Fig. 1. Variation in leaf shape of A.cannabinum is illustrated in Fig. 2. A seedling withcotyledons is shown in Fig. 3. A photo of a floweringplant is seen in Fig. 4. Woodson (1930) providesillustrations of leaf, root and stem anatomy, as well asthe general habit of various segregate taxa.

3. Economic Importance(a) Detrimental * Apocynum cannabinum infestationscan result in significant crop losses, affect harvestingefficiency and result in livestock poisoning. Crop lossesin corn (Zea mays L.) due to hemp dogbane ranged from9% in irrigated systems in Nebraska (Schultz andBurnside 1979a) to 15% in non-irrigated systems inNebraska (Evetts and Burnside 1973) and Pennsylvania

(Curran et al. 1997). Soybean (Glycine max L.) lossesranged from 30 to 38% in Nebraska (Schultz andBurnside 1979a; Furrer et al. 1983) with Webster et al.(2000a) reporting a loss of 75% in an Ohio study whenshoot density was 28 shoots m�2. Sorghum [Sorghumbicolor (L.) Moench] losses were similar, ranging from37% to as high as 45% in Nebraska (Schultz andBurnside 1979a). In Ohio, Loux and Berry (1991)ranked A. cannabinum as the second most importantperennial broadleaf weed after Canada thistle [Cirsiumarvense (L.) Scop.] in corn and soybeans. However, inmany cropping situations, the concern over A. cannabi-num is more one of appearance than actual crop loss(Robison and Jeffery 1972; Evetts and Burnside 1973).For example, studies in irrigated corn in Nebraskafound little effect on yield, whereas dryland corn andsorghum in the same area experienced 15 and 30% yieldlosses due to this weed, respectively (Evetts and Burn-side 1973). Schultz and Burnside (1979a) found thatA. cannabinum consistently reduced corn and sorghumpopulations in Nebraska, but that effects on biomass,seed weight and protein content were inconsistent.Owing to its perennial nature and extensive root system,A. cannabinum often grows in isolated circular patcheswithin a crop field, which can be particularly detri-mental to crop yields (Evetts and Burnside 1973; Triplett1985). Certain crops may be more susceptible toinfestations: Schultz and Burnside (1979a) foundA. cannabinum was most prevalent in oat (Avena sativaL.) and soybean crops in Nebraska and least prevalentin alfalfa (Medicago sativa L.), pasture, and winterwheat (Triticum aestivum L.).As with many other perennial weeds, increases in the

abundance of A. cannabinum have been linked toincreased use of reduced tillage systems (Triplett andLytle 1972; Triplett 1985; Loux and Berry 1991; Websterand Cardina 1999). In a greenhouse study, Wyrill andBurnside (1976a) found the allelopathic effects ofleachate from live A. cannabinum reduced sorghumgrowth by 33% over a 20-d growth period. Schultzand Burnside (1978h) found a consistently greaterdepression of yield when corn, soybeans or sorghumwere grown with live versus dead A. cannabinum plants.Hafemann and Jones (1986), however, found no allelo-pathic effect on corn, soybeans or sunflowers in agreenhouse study. Other cropping system changes,such as a reduction in alfalfa as a component inrotations, have lead to increased infestation by A.cannabinum in Wisconsin (Doll 1995).Apocynum cannabinum is in full bloom during winter

wheat harvest, which causes the milky sap to ‘‘coatcombine parts and adhere dirt and chaff’’ (Doll 1994).The sap contains a cardiac glycoside similar to thatfound in Digitalis species (Genkina et al. 1974; Furreret al. 1983), as well as various kinds of alkaloids, but isnot tanniferous (Cronquist 1981). The sap is toxicand 15�30 g of green leaves can kill a horse orcow (Muenscher 1951), but animals find the plant

Fig. 4. Terminal flower clusters of Apocynum cannabinum;flowers are small and bell-shaped, and range in colour fromwhite to greenish-white.


unpalatable and animal poisonings are rare (Fyles 1920;Furrer et al. 1983; Doll 1994; Dalby 2004).

(b) Beneficial* The strong roots of A. cannabinum havebeen used by North American natives as a source offibre for a number of applications such as bowstrings,nets, rope, thread, and cloth (Woodson 1930; Hill 1952;USDA-NRCS 2004). Each plant can yield up to 0.75 mof fibre with a tensile strength of hundreds of kilograms(Turner et al. 1980). In Idaho, Oregon and Washington,Native Americans used the cordage for ritual basketmaking, ceremonial bags and cloth weaving, and inCalifornia, the weed is still harvested for its fibre today(USDA-NRCS 2004). The fibres were an importantcommodity to the native tribes of the ColumbianPlateau, including the Nez Perce, Spokane, Umatillaand others for baskets and fishing nets (USDA-NRCS2004) and the Chemeweve made snares for otter andrabbits from the root fibres (Merriam 1955). TheLuiseno of southern California used it to construct theirdance regalia, by incorporating golden eagle feathersinto a netting of fibres (Merriam 1955). Another use ofgreat cultural significance for the Columbian peopleswas the creation of the itatamat or ‘‘counting the days’’ball, wherein a woman would record events in her life bytying a knot in a length of cordage (USDA-NRCS2004).Apocynum cannabinum is effective in erosion control

and is attractive to bees and butterflies producing a finegrade of honey (Pellett 1976; Turner et al. 1980; Dalby2004; USDA-NRCS 2004). Foster and Karpiscak (1983)listed A. cannabinum, along with milkweed (Asclepiasspp.) and rabbit bush [Chrysothamnus nauseosus (Pursh)Britton], as a potential biofuel crop, particularly forpotential use in the Intermountain/Rocky Mountainregion of the United States where it can grow under aridconditions. This species may also provide a source oflatex for rubber production, with plant tissues contain-ing more than 5% latex (Woodson 1930; Hill 1952). Apermanent brown to black dye was produced from theplant by decoction (i.e., extraction of plant material)(Millspaugh 1887) and the dye is still used today(Anonymous 2009).Because of its production of glycosides and alkaloids,

