Organic Weed Control Products for Use in Lawns and Gardens

Tim Miller WSU NWREC

Natural Herbicides for Weed Control

• In today’s class, we’ll talk about: – Preplant incorporated and

preemergence products (applied to soil as an amendment or as a surface treatment)

– Postemergence products (applied to weeds that are up and growing either as a foliar or a drench)

Organic Amendments for Weed Control

• PRE/PPI products –Corn gluten meal –Wheat gluten –Mustard seed meal –Distillers grain

Corn Gluten Meal • Protein fraction of corn grain extracted by the

wet-milling process (high fructose corn syrup) • Root formation during germination is inhibited

in susceptible species – Some level of weed control achieved by water

stressing seedlings • Contains 10% nitrogen • Sold as organic amendment for use in turf

and small fruits/vegetable gardening

Corn Gluten Meal • Greenhouse Trial

– Rates equivalent to 2900, 5800, and 8700 lbs/a (67, 133, and 200 lbs/1000 ft2)

– Weed control ranged from 97% for purslane to 0% for quackgrass (seedling) at the low rate and from 100% for purslane, green foxtail, dandelion (seedling), and black nightshade to 45% for velvetleaf at the high rate

– Roots were more inhibited than shoots, control improved by incorporating the product

Wheat Gluten

• Two wheat proteins (gliadin and gluteanin) become elastic when mixed with a liquid and kneaded – Gluten traps carbon dioxide formed from

fermentation by yeast in dough, causing bread to rise

• Like corn gluten, wheat gluten is a source of nitrogen (12%)

• Sold as a food product to bakeries

Wheat Gluten • Greenhouse Trial

– Rates ranging from 900 to 8000 lbs/a (21 to 184 lbs/1000 ft2)

– Weed seed germination was reduced by 50% or more for 8 species at 900 lbs/a and for 12 species at 2700 lbs/a

– Shepherd’s-purse, henbit, annual bluegrass, and Canada thistle (seedling) were most sensitive

– Root inhibition was greater than shoot inhibition

Mustard Seed Meal • Dry portion of mustard seed after the

crushing process removes the oil • Contains high quantities of glucosinolates

(sulfur-bearing molecules) – When moistened, enzymatic degradation of

glucosinolates (myrosinase) releases isothiocyanate, a volatile biocide

– Some level of weed control achieved by killing germinating seedlings

• Contains ~7% nitrogen

Mustard Seed Meal • Field and Greenhouse Trials

– Rates up to 2000 lbs/a (46 lbs/1000 ft2) – Weed control ranged from 25% to over

90%, depending on species – Crop injury

• Killed certain vegetable seedlings • Slowing of growth of perennials

– Also provided some control of fungal pathogens and destructive insects in soil

So…Do They Work?

So…Do They Work? • Corn Gluten Meal

– Not very well – In my trials, I’ve never seen greater

than about 20% weed control from applications of corn gluten meal

– Applications may encourage slugs, crows, and dogs

– Research continues with Hydrolyzed CGM • Amino acids that are not active are

removed, leaving those dipeptides that are active

So…Do They Work?

• Wheat Gluten – Not very well – I saw no weed control in my trials – As with corn gluten meal,

applications may attract slugs

So…Do They Work?

• Mustard Seed Meal – Yes, perhaps too well – Very “hot” on seedling weeds…

…but also on seeded crops (veggies) and on perennials like strawberries

– A similar effect has also been observed from cover cropping mustard family species

Organic Amendments and Herbicides in Strawberry

• Corn gluten meal, mustard seed meal (high and low glucosinolate), and wheat gluten applied post-transplant in 2000 and 2001 – Goal, Prowl, and Goal + Prowl also applied

post-transplant – All treatments applied again in spring of 2001

and 2002

• Hand weeding times noted throughout the growing season

• Berries harvested, 2001 and 2002

Corn Gluten Meal 435 lbs/a (=1500 lbs/a in the band)

Corn Gluten Meal 870 lbs/a (=3000 lbs/a in the band

Mustard Seed Meal 575 lbs/a (=1980 lbs/a in the band)

Effect of organic amendments and herbicides on weeding time of strawberries.

