The document discusses green herbicides, defined as substances with the least potential hazards to individuals and the environment, exempt from EPA registration or classified as bioherbicides. It details the two types of green herbicides: pre-emergence, primarily using corn gluten meal, and post-emergence, which includes contact, non-selective herbicides and selective options for turf. Additionally, it highlights the need for careful consideration and planning before large-scale use of green herbicides due to varying efficacy and environmental factors.

1. Green Herbicides
Dr. Michael Pfeiffer
Herbicides are defined by EPA as "substances
intended to kill weeds or other plants that grow where
they are not wanted". So what makes an herbicide a
green herbicide? That depends on who you talk to.
Some would say that Roundup© is a green herbicide
because of it’s low mammalian toxicity, few non-target
problems, low potential to leach into ground water and
quick degradation. Others would state that Roundup®
can not possibly be a green herbicide because it is
synthetic. Any type of classification system is by
definition, artificial. For our purposes “green”
herbicides will be products which pose the least
potential hazards to individuals and the environment
and are exempt from registration with EPA under
FIFRA 25(b) or are considered bioherbicides. See the
citations for further details on exemption from
registration with the EPA and what EPA considers
bioherbicides/biopesticides. Be aware that the
assignment 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, preemergence 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 CGM
should 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 as
a 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 applied
may 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 816 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 will
supply the same level of control as “synthetic” preemergence herbicides however clients may not care! If
the turf is uniform and green many people don’t care
about a few weeds.
POST EMERGENCE HERBICIDES:
There are a number of green herbicides which are
applied to plants after emergence. The majority of
which are contact, non-selective type herbicides.
Contact, non-selective herbicides control weeds by
destroying plant foliage. They work best on small weeds
and are much less effective on older plants. In general,
control of broadleaf weeds appears better than control of
grasses with these products. If control of perennials such
as bermudagrass is needed, these products will need to
be continually applied as they do not kill roots of perennials. If these types of products are used in turf, the
turf will also have burned foliage. They provide no preemergence activity and thus will need to be reapplied
with new flushes of weeds. Good spray coverage is
essential for success and higher spray volumes (50-70
gallons of water per acre) appears to increase efficacy.
Addition of a spray adjuvant designed to increase
coverage of the foliage may increase activity of these
products.
There are two green herbicide products on the
market which claim to be selective in turf. One product,
Agralawn Crabgrass Killer list cinnamon bark as the
active ingredient. It claims to control crabgrass,
basketgrass, chickweed, clover and other similar weeds
in St. Augustine, Bahia, Common Bermuda and
Centipede grasses. This product is applied as a dust to

2. wet foliage of weeds. There are precautions about being
rain free for several days after application and that the
product will stain concrete.
The other products which claim to be selective in
turf all contain the same active ingredient chelated iron,
FeHEDTA (iron hydroxyethylenediaminetriacetic acid).
Claims are that products containing FeHEDTA will
control 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 on
bentgrass and there are cautions about using products
containing FeHEDTA when the temperature will exceed
85ºF. I have seen no information on FeHEDTA’s safety
on Bermudagrass or zoysia. EPA considers products
which contain FeHEDTA to be biopesticides so
registration is required.
There are other bioherbicides (plant pathogens)
which have been used on a limited basis. The environment, i.e. temperature, rainfall, humidity, UV light
for which we usually have no control over, can have a
profound effect on the activity of these bioherbicides.
There likely will be more bioherbicide products released
in the future.
Biological control of weeds with insects has been
less than satisfactory on a small scale. The best control
(usually not eradication) of weeds with insects has been
on large scale infestation such as rangeland or aquatic
sites. It is important to remember that for control of
weeds with insects, the insects must become established
on a large scale. This is often difficult to achieve.
Although control of weeds with insect has had limited
success, do not eliminate this option from the arsenal.
There have been few if any University type trials
conducted (exception: Patton & Weisenberger, 2012) to
determine the efficacy of many of these green herbicides. Caution should be exercised before committing to
use of green herbicides on a large scale without careful
investigation and planning.
A list of active ingredients and type of activity of green herbicides is provided below. This list should
be considered non-inclusive as new green herbicides products are likely in the future.
Active Ingredient
Types of Activity
Acetic acid (vinegar)
Ammoniated soap of fatty acid
Cinnamon Bark
Cinnamon oil, rosemary oil
Citrus oil (d-limonene)
Citric acid
Clove oil
Corn Gluten Meal
Ethanoic acid
FeHEDTA
Octanoic acid
Pelargonic acid
Potassium salts of fatty acids
Thyme oil
Postemergence, Nonselective.
Postemergence, Nonselective
Postemergence, Selective weed control in turf
Postemergence, Nonselective
Postemergence, Nonselective
Postemergence, Nonselective
Postemergence, Nonselective
Pre-emergence, Nonselective
Postemergence, Nonselective.
Postemergence, Selective weed control in turf
Postemergence, Nonselective
Postemergence, Nonselective
Postemergence, Nonselective
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.htm
EPA. 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
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