A. cannabinum has been investigated as a source ofmedical products (Zaitseva and Feofilaktov 1950;Babcock and Carew 1962; Lee et al. 1972). Cymarin, amajor constituent of A. cannabinum, has been used in thetreatment of congestive heart failure (Burger 1960;Tashmukhamedova et al. 1968). Cymarin and apocanno-side are also cytotoxic against certain human carcinomas(Kupchan et al. 1964). Other historical medicinal usesinclude use as a cathartic, a diuretic, a stimulant, adiaphoretic, a febrifuge (lowers fever), a rheumatismremedy, and a treatment for gallstones (Dalby 2004). Itis held in regard by homeopathic practitioners and Felter(1922),while describing its uses, stated: ‘‘No remedy in theEclectic materia medica acts with greater certainty than

does apocynum.’’ Some North American natives wouldmake a tea to treat heart palpitations and a variety ofother medicinal purposes such as to treat colds, earaches,headaches, nervousness, dizziness, or worms, or as anemetic, antispasmodic, catharti, anodyne, hypnotic, orlaxative (Moerman 1998). Additionally, a shampoo canbe produced from crushed roots that would removedandruff and head lice. Poultices made from leaves wereused to treat various skin inflammations and eye diseases(USDA-NRCS 2004).

(c) Legislation * Apocynum cannabinum is not listed asa noxious weed on any Canadian federal or provincialweed legislation. It is listed as a noxious weed federallyin the United States due to its toxicity to livestock andinvasive characteristics (USDA-NRCS 2004).

4. Geographical DistributionA native plant of North America, A. cannabinum ispresent in all Canadian provinces and territories exceptNunavut and the Yukon (Fig. 5); however, Cody (1996)included it in his Flora of the Yukon Territory,indicating that its presence in that region is to beexpected. In the United States, A. cannabinum is presentin all 48 contiguous states (Anonymous 1970; Kartesz1999), although it is most common in the upperMississippi River Valley and east to the Atlantic coast(Becker 1981). The abundance of this species has beenincreasing in other regions, such as the Missouri Riverarea in Iowa, Nebraska and Kansas (Becker 1981). Ithas also been introduced to Germany (Radkowitsch1999).

5. Habitat(a) Climatic Requirements * Able to grow in numeroushabitats, A. cannabinum is highly variable and adaptable(Balbach 1965). Optimal growing conditions include amaximum annual precipitation of approximately 152cm, and a minimum of 100 frost-free days (USDA-NRCS 2004). It has been found to grow at elevations upto 5000 m in California (USDA-NRCS 2004). Evettsand Burnside (1971) found that common milkweed(Asclepias syriaca L.) was less sensitive to moisturestress than A. cannabinum.

(b) Substratum * Information from the labels ofherbarium specimens collected in Canada indicates awide variety of soil types including, sandy soil, dry soil,alluvial gravel, rocky soil, silt, calcareous soil, loam,clay, mineral soils around hot springs, and evenoccasionally peaty soil. Johnson et al. (1998) statedthat A. cannabinum usually grows in sandy or loamysoils; however, according to Robison and Jeffery (1972),the plant is seldom found in sandy soils with lowfertility, and grows best in fertile, medium- or heavy-textured soils. Dense infestations tend to be found inlow-lying, wet areas, but fairly high densities may stilloccur on drier upland soils (Becker 1981; Voss 1996).


Fig. 5


The range of soil pH tolerated is reported to be 4.5�7(USDA-NRCS 2004); however, based on the numerousherbarium specimen labels indicating that this speciesgrows in ‘‘calcareous soil’’, a higher pH tolerance islikely.

(c) Communities in Which the Species Occurs * InCanada, the natural habitat of A. cannabinum appearsto be mainly along shores and flood plains. Informationon herbarium specimens indicates it growing in a widevariety of habitats including thickets, marshes, mea-dows, disturbed ground, roadside ditches, forest edgesand openings (coniferous, mixed, deciduous and planta-tion), alvars, old fields, cultivated fields (oat, corn,soybean, potatoes, etc.), intertidal zones, prairies, aban-doned farm fields, sand dunes, ditches, talus slopes andsloughs. The plant is rarely a problem in conventionaltillage systems because tillage constantly breaks therhizomes and roots (Curran et al. 1997). Buhler et al.(1994) found similar results with higher dogbane den-sities in continuous corn than with a corn/soybeanrotation in both ridge- and no-till systems. Apocynumcannabinum grows best in open spaces (Gleason andCronquist 1991) and poorly in full shade (Balbach1965). In non-agricultural settings, Muenscher (1951)found plants in gravelly fields, meadows, waste places,and along streams, whereas Johnson et al. (1998) foundplants mainly in prairie river flood plains, terraces, androadside ditches.

6. HistoryAs mentioned in Section 3b, changing cropping systemshave favoured increased infestation in some regions inrecent years. For example, in Wisconsin, 6% of agri-cultural extension agents ranked it as serious or veryserious in row crops and small grains in 1977, but thishad increased to 15% by 1994 (Doll 1995). Practicescontributing to this trend may include reduced tillage,decreased use of phenoxy herbicides, reduced ratesof triazine herbicides and continuous row cropping(Robison and Jeffery 1972; Wyrill and Burnside 1976b;Schultz and Burnside 1980; Becker 1981; Webster andCardina 1999).

7. Growth and Development(a) Morphology * The development of the extensiveroot and rhizome system (Fig. 1B) is important for theweed’s survival. The aggressive spread and persistence ofthe weed is most likely due to the large reserves ofcarbohydrates in the root system (Balbach 1965; Beckerand Fawcett 1998), which serve as a major resource for

colony survival and spread. The main root is 2�3 (10)mm in diameter and can extend a number of meters.The root system consists of a vertical primary and one

to many lateral horizontal roots from which secondaryvertical roots may arise (Frazier 1944). Primary roots of2-yr-old A. cannabinum have been found to penetrate todepths of 4 m, while extending as much as 6 m from theparent plant (Frazier 1945; Becker 1981; Orfanedes et al.1993). Lateral roots rarely attain a depth of more than17 cm (Orfanedes et al. 1993) and grow 5�30 cm beforesecondary vertical roots develop (Doll 1994). Secondarygrowth occurs from buds along the lateral roots (Frazier1944). Crown buds, located at the base of the stem, arealso important for vegetative reproduction and begingrowth each spring or following damage to the stem.