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Corn gluten (low)Corn gluten (high)Wheat glutenMustard seed (low)Mustard seed (high)UntreatedGoalProwlGoal + Prowl

Natural Herbicides for Weed Control

• POST products –Vinegar (acetic acid) –Pelargonic acid –Clove/Cinnamon oil –Pine oil –Limonene –Lemongrass oil

Use of Acetic/Citric Acid for Blackberry Control

• Various products tested for blackberry control (conducted in 2002) – Blackberry & Brush Blocker, Crossbow,

Roundup, and Finale tested

• Products were applied as cut-stem (crowns mowed to 6”) and foliar treatments in mid-February (late dormant)

• Foliar B&BB, Roundup, and Finale applied again in late May (non-dormant)

Untreated Blackberries

Cut canes

Uncut canes

Blackberry & Brush Blocker

Full strength, cut stem 1/3 qt/400 ft2

Diluted, foliar 6.4 fl.oz./400 ft2

Blackberry & Brush Blocker Summer Treatment

Full strength 1/3 qt/400 ft2

Diluted, foliar 11.3 fl.oz./400 ft2

Blackberry control after cut-stem treatment with several products

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B&BB cutCheck cut

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Blackberry control after foliar treatment with several products

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B&BBUntreated

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Full strength, 1.3 qt/400ft2

Blackberry & Brush Blocker Summer Treatment, Bittersweet Nightshade

Scythe Foliar, 10%

Interceptor Foliar, 20%

Using Vinegar Drenches • Blackberry work (mine)

– One pint of B&BB reduced re-sprouting of clipped blackberry crowns by 67%

– At the price of $20/gal = $2.50 per crown

• Cranberry work (Kim Patten, WSU Long Beach) – One gallon vinegar (5% acetic acid) per

weed in cranberry bogs gave excellent control of some perennial weeds

– At Safeway price $3.50/gal (per weed)

Control of Annual Weeds With Vinegar Products

• Two household vinegars (5% acetic acid) – White distilled – Cider

• Blackberry & Brush Blocker • Study conducted in 2003

Effect of organic herbicides for annual weed control

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B&BBearly

B&BB late Pine oilearly

Pine oillate

2 DAT4 DAT8 DAT15 DAT

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Effect of vinegar for annual weed control

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Whiteearly

White late Ciderearly

Cider late

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Control of Annual Weeds from Two Applications of Organic Herbicide

• Vinegar (Weed Pharm, 20% acetic acid) • Clove oil (Matran 2, 10 and 20%) • Pine oil (Interceptor, 10 and 20%) • Study conducted in 2004 • Products applied twice, one week apart

– First application to cotyledon-stage annual weeds

• Tested with and without two surfactants

Effect of organic herbicides for weed control

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Weed Pharm Matran Interceptor Alone, with Humisol, with Yucca Extract

Multiple Applications of Organic Herbicides for Control of Canada

Thistle in Raspberry

• Various products tested for Canada thistle control (conducted in 2006) – Organic herbicides

• Clove oil (20%), pine oil (20%), and vinegar (20% acetic acid)

– Conventional herbicides • Gramoxone, Goal, and Finale

Organic Herbicides for Control of Canada Thistle in Raspberry

• Applied as foliar treatments – Organics applied five times

• Four pre-harvest, at weekly intervals from early May through early June

• Once post-harvest, in August

– Conventional products • Once pre-harvest in early May • Once post-harvest in August

– All Canada thistle plants removed by hand just prior to harvest (early July)

Untreated

Untreated

Paraquat

Clove oil

Pine oil

Vinegar

Effect of organic herbicides on Canada thistle

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Initial burn Mid-season Season long

Clove oilPine oilVinegarParaquat

%

Organic Herbicides • Of the several non-selective herbicides

evaluated for organic weed control: – Acetic acid: General consensus is that products

should contain a minimum of 20% acetic acid; acetic acid and limonene should be used full strength (no dilution)

– Clove oil and pine oil: General consensus on rate is for 15 to 20% of the product in water

– Pelargonic acid: New formulation may become organically certified. General consensus on rate is for 5 to 10% of the product in water

Bottom Line: Organic Herbicides • PPI products can be active on seedling

weeds – Mustard Seed Meals can be hot enough to

injure certain vegetable crops and perennials

• POST products can be active on seedling weeds – Acetic acid and oil extracts are best applied on

warm, dry days and generally require repeat applications to effectively control weeds over time

– Injury to adjacent plants may occur