8/12/2019 Green Herbicides

1/2

Green Herbicides

Dr. Michael Pfeiffer

Herbicides are defined by EPA as "substancesintended to kill weeds or other plants that grow wherethey are not wanted". So what makes an herbicide agreen herbicide? That depends on who you talk to.Some would say that Roundup is a green herbicidebecause of its low mammalian toxicity, few non-targetproblems, low potential to leach into ground water andquick degradation. Others would state that Roundupcan not possibly be a green herbicide because it issynthetic. Any type of classification system is bydefinition, artificial. For our purposes greenherbicides will be products which pose the leastpotential hazards to individuals and the environmentand are exempt from registration with EPA underFIFRA 25(b) or are considered bioherbicides. See thecitations for further details on exemption fromregistration with the EPA and what EPA considersbioherbicides/biopesticides. Be aware that theassignment of green in this publication does not

mean absence from risk. There is risk associated

with any pesticide to include herbicides described as"green".

There are two types of green herbicides, pre-

emergence herbicides and post emergence herbicides.PRE-EMERGENCE HERBICIDES:

The only current green pre-emergence products

on the market all contain the same active ingredient,

corn gluten meal (CGM) . Products containing CGMshould be used primarily for weed control in turf and

possibly around established perennials. Corn gluten

applied at 20 lbs/1000 square feet in turf on a regular

basis has provided variable control of weeds such as

chickweed, crabgrass and dandelion. The weed control

effect from CGM are likely from two mechanisms. One

mechanism is the release of organic molecules(peptides) from CGM which have a "root stunting"

regulatory effect on weeds and other plants when in the

seedling stage. Corn gluten meal also contains 9-10% by

weight nitrogen which may make the turf more

competitive. A 20 lb/1000 sq. ft. rate of CGM would

supply 1.8-2.0 lbs of actual nitrogen. That amount of

nitrogen could cause unanticipated side effects such asa need for more frequent mowing, more biomass to

dispose of, more turf disease, increased growth of

existing weeds and the danger of pushing turf too hard

with nitrogen in hot weather. Rates of CGM appliedmay need to be adjusted downward depending on

location. The best control with CGM has been obtained

when there is a dry period after application so irrigated

turf likely will be problematic. The price differential for

corn gluten vs synthetic herbicides will be substantial

Two applications of CGM needed for 8-12 weeks of

control at a rate of 10-20 lbs per 1000 sq. ft. will be in

the $30.00-$60.00 range. Pre-emergence herbicides

such as pendimethalin will be in the $1.50 range for 8-

16 weeks of control. Control of weeds with corn gluten

meal has been very erratic depending on location(Chalker-Scott,2008). Best success with CGM has been

in the Midwest. It is unlikely that corn gluten wil

supply the same level of control as synthetic pre-

emergence herbicides however clients may not care! If

the turf is uniform and green many people dont care

about a few weeds.

POST EMERGENCE HERBICIDES:There are a number of green herbicides which are

applied to plants after emergence. The majority ofwhich are contact, non-selective type herbicides

Contact, non-selective herbicides control weeds bydestroying plant foliage. They work best on small weedsand are much less effective on older plants. In generalcontrol of broadleaf weeds appears better than control ofgrasses with these products. If control of perennials suchas bermudagrass is needed, these products will need tobe continually applied as they do not kill roots of per-ennials. If these types of products are used in turf, theturf will also have burned foliage. They provide no pre-emergence activity and thus will need to be reappliedwith new flushes of weeds. Good spray coverage isessential for success and higher spray volumes (50-70

gallons of water per acre) appears to increase efficacyAddition of a spray adjuvant designed to increasecoverage of the foliage may increase activity of theseproducts.

There are two green herbicide products on themarket which claim to be selective in turf. One productAgralawn Crabgrass Killer list cinnamon bark as theactive ingredient. It claims to control crabgrassbasketgrass, chickweed, clover and other similar weedsin St. Augustine, Bahia, Common Bermuda andCentipede grasses. This product is applied as a dust to

8/12/2019 Green Herbicides

2/22012 PESTICIDE TRAINING RESOURCES

wetfoliage of weeds. There are precautions about beingrain free for several days after application and that theproduct will stain concrete.

The other products which claim to be selective inturf all contain the same active ingredient chelated iron,FeHEDTA (iron hydroxyethylenediaminetriacetic acid).Claims are that products containing FeHEDTA willcontrol or suppress 23 different broadleaf weeds in cool

season turf. There are no claims of control of grasses.Products containing FeHEDTA should not be used onbentgrass and there are cautions about using productscontaining FeHEDTA when the temperature will exceed85F. I have seen no information on FeHEDTAs safetyon Bermudagrass or zoysia. EPA considers productswhich contain FeHEDTA to be biopesticides soregistration is required.

There are other bioherbicides (plant pathogens)which have been used on a limited basis. The environ-ment, i.e. temperature, rainfall, humidity, UV light

for which we usually have no control over, can have aprofound effect on the activity of these bioherbicidesThere likely will be more bioherbicide products releasedin the future.

Biological control of weeds with insects has beenless than satisfactory on a small scale. The best contro(usually not eradication) of weeds with insects has beenon large scale infestation such as rangeland or aquatic

sites. It is important to remember that for control ofweeds with insects, the insects must become establishedon a large scale. This is often difficult to achieveAlthough control of weeds with insect has had limitedsuccess, do not eliminate this option from the arsenal.

There have been few if any University type trialsconducted (exception: Patton & Weisenberger, 2012) todetermine the efficacy of many of these green herbi-cides. Caution should be exercised before committing touse of green herbicides on a large scale without carefulinvestigation and planning.

A list of active ingredients and type of activity of green herbicides is provided below. This list shouldbe considered non-inclusive as new green herbicides products are likely in the future.

Active Ingredient Types of ActivityAcetic acid (vinegar) Postemergence, Nonselective.Ammoniated soap of fatty acid Postemergence, NonselectiveCinnamon Bark Postemergence, Selective weed control in turfCinnamon oil, rosemary oil Postemergence, NonselectiveCitrus oil (d-limonene) Postemergence, NonselectiveCitric acid Postemergence, NonselectiveClove oil Postemergence, Nonselective

Corn Gluten Meal Pre-emergence, NonselectiveEthanoic acid Postemergence, Nonselective.FeHEDTA Postemergence, Selective weed control in turfOctanoic acid Postemergence, NonselectivePelargonic acid Postemergence, NonselectivePotassium salts of fatty acids Postemergence, NonselectiveThyme oil Postemergence, Nonselective

Citations:Chalker-Scott, L. 2008. The Myth of Weed-Killing Gluten: Article. Washington State University. http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Corn%20gluten.pdf

EPA. Minimum Risk Pesticides, Criteria for FIFRA 25(b) Exemption.(2012).

http://www.epa.gov/oppbppd1/biopesticides/regtools/25b_list.htmEPA. Regulating Biopesticides. (2012). http://www.epa.gov/opp00001/biopesticides/.

Patton, A. & D. Weisenberger. 2012. Efficacy of Current Postemergent Weed Control Options in Turfgrass Systems.Annual Report. Purdue Univ. Turfgrass Science Progress. PP 26-30.http://www.agry.purdue.edu/turf/report/2011/PDF/06_AGRY_Patton_organic%20weed%20control.pdf

www.ptrpest.com