(b) Perennation * A perennial species, A. cannabinum,over-winters as a below-ground system of crown buds,rhizomes and roots, reemerging from buds each spring(see Section 7d). Becker (1981) noted a University ofNebraska study indicating that hemp dogbane seedlingsare capable of perennial regrowth within 10 to 41 d ofemergence, but various other studies suggest that theactual time is closer to 6 wk (Robison and Jeffery 1972;Furrer et al. 1983). Schultz and Burnside (1979a)observed a cyclical pattern in root and aerial stemregenerative capacity, with the highest activity occurringin spring and late fall and lowest activity occurring insummer and early fall.

(c) Physiology * In common with other perennialweeds, protein and carbohydrate levels peak in thespring and fall, and are relatively low in the summer.Schultz and Burnside (1979a) showed that protein levelsranged from 4 to 5% in summer to from 7 to 9% in fall.Protein and non-structural carbohydrate levels arehigher in lateral roots than crown roots (Schultz andBurnside 1979a).The thickness of the cuticle (measured as 305 mg cm�2)

on the adaxial leaf surface and (458 mg cm�2) on theabaxial surface and the presence of stomata only onthe abaxial surface (320 stomata mm�2) (Wyrill andBurnside 1976b) likely contribute to drought toleranceand resistance by the plants to penetration by herbi-cides. Moreover, these workers measured epicuticularwax thickness of 85 mg cm�2 on the adaxial surface and56 mg cm�2 on the abaxial surface, noting the amount ofwax was 2.8 times greater and cuticle thickness 1.6 timesgreater in A. cannabinum than in common milkweed.They attributed the lower absorption of glyphosateand 2,4-D in A. cannabinum compared with common

Fig. 5. Distribution of Apocynum cannabinum in Canada plotted from specimens examined from the herbaria ACAD, ACK, ALTA,CAN, DAO, HAM, MMMN, MT, NFLD, NSPM, QFA, QUE, SASK, TRT, TRTE, UNB, UVIC, UWO, UWPG, WAT, WIN,and V. Acronyms according to Holmgren et al. (1990). A. distribution of A. cannabinum var. cannabinum and var. hypericifoliumplotted from 1642 specimens; B. distribution of Apocynum cannabinum var. cannabinum plotted from 230 specimens, including thosewhich could not be confidently placed in either subspecific taxon; C. distribution of putative hybrids between Apocynum cannabinumand Apocynum androsaemifolium (A.�floribundum) plotted from 88 specimens.


milkweed to the greater thickness of the cuticle andepicuticular wax.Apocynum cannabinum has been shown to be more

susceptible to herbivory following touching or handlingof field plants by humans (experimenter) or by theexperimenter visiting test plots containing the plant(Cahill et al. 2001, 2002; Niesenbaum et al. 2006). Thelatter is believed to reduce above-ground competition forlight through trampling of neighbouring vegetation, thusincreasing the visibility or apparency of A. cannabinumplants to herbivores. Touching or handling the plantsincreased oviposition by herbivores and altered thedefense chemistry or nutrition of plants, with the latterleading to increased emergence of the stem borer Papai-pema baptisiae Bird (a noctuid moth) (Cahill et al. 2002).Mid-vein cutting has been suggested to have evolved

in insects as a countermeasure to deactivate induced leaflatex or cardenolide defences in milkweeds and dog-banes. In field experiments in Nebraska, transversecutting of the leaf mid-vein in A. cannabinum reducedseveral gas exchange parameters, but only downstream(distal) from the injury location (Delaney and Higley2006). Injury was most severe as the location ap-proached the petiole, suggesting that the effects of veincutting on leaf physiology would be more severe thanjust partial leaf tissue consumption of the plant.Lee et al. (1972) maintained tissue cultures of A.

cannabinum initiated from seedlings for 3 yr on solidmedia, in flask suspension, and in a New Brunswick 5-Lfermentor. They recorded an average growth rate of5.33 gL�1 d�1 in the fermentor. The tissue culturesproduced glycosides such as cymarin, which may beuseful as medicines (see Section 3b).

(d) Phenology * Growth is initiated each spring fromcrown buds and rhizomes (Robison and Jeffery 1972).Emergence from crown buds was observed to occur atsoil temperatures of 17�198C in Nebraska (Schultz andBurnside 1979a). Within 28 wk from the seedling stage,roots were as long as 3 m; after two years of growth, thesame plants’ roots extended up to 4 m in length and 5.4m radially (Robison and Jeffery 1972). Hitchcock andClothier (1898) recorded a single horizontal root mea-suring 8.8 m long. The flower bud stage occursapproximately 4�7 wk after shoot emergence (Schultzand Burnside 1979a; Becker 1981). In Colorado,Johnson et al. (1998) observed that flowering began inmid-June (sometimes in May) and continued throughAugust or until September. Plants do not flower in thefirst year after seedling emergence, but flower readily inthe second season (Frazier 1945). Webster and Cardina(1999) observed that 75% flowering was attained at asimilar calendar date in 1997 (July 02) and 1998 (June30) in Ohio, despite a large variation in accumulatedgrowing degree units (GDU). Early flowering, fullbloom and follicle initiation proceed at 1-wk intervalsfollowing the flower bud stage and the first viable seeds

occur 10 wk after full bloom (Schultz and Burnside1979a; Becker 1981).Carbohydrate levels in the roots, mainly in the form

of starch, decline with vegetative growth in the spring,reach seasonal lows during flowering, and increase untildormancy when levels are the highest (Becker andFawcett 1998). The decline of stored carbohydrateduring vegetative growth and flowering correlates withthe high energy demands of these life stages (Becker andFawcett 1998).

(e) Mycorrhiza * Rickerl et al. (1994) includedA. cannabinum in a study investigating mycorrhizalcolonization of wetland plants in South Dakota. Theyrecorded approximately 29% mycorrhizal colonizationof A. cannabinum in a hydric soil (but without surfacewater).

