Classification of weeds

Weed classification groups

Habitat

Life Cycle

Taxonomic Group

Physiology

Habitat

Terrestrial Live on land

Crops, rangeland, turf, forests, etc. Aquatic

Structurally and physiologically modified to live on, near or around water

Aerial Not rooted in the soil an use other plants for support

Parasitic: Mistletoe, dodder Epiphytic: Orchids, Spanish moss

Life Cycle

Herbaceous plants Plants with non-woody aboveground stems Annuals

Plants that live for one growing season only Re-establish from seed

Biennials Plants that require parts of two growing seasons to complete their life cycle Reproduce from seed

Perennials Plants that live indefinitely In addition to seed, some of these plants produce other structures that

allow them to overwinter and/or reproduce Modified stems (stolons, rhizomes,corm, bulb, tuber), leaves (part of a bulb) or

roots (tuber)

Life Cycle

Woody plants Plants with woody aboveground stems that persist from

year to year Trees

Woody perennials with a single main stem or trunk Shrubs

Woody perennials with more than one principal stem arising from the ground

Taxonomic Group

Dicots Plants whose seedlings produce two cotyledons (seed leaves) Typified by:

Net leaf venation Flower parts in multiples of 4 or 5 Called “broadleaves” Examples?

Monocots Plants whose seedlings bear only one cotyledon Typified by:

Parallel leaf venation Flower parts in multiples of 3 Called “grasses” or “grass-like plants”

Physiology

Photosynthesis is the process by which plants convert CO2 to glucose (carbon fixation)

Plants can vary in how they convert CO2 to glucose (carbon fixation) C3 plants

First stable product of the Calvin Cycle is a 3 carbon acid C4 plants

First stable photosynthetic product is a 4 carbon acid More efficient at photosynthesis under high temperatures Many weeds exhibit this photosynthetic pathway

How weeds interfere with desirable plants

Interference

Not all interference is negative Weeds interfere with growth of desirable plants via three

mechanisms: Competition Allelopathy Parasitism

Mistletoe, dodder, broomrape, witchweed

Competition

The ability of weeds to compete for light, water and nutrients depends on three main factors:

Timing of weed emergence The first plant to emerge has a

distinct competitive advantage Why?

Can access resources Can negatively affect the other

plants Critical weed-free period

concept

Crop Critical Weed-Free Period

Soybean 1st-3rd trifoliate-leaf stage (V2-V3)

Spring cereals 1-3-leaf stage (Zadok's 10-13)

Forages Year of establishment: 4-6 weeks after planting

Canola Emergence to 6-leaf stage

Competition

Growth form Growth habit and rate of development Which are more competitive, broadleaves or grasses?

Stem height, leaf area, branching, leaf angle Root development

Weed density Increased weed density = decreased crop yield Are there exceptions to the rule??

Allelopathy

Allelopathy refers to the chemical inhibition of one species by another The "inhibitory" chemical is released into the environment where it affects the

development and growth of neighboring plants

Allelopathic chemicals can be present in any part of the plant May inhibit shoot/root growth May inhibit nutrient uptake May attack a naturally occurring symbiotic relationship thereby destroying the

plant's usable source of a nutrient

Has proven difficult to study The case of spotted knapweed and catechin

2003 study showed that the plant produces catechin Found that native grasses were more susceptible to catechin than European grasses

2005 study unable to detect catechin in soils infested with spotted knapweed

How weeds persist

Weed seeds

Large numbers Weeds can produce thousands of seeds per plant Small seeded species are usually more prolific

Weed species Weed seeds/plantBarnyardgrass 7,000Common ragweed 15,000Shepherd’s purse 38,500Common lambsquarter 72,000Redroot pigweed 117,000Russian thistle 200,000Adapted from: Ross, M. A., & C.A. Lembi. Applied weed science. 3rd Edition. 2009.

Weed seeds

Can survive and remain viable under adverse conditions Well-protected by seed coats resistant to breakdown

Resist freezing, drought, fire, animal predation Inactive seeds cannot be controlled by herbicides

Have adaptations that help with dispersal Hooks and spines adhere to animals Floating on water, carried on wind Are of similar size and shape to crop seeds so they cannot be removed

Seed banks

Weed seeds build up in the soil over time

One study 7 million weed seedlings/ac = 280

million seeds per acre-furrow slice (6”deep)

Weed seed bank dynamics dependent on Number of seed present Species composition Depth of distribution Seed viability

Seeds can remain viable for years

Weed Seed Viability in Years

Quackgrass 1-6Wild oat 4-7Foxtail 20Field bindweed 20+Canada thistle 21Common lambsquarter

40

Redroot pigweed 40Adapted from: Ross, M. A., & C.A. Lembi. Applied weed science. 3rd Edition. 2009.

Seed banks

Dormancy is a temporary condition where viable seeds fail to germinate even when environmental conditions are adequate

Seed dormancy ensures continued germination for many years Primary dormancy

Seeds are unable to germinate after they first mature Secondary dormancy

Induced over a period of time by environmental conditions Quiescence

A period of inactivity cause by the lack of a necessary factor (e.g., oxygen, water, light) Afterripening

Transition from dormancy to a more readily germinable state Stratification (exposure to cooler temperatures for a period of time) Scarification (breaking of seed coat) Light-regulated

Vegetative Reproductive Structures

Major food storage organs Possess numerous buds capable of starting new plants Rhizomes, tubers, bulbs, stolons and creeping roots

Serve as overwintering mechanisms Serve as a second means of propagation Allow plants to grow more quickly than those from seed Help with survival after disruption

½ inch piece of Canada thistle can start a new plant

Questions!

1. Dormancy is a temporary condition where viable seeds fail to germinate even when environmental conditions are adequate. True or False?

2. Biennial weeds complete their life cycle in ____________ year(s)?a. Twob. Onec. Every other d. More than two

3. Which are more competitive, broadleaf or grass weeds? Why?

Wild buckwheat Scientific name: Fallopia convolvulus Family: Polygonaceae Life cycle: Annual Where found: Cropland Physical description:

Seed: black, three-angled Cotyledon: linear to oblong True leaves arrowhead to heart shaped

Alternate arrangement Growth habit: Prostrate/climbing Leaves possess small stiff hairs making it rough to the touch

Interesting facts: Latin name describes resemblance to bindweed

convolvulus=refers to species associated with Convolvulaceae family (bindweeds), meaning “to intertwine”

Bedstraw aka cleavers, goosegrass Scientific name: Galium aparine Family: Rubiaceae Life cycle: Annual Where found: Range/Cropland Physical description:

Seed: ball-shaped with stiff, hooked bristles Cotyledon: ovate to round, apex indented True leaves linear with barb at the tip

Whorled arrangement, 4-5 leaves at first nodes, then 6-8 Leaf surface has short stiff hairs that help it cling to

other plants Interesting facts:

Square stems Latin name describes low, dense growth habit

Galium= milk, aparine= seizer Rumored to have once been used as a strainer

in cheese making