8. ReproductionReproduction occurs via seed and an extensive system ofroots and rhizomes (Orfanedes et al. 1993). Localpopulations may propagate through a mixture of sexualand vegetative reproduction or entirely clonally (Ran-som et al. 1998b).

(a) Floral Biology *The small, pale flowers (Fig. 1C, 4)are not adapted for specialized pollinators. Lipow andWyatt (1999) provided a definitive description of thefloral biology, and noted that pollination is onlymediated through insects attracted to the fragrantflowers and abundance of nectar. Autogamy is pre-vented by the morphology of the pistil, where the stigmais located on the underside of the style head; however, ifself-pollination occurs, late-acting self-incompatibilityresults in termination of embryo development (Lipowand Wyatt 1999). Although there are large numbers ofinsect visitors, they represent only 25 species and mostcarry little or no pollen (Johnson et al. 1998). Lipow andWyatt (1999) determined that floral structure limits thenumber of effective pollen distributors. Since the insectproboscis must be slipped between the stamens, con-tacting the stigma first, previously collected pollen isdeposited before new additional pollen is picked up.Often the narrow space does not allow enough room towithdraw the proboscis and insects have been founddead within the flowers of A. cannabinum (Lipow andWyatt 1999).

(b) SeedProduction andDispersal*Seeds (Fig. 1E,F) areset in summer andmature in the fall (USDA-NRCS 2004)in long pods (follicles) (Fig. 1D). The folliclesmay containup to 200 seeds (Doll 1994). In Nebraska, the number ofdeveloping seeds per follicle increased rapidly betweenweeks 4 and 5 after flowering, with seed weight triplingfrom 4 to 13 wk after flowering (Schultz and Burnside1979a). Evetts and Burnside (1972) found a range of 80 to200 seeds per follicle in Nebraska. Schultz and Burnside(1979a) recorded an average number of 81 seeds per


follicle in naturally pollinated field-grown plants fromNebraska. The average number of follicles per plantproduced in Nebraska ranged from 150 in a monotypicnursery stand to 2 in soybean fields (Schultz and Burnside1979a). Lipow and Wyatt (1999) recorded an average of46.7933.8 seeds per follicle in hand-pollinated green-house plants in Georgia. This lower number relative toresults from Nebraska may be due to the inefficiency oftheir hand pollination since 162.9965.7 seeds developedin two naturally pollinated follicles within a flower whenthe greenhouse plants were placed in the field. Lorenziand Jeffery (1987) estimated production of approximately200 seeds per follicle in populations from across theUnited States. When grown in competition with mostagronomic crops, A. cannabinum does not produce manyviable seeds (Schultz and Burnside 1979a) and dependsprimarily on vegetative reproduction (Gerhards et al.1997).The seeds with their tuft of white silky hairs are

approximately 4-6 mm long and 0.75 mm wide andadapted to wind dispersal (Robison and Jeffery 1972;Doll 1994). The ability of seeds to float facilitatespotential dispersal by water (Evetts and Burnside1973). Burnside et al. (1981) concluded that owing topoor survival of seed in soil, seed dispersal plays a rolein establishing new populations, but new plants areprimarily established by vegetative propagation.

(c) Seed Banks, Seed Viability and Germination *Jeffery and Robison (1969) found that in Nebraska ca.50% of the seeds initially released from the follicle aredormant. In contrast, Burnside et al. (1981) found thatA. cannabinum seeds from Nebraska had ‘‘essentially noseed dormancy’’, with most buried seeds germinatingwithin a year of burial. Four years of burial reduced thegermination of seeds to below 1%. In Nebraska, Schultzand Burnside (1979a) examined seed weight and viabi-lity for seeds harvested at weekly intervals after flower-ing. They recorded an increase in seed weight from 199mg per 100 seeds 4 wk after full bloom to 736 mg per 100seeds 13 wk after full bloom. The first seeds to germinatewere those harvested 10 wk after full bloom, and themaximum germination attained was 38% for seedsharvested 14 wk after full bloom. Lipow and Wyatt(1999) obtained germination of 93.7% of the seedsproduced via hand-pollination.After 21 mo of burial in a Nebraska field study, Evetts

et al. (1972) found that no A. cannabinum seedsgerminated. Germination increased 41% when seedswere placed in the light, without any scarification, butscarifying seeds and subjecting them to either light ordark increased germination (Robison and Jeffery 1972).Unfortunately, these workers did not indicate whetherthey used freshly harvested seeds for these studies. Seedsplanted at 1 cm depth resulted in 70% emergence, whileemergence is greatly reduced at increasing depths(Robison and Jeffery 1972; Thompson 1972). Evettsand Burnside (1972) observed maximum emergence at

4.3 cm in coarse-textured soils, but only 3.3 cm in fine-textured soils. Although seeds may germinate between10 and 408C, Evetts and Burnside (1972) reportedoptimal seedling growth between 20 and 258C. Thehighest percentage seed germination recorded byThompson (1972) was at 358C. Similarly, Webster andCardina (1999) found maximum germination occurredat 348C when seeds from Ohio populations weresubjected to alternating temperatures. They also foundthat germination followed a sigmoidal pattern forconstant temperatures, whereas with alternating tem-peratures there was a linear relationship betweengermination and maximum daily temperature between15 and 348C.Evetts and Burnside (1972) obtained a high germina-

tion percentage with ample moisture (91% germinationat 0 MPa of osmotic pressure), but seeds failed togerminate at �0.91 MPa or higher. Similarly, Websterand Cardina (1999) observed a linear decline in germi-nation going from 80% at 0 MPa to 4% at �1.0 MPa,with seed germination decreasing rapidly at osmoticpotentials below �0.2 MPa.

(d) Vegetative Reproduction * Extensive vegetativereproduction and spread occurs by the horizontal rootsand rhizomes (Woodson 1930). New shoots may emergefrom either crown buds or from buds forming on thehorizontal roots and rhizomes (Woodson 1930; Frazier1944; Orfanedes and Wax 1991). Frazier (1944) con-ducted a series of experiments to elucidate the vegetativegrowth of A. cannabinum. Small feeding roots that grewalong the main vertical root that were in a suitableposition (i.e., mainly in the top 18 cm of soil) grew morerobust and became permanent lateral roots. Budsproduced along this permanent lateral root formed asecondary vertical root along with a shoot or rhizome.After 28 wk of growth without competition, Frazier(1944) found four permanent lateral roots with a rangeup to 3.5 m. By the second year, the roots hadpenetrated to 4.2 m deep and 5.9 m outwards (Frazier1945). Webster and Cardina (1999) demonstrated thatshoot appearance could be predicted based on accumu-lated growing degree units (GDU), with 50% of shootsemerging after about 280 GDU.

9. HybridsHybridization occurs frequently among species in thegenus Apocynum, complicating taxonomic classificationand increasing intraspecific variability (Woodson 1930;Anderson 1936; Balbach 1965; Voss 1996). Hybridsderived from Apocynum androsaemifolium and Apocy-num cannabinum, are sometimes designated as A.�floribundum Greene (�A.�medium Greene) and arefound throughout the sympatric distribution of theparental species (Fig. 5C). Plants identified by Anderson(1936) as either A. cannabinum or A. androsaemifoliumbred true, whereas offspring of A.� floribundum(F2 generation) segregated and displayed some traits


that resembled those of the parental species, but othertraits (e.g., pollen viability) were substantially different.Johnson et al. (1998) identified six isozyme loci diag-nostic for A. cannabinum and A. andorsaemifolium, andfound four putative hybrids which were heterozygous.Plants identified as hybrids exhibited lower seed set andproduced smaller sized seed than parental species.Balbach (1965), on the other hand, was unable togerminate seeds from plants intermediate between A.cannabinum and A. androsaemifolium.

10. Population DynamicsPopulations of A. cannabinum have been increasingthroughout the north-central, eastern, and the GreatPlains regions of the United States (Robison and Jeffery1972), but increased prevalence of this species in Canadahas not been reported. Usually, A. cannabinum isdistributed in patches, and is rarely found to infestentire fields (Robison and Jeffery 1972; Schultz andBurnside 1979a). Coffman and Frank (1988) observedthat A. cannabinum patches in no-till corn in Marylandtended to remain in approximately the same locationsover a 5-yr period. Patch growth and movement hasbeen studied fairly intensively. Webster et al. (2000b)found that patches in Ohio with an initial area less than23 m2 doubled in one year, whereas Gerhards et al.(1997) observed that patch sizes remained stable for 4 yrin Nebraska crop fields. In the latter study, A. canna-binum was more stable than the three annual weedsmonitored, being largely unaffected by various weedcontrol practices (Gerhards et al. 1996). The movementof A. cannabinum between fields is often facilitated bytillage equipment dragging roots, rhizomes and crownfragments (Schultz and Burnside 1979a). The patchborders are established early in the growing seasonand although the patches grew 2.4�3.5% per day in Mayand June, 50% of the final patch size was alreadydetermined in mid-May (Webster et al. 2000b). Thepotential for a serious infestation can be seen in therelative abundance, where 60% of the fields evaluated inNebraska had a population of 1�25 plants ha�1, 25%had 26�250 plants ha�1, and 15% had 251�2500 plantsha�1 (Schultz and Burnside 1979a). The ability ofindividual plants to spread may be strongly influencedby genotype, as more aggressive plants were shown tooccupy 19 times more area than the least aggressivewhen grown in common garden experiments in Illinoisand Michigan (Ransom et al. 1998a).Although A. cannabinum ordinarily attains heights

between 30 and 60 cm, it may occasionally produceshoots over 150 cm, with its growth strongly affected byenvironmental conditions (Woodson 1930). Robisonand Jeffery (1972) recorded effects of soil type andfertility level on plant growth in a greenhouse study.Higher fertility generally resulted in taller and heaviershoots, longer branches, more leaf pairs, and heavierroots. For one of the soil types tested (sandy soil),however, the moderate fertility treatment (112 kg N

ha�1) resulted in greater stem growth and more leafpairs than the high fertility level treatment (224 kg Nha�1) (Robison and Jeffery 1972). Ransom et al.(1998a) collected plants from 16 populations in bothMichigan and Illinois and subsequently planted them ina common garden in each state. These workers recordedgreater shoot number, shoot height and ground areacovered for ecotypes grown in Michigan as comparedwith those grown in Illinois. Mean shoot dry weightswere greater in Illinois plants, although considerablevariation was observed within ecotypes from Illinois.Leaf shape also differed, with Michigan-grown plantsexhibiting longer leaves in proportion to width ascompared with leaves produced in Illinois, and waslikely due to site differences. In general, Ransom et al.(1998a) observed a great deal of variation in almostevery morphological attribute they measured, presum-ably reflecting both a high degree of phenotypicplasticity and considerable genetic differences amongecotypes.Apocynum cannabinum is not a strong competitor in

small grains, hay, alfalfa or winter wheat (Becker 1981),but tends to grow faster than row crops, especially inspring (Furrer et al. 1983). In Nebraska, corn alsocompetes well with this weed whereas soybeans andgrain sorghum do not (Schultz and Burnside 1979a).Within corn, sorghum and soybean fields, the averagenumber of plants ranged from 39 700 to 76 500 ha�1

with irrigated fields containing approximately 25 000more plants ha�1 than dryland fields (Schultz andBurnside 1979a). The latter authors also found thatthe highest infestations were in oats and soybeans andthe lowest in alfalfa and winter wheat. In weedy checkplots maintained by Glenn et al. (1997) within a corncrop in Maryland, A. cannabinum populations increasedonly 10% over 2 yr, as compared with 123% forAllegheny blackberry (Rubus allegheniensis Porter) and177% for lamb’s-quarters (Chenopodium album L.).Current crop production systems favour the increasein A. cannabinum populations. Removal of annualweeds via pre-emergence herbicides decreases competi-tion and encourages abundant growth of this perennialspecies (Schultz and Burnside 1979a; Orfanedes andWax 1991).Thompson (1972) reported that the number of days

needed for the first shoot to occur after clippingdecreased as days from planting to clipping increased;the regrowth percentage of the weed clipped at thesoil line increased as days from planting to clippingincreased.

11. Response to Herbicides and Other ChemicalsNo specific Canadian studies on herbicidal control of A.cannabinum have been reported; thus, all findings arebased on studies carried out in the United States.There is a lack of consistency in the reported

effectiveness of chemical applications on A. cannabinum,and herbicides often fail to control the weed (Evetts and


Burnside 1973; Schultz and Burnside 1979b; Glennand Anderson 1993; Ransom and Kells 1998). Schultzand Burnside (1980) attributed the differences in controlto various edaphic, climatic and biotic factors.Welch and Ross (1997) conducted a common garden

pot study of root and shoot growth in response to theherbicides glyphosate, glufosinate, and 2,4-D. Shootgrowth was reduced more than root growth, and somehealthy root tissue remained in all treatments. However,single applications of either glyphosate (1262 g a.i.ha�1) or glufosinate (400 g a.i. ha�1) resulted in moreshoot dry weight than untreated plants. There was areduction in shoot dry mass from a single application of2,4-D (1121 g a.i. ha�1). The only treatment thatprevented the formation of new shoots was a splitapplication of glyphosate, where a second application of1262 g a.i. ha�1 was applied to surviving shoot growth.For all other treatments, Welch and Ross (1997)concluded that even if A. cannabinum was controlledduring the growing season, it would be capable ofre-establishing new plants subsequently from survivingroot segments.

Glyphosate *Control of A. cannabinum with glyphosatemay be limited, when compared with other perennialweeds, due to reduced foliar uptake. Wyrill and Burn-side (1976b) recorded that only 0.1% of appliedglyphosate diffused through the leaf cuticle of 5- to6-wk-old seedlings. Richard and Slife (1979) found thatadjuvants increased the amount of glyphosate absorbedin the first half hour by 3.75 times, but did not extendthe period or amount of absorption. Barnes andBrenchley (1972) observed that glyphosate (1700and 3400 g a.i. ha�1) was effective when applied toplants in the early bud stage (plants 1.2 to 1.5 m tall),but was ineffective at the late flowering stage (plants 1.5to 2.1 m tall). The addition of ethoxylated amines at upto 0.3% (wt/vol) concentrations acted as effectivesurfactants, improving uptake and increasing glyphosatetoxicity (Wyrill and Burnside 1977). In contrast, Doll(1997) found that glyphosate applied to A. cannabinumin the vegetative and bud stages was less effective at allrates used (630�1680 g a.e. ha�1) than glyphosateapplied in the early and full flower stages. The 630 ga.e. ha�1 rate seldom gave 90% control, while rates of840 g a.e. ha�1 or more almost always provided morethan 90% control. Glyphosate application (1130 or 2260g a.i. ha�1) resulted in approximately 90% and �85%control when applied in early summer and fall, respec-tively (Fawcett and West 1978; Curran et al. 1997).Increasing glyphosate rates did not improve control incorn, nor did applications in combination with 2,4-Dand dicamba (Curran et al. 1997). Studies conducted bySchultz and Burnside (1978b,c,d) found that impuritiesin water, especially high levels of Ca2� reduced thephytotoxicity of glyphosate, whereas monovalentcations had no effect. In general, however, glyphosate

is one of the most effective herbicides against A.cannabinum (Webster and Cardina 1999).

2,4-D and Dicamba * The herbicides 2,4-D (1120 g a.i.ha�1) and dicamba (280 g a.i. ha�1) together work wellto suppress growth of A. cannabinum, but may require anadditional application in the second year to maintainsuppression (Roeth 1977). In one study, 2,4-D alone (560g a.i. ha�1) was found to control 79�89% of A.cannabinum (Orfanedes and Wax 1991). However, as inthe case of glyphosate, diffusion of 2,4-D across thecuticle is limited (Wyrill and Burnside 1976b). Wyrill andBurnside (1976b) pointed out that the lack of 2,4-Dmetabolism in the roots of A. cannabinum explains itssusceptibility to the chemical. In corn, Bradley andHagood (2001) found that 2,4-D with either primisul-furon-methyl (39 g a.i. ha�1) or nicosulfuron (35 g a.i.ha�1) provided better control than dicamba (560 g a.i.ha�1) with either primisulfuron-methyl or nicosulfuron.Glenn and Anderson (1993) found that tank mixtures ofnicosulfuron (31�62 g a.i. ha�1) with 2,4-D (560 g a.i.ha�1) or dicamba (280 g a.i. ha�1) gave 72�100%control, while triclopyr (140�280 g a.i. ha�1) and 2,4-D(280�560 g a.i. ha�1) mixtures gave 72�98% control.Kalnay and Glenn (2000) agreed with Glenn andAnderson (1993) and showed that dicamba (70 g a.i.ha�1)�nicosulfuron (35 g a.i. ha�1) increased translo-cation of both herbicides more than either one alone. Theester form of 2,4-D�dicamba provided erratic control,but two studies (Schultz and Burnside 1979b; Ransomand Kells 1998) found that the 2,4-D ester (40�140 g a.e.ha�1) performed better than the 2,4-D amine (70�280 ga.e. ha�1). For best control, Becker and Fawcett (1980b)recommended applying in late June, while Roeth (1977)recommended September applications.

Paraquat * Bradley and Hagood (2001) recommendusing paraquat (260 g a.i. ha�1) to control A. cannabi-num in established alfalfa fields.

Fluroxypyr and Clopyralid * Although sensitive tofluroxypyr, A. cannabinum is not sensitive to clopyralid(560 g a.e. ha�1), possibly because of differences at theactivity site (Orfanedes and Wax 1991; Orfanedes et al.1993) or, because the latter is highly water-soluble, itspenetration of the thick cuticular wax is inhibited(Orfanedes et al. 1993). Resprouting of A. cannabinumplants near the base of controlled shoots has beenobserved 4�8 wk after application of fluroxypyr (70�280g a.i. ha�1) (Orfanedes and Wax 1991).

Thiocarbamates * Thiocarbamates may suppress re-growth from either crown or lateral roots (Schultz andBurnside 1979b; Becker 1981). According to Schultz andBurnside (1979b), preplant applications of thiocarba-mate herbicides (e.g., butylate) aided in control of theweed by delaying emergence, suppressing vigour, and


possibly allowing subsequent foliar applications of 2,4-D to be more effective, with these effects persisting forabout 2 mo.

Nicosulfuron*Nicosulfuron (35 g a.i. ha�1) was shownto have similar efficacy as dicamba (560 g a.i. ha�1),and the efficacy of both was enhanced in combinationwith 2,4-D (280 g a.i. ha�1) (Glenn et al. 1997).

Primisulfuron*Doll (1994) found 80�90% control withprimisulfuron (20 g a.i. ha�1) when plants were 12�30cm and better at stem heights up to 61 cm. Curran et al.(1997) also tested primisulfuron (20 g a.i. ha�1) withdicamba (282 g a.e. ha�1) and found control equal tofall applied glyphosate (�85%). Glenn et al. (1997)reported that primsulfuron (20 g a.i. ha�1) was lesseffective than nicosulfuron (35 g a.i. ha�1) or dicamba(560 g a.i. ha�1).

Glufosinate * Although a single application of glufosi-nate (400 g a.i. ha�1) in a greenhouse experiment failedto provide adequate control, a split application, wherebythe same herbicide dosage was re-applied to survivingshoots, resulted in a 79% reduction in shoot volume(Welch and Ross 1997).

Amitrole * Amitrole (560�2240 g a.i. ha�1) providedpoor control in all studies (Schultz and Burnside 1978g;Schultz and Burnside 1979b).

Timing of Application of Certain Herbicides * Moststudies have concluded that fall application is moreeffective than spring (Robison and Jeffery 1972; Roeth1977; Becker 1981; Furrer et al. 1983; Curran et al. 1997;Becker and Fawcett 1998). Fawcett and West (1978)found almost no control if herbicides are applied aftercorn harvest and Furrer et al. (1983) cautioned againstspraying after frost- or drought-induced yellowing in theleaves. Doll (1994) found that spot treating or applyingpre-harvest in soybeans was effective. Spring sprayinghas been recommended during the early reproductivestage to early flowering stage when most of the growthand phloem movement is occurring (Furrer et al. 1983;Orfanedes and Wax 1991; Orfanedes et al. 1993). Earlypost-emergence applications of herbicides will notinhibit regrowth from perennial roots (Becker 1981;Furrer et al. 1983; Ransom and Kells 1998). On theother hand, seedlings were susceptible to a wide range ofsoil-applied herbicides, i.e.,�80% control was achievedwith clomazone (840 g a.i. ha�1), sulfentrazone (220 ga.i. ha�1), metribuzin (280 g a.i. ha�1), atrazine (840 ga.i. ha�1), flumetsulam (40 g a.i. ha�1), cloransulam (30g a.i. ha�1), imazaquin (140 g a.i. ha�1), and metribuzin(180 g a.i. ha�1) plus chlorimuron (30 g a.i. ha�1)(VanGessel 1999). Often, the timing of pre-emergentherbicide application does not correlate with susceptiblestages of perennial weeds, therefore, only the top growthof the weeds is affected, while the weed continues to re-

grow from dormant below-ground buds (Glenn andAnderson 1993).

12. Response to Other Human ManipulationsNo-till systems, more than chisel or moldboard, oftenexperience infestations because the under-ground rootsystem is left intact, which allows new plants to developfrom lateral root buds (Becker 1981; Buhler et al. 1994).However, reduced density of established A. cannabinumpopulations through tillage may be accompanied byincreased area of infestation due to root fragmentationand dispersal throughout the tilled area (Woodson 1930;Robison and Jeffery 1972; Schultz and Burnside 1979b).Doll (1995) recommended using no-till systems wherepossible because tillage may increase the already vari-able early emergence, while no-till systems allow theplants to reach a more susceptible stage when postemer-gence herbicides are normally applied. Likewise, Beckerand Fawcett (1980a) concluded that no tillage, ascompared with moldboard plough, chisel plough ordisk systems would result in the least persistence interms of numbers of plants (but not plant vigour). Fallchisel ploughing may only improve early-season control,while spring chisel ploughing may increase biomasswhen herbicides are not used (Curran et al. 1998).Postemergence herbicide applications do not tend tocorrespond with susceptible stages of A. cannabinum,and thus reduced tillage systems may favour this speciesover other weeds (Triplett 1985). Similarly, cultivationshould be timed to correspond to the early seedling stage(B 25 cm tall) before they are capable of forming newshoots from root buds (Doll 1995). Plant vigour, rate offlowering, and maturation are decreased with increasingseverity of tillage (Becker and Fawcett 1980a). Shallowcultivation at 2- to 3-wk intervals for 2 to 3 yr willeventually deplete the energy reserves in the roots(Schultz and Burnside 1979b; Becker 1981).Crop rotation with alfalfa and frequent clipping for

hay will diminish the competitive effects of A. cannabi-num (Evetts and Burnside 1973; Becker 1981). In agreenhouse clipping experiment, Robison and Jeffery(1972) found that regrowth occurred only after plantswere 41 d old and that plants less than 10 cm in height orwith fewer than eight leaf pairs did not regrow. Schultzand Burnside (1978e,f) found that regrowth of clippedplants in a greenhouse decreased if clipping occurred 1 dafter spraying with either 2,4-D or glyphosate, andremained depressed if clipping was delayed up to 2 wkafter application of high rates of 2,4-D (105 g ha�1).Rotation with winter wheat allows for increased controlof A. cannabinum patches (Evetts and Burnside 1973;Furrer et al. 1983). Evetts and Burnside (1973) recom-mended that if this species still persisted in a field aftercompetition with winter wheat, periodic cultivationcould serve to further deplete food reserves and even-tually cause mortality.


13. Response to Herbivory, Disease and HigherPlant Parasites(a) Herbivory

(i) Mammals * No information was located.

(ii) Birds * No information was located.

(iii) Insects * Schultz and Burnside (1978a) in Nebraskaobserved four different Lepidoptera feeding on the foliageof A. cannabinum, which caused enough defoliation insome cases to decrease the effectiveness of foliar appliedherbicides. The four insects found were the woolly bearmoth Ammalo teneraHubner (Arctiidae), Pyrausta futia-lis Lederer (Pyralidae), the leafroller Aphelia palloranaRobinson (Tortricidae), and the milkweed tiger mothEuchaetes egleDrury (Arctiidae).Most foliar damagewascaused by A. tenera and P. futialis and the extent ofdefoliation varied from 2 to 95% (Schultz and Burnside1978a). Another lepidopteran specializing on A. cannabi-num (and the genus Asclepias), the dogbane tiger mothCycnia teneraHuebner (Arctiidae) was observed causingconsiderable damage to A. cannabinum in Pennsylvania(Cahill et al. 2002). Cahill et al. (2002) also recorded thedogbane beetleChrysochus auratusFabricius (Chrysome-lidae) feeding on A. cannabinum in Pennsylvania. Theyalso noted that A. cannabinum damage due to theseherbivores increased when nearby vegetation wastrampled and that touching or handling ofA. cannabinumplants resulted in higher incidence of emergence holesfrom the lepidopteran stem borer Papaipema baptisiaeBird (Noctuidae). Panic moth, Saucrobotys futilalisLederer (Pyralidae), larvae have also been documentedto feed on A. cannabinum foliage in central New YorkState (Grant 2007). Other lepidopterans documented touse A. cannabinum as a host in Nearctic regions of NorthAmerica include Marmara apocynella Braun (Gracillar-iidae), zebra caterpillar moth Melanchra picta Harris(Noctuidae), and the snowberry clearwing mothHemarisdiffinis Boisduval (Sphingidae) (Natural History Mu-seum 2008). Lepidopterans reported to use Apocynumspp. as host plants in Nearctic regions include the OregonCycniamothCycnia oregonensis (Stretch) (Arctiidae) andthe six-spotted gray moth Spargaloma sexpunctataGrote(Noctuidae) (Natural History Museum 2008).

(iv) Nematodes and Other Invertebrates * No informa-tion was located.

(b) Diseases

(i) Fungi * Fungi reported from A. cannabinum(including A. sibiricum) in Canada are Cercosporellaapocyni (Ellis & Kellerm.) Trel., Cylindrosporium apoc-yni Ellis & Everh., C. sibiricum Dearn. & Bisby, Pucciniaseymouriana Arthur and Septogloeum apocyni Peck(Conners 1967). Farr et al. (2004) listed additionalspecies recorded on A. cannabinum elsewhere in North

America: Aecidium apocyni Schwein. (North Carolina),Bionectria apocyni (Peck) Schroers & Samuels (�Nec-tria apocyni Peck) (New York), Colletotrichum trunca-tum (Schwein.) Andrus & W.D. Moore (Illinois),Fusarium oxysporum Schltdl. (Delaware), Macropho-mina phaseolina (Tassi) Goid. [�M. phaseoli (Maubl.)S.F. Ashby] (Illinois), Pestalozziella andersonii Ellis &Everh. (Montana), Phyllosticta apocyni Trel. (Florida,Mississippi, New Jersey, New York, Oklahoma, Wis-consin), Phymatotrichopsis omnivora (Duggar) Henne-bert (�Phymatotrichum omnivorum Duggar) (Texas),Puccinia smilacis Schwein. (Illinois, Kansas, Maryland,North Carolina, Tennessee, Virginia, Wisconsin), Sep-toria littorea Sacc. (Kansas, Mississippi, North Dakota,Nebraska, Ohio), Stagonospora apocyni (Peck) Davis(Indiana, Minnesota, New York, South Dakota, Virgi-nia, Wisconsin) and Thanatephorus cucumeris (A.B.Frank) Donk. (Washington). Venkatasubbaiah et al.(1992) identified a fungus found extensively on A.cannabinum in Virginia as Stagonospora apocyni. All ofthe toxins produced by S. apocyni (citrinin, mullein,tyrosol and a�actyloricinol) caused necrosis on leaves ofhemp dogbane. Plants inoculated with S. apocyni hadsmall black lesions, and heavily infected leaves abscised.

(ii) Bacteria * No information was located.

(iii) Viruses * No viruses have been recorded for anyspecies of Apocynum in the VIDE database (Brunt et al.1996 onwards).

(c) Higher Plant Parasites * No information waslocated.

ACKNOWLEDGEMENTSThe curators are thanked for the loan of specimens fromthe following herbaria: ACAD, ACK, ALTA, CAN,DAO, HAM, MMMN, MT, NFLD, NSPM, QFA,QUE, SASK, TRT, TRTE, UNB, UVIC, UWO,UWPG, WAT, WIN, and V. The help provided byScott Morris and Marisa Isaacson to locate references isalso acknowledged.

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Key to Apocynum in Canada.1. Flowers terminal and often axillary; calyx usuallyabout 1/3 as long as corolla; corolla (5) 5.5�8 (10) mmlong, petals strongly spreading to recurved, pink to rosy(usually at least pink to reddish on the veins within);leaves ovate to broadly elliptic, widely spreading ordrooping; seedsB3 mm long . . . . . . . . . . . . . . ...A. andro-saemifolium

1. Flowers terminal, rarely axillary; calyx about 1/2 ormore as long as corolla; corolla (2) 2.5�4.5 (6) mm long,petals erect, white to greenish; leaves oblong to broadlylanceolate, usually ascending or slightly spreading, butnever drooping; seeds�3 mm long . . . . . . . . . . . . . . ..A.cannabinum. Go to 2

2. Leaves in the middle of the stem with blades taperingto a petiole�3 mm long . . . . . .A. cannabinum var.cannabinum

2. Leaves in the middle of the stemwith blades rounded tocordate at the base, often clasping the stem, petioleusuallyB3 mm long . . ..A. cannabinum var. hypericifo-lium


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