Weed Management for Organic Farmers - Transition to Organic Partnership Program

Organic farmers use a wide variety of tools and strategies to control weeds without synthetic chemicals. Those tools and strategies and their effects on soil quality are discussed.
Midwest

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2003

Weed Management
F O R

Organic farmers

O R G A N I C

F A R M E R S

Organic Farming Requires Weed Management

use a wide variety
of tools and

Organic farmers use a wide variety of tools and strategies to control weeds without

strategies to control

synthetic chemicals. Successful organic farmers continually adapt their weed

weeds without

management practices as weed populations shift. Producers should have a good

synthetic chemicals.

understanding of the philosophies and legalities of organic farming before they plan
their weed management strategies. A brief overview of organic agriculture follows;
for further details, see Iowa State University Extension publication Organic
Agriculture (PM 1880). (See page 8 for ordering instructions.)

K. DELATE

Mechanical tillage
is an important
component of organic
weed management.

PM 1883 August 2003

Weed Management
F O R

O R G A N I C

Why Organic?

F A R M E R S

Weed Productivity and Ecology

Environmental, economic,
and food safety concerns
are among the many reaEnvironmental,
sons why some farmers
choose organic production.
economic, and
For example, certified
food safety concerns
organic soybeans in Iowa
averaged a 200–300 perare among the many
cent premium price over
reasons why
conventional soybeans. On
the environmental front,
some farmers choose
organic farmers hope to
organic production.
reduce the 240 million
pounds of herbicides
applied each year in the
Midwest. Organic farmers
also express concerns
about weed resistance to herbicides and the
transfer of herbicide-resistant genes to wild plants
that may occur with increasing reliance on
herbicide-resistant crops.
According to the federal Organic Farming
Production Act (OFPA) of 1990, and the USDA
National Organic Program (NOP), anyone selling
products as “organic” must follow a set of prescribed practices that includes avoiding synthetic
chemicals in crop and livestock production and in
the manufacturing of processed products. Organic
certification agencies, serving as the required third
party certification, include private agencies, such as
the Organic Crop Improvement Association (OCIA),
and public programs, such as the State of Iowa
Department of Agriculture and Land Stewardship
(IDALS) organic certification program. For more
information, see Organic Agriculture (PM 1880).

A weed is simply a plant that takes advantage of
unused resources made available by cropping
practices. Nutrient availability is usually high at the
beginning of the season and decreases during the
season. Water availability depends on the weather
pattern of the particular year, but it is typically
higher in the beginning and lower at the end of the
season. Light under the crop canopy that is
available for weeds is high at beginning, low in the
middle, and generally high again at the end of the
season when crop leaves are senescing. Knowing
the ecology (germination and growth period) of
the weeds on your farm is key to organic weed
management.

K. DELATE

Natural mulches can regulate
the soil temperatures and moisture,
improve soil quality, and suppress
weeds in organic crops.

under any remaining rye plant residues to avoid
competition with the germinating crop.
There is a strong correlation between biomass,
tillering (multiple stems), and weed competitiveness. Barley, for example, has a more extensive
tillering system and is more competitive with
weeds than wheat. Because small grains also are
planted in narrower rows, these crops are more
competitive with weeds than corn or soybeans.
Many organic farmers opt to fallow a field to a rye
for an entire season if weeds have presented a
persistent problem in the past.
Maintaining soil fertility through crop rotations,
cover crops, intercrops, and biologically-based
fertilizers will enhance the competitiveness of the
crop plant and inhibit weed growth. Reports
indicate that humic and fulvic acids in compost may
mitigate weed seed germination. Small-seeded
weeds also may be more susceptible to pathogens
associated with high organic matter in compost.
Compost placed close to the crop plant reduces the
amount of nutrients available to weeds between
crop rows. Mulch also is effective in suppressing
weed establishment.

Ecological Weed Managemnet

Most organic farmers rely
on multiple tactics for their
weed management. Ecological weed management
promotes weed suppression, rather than weed
elimination, by enhancing
crop competition and phytotoxic effects on weeds.
Specific methods include
the following:

Crop Rotations

Ecological weed
management
promotes
weed suppression,
rather than
weed elimination,
by enhancing

K. DELATE

Crop rotations are the
crop competition and
foundation of organic
phytotoxic effects
farming. Organic certification requires that a small
on weeds.
grain and/or legume be
planted after row crops to
maintain soil health and
biologically based pest management. As an
example, if the legume is plowed under as a cover
crop in the fifth year, four years of row crops could
be grown prior to the green manure crop year. The
same crop cannot be grown in sequential years;
thus, soybeans cannot be grown in the same field
year after year. The ideal crop preceding soybeans
is winter rye. Soybean fields are rotated to a small
grain (oats, barley, wheat, or rye) or corn.
A typical crop rotation in Iowa is corn followed
by a winter cover of rye, soybeans, and oats with an
underseeding of alfalfa or red clover in the third
year. Rye, with its allelopathic properties, will help
prevent weed establishment. In the spring, rye that
is less than 8″ in height can be killed with a field
cultivator. Taller rye plants should be mowed or cut
with a stalk chopper before cultivating. A second
cultivation or disking may be necessary to turn

Organic crops are required to be grown in rotations, as
demonstrated by the corn-soybean-oat-alfalfa rotation,
shown at the ISU Neely-Kinyon Farm.

Weed Management
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O R G A N I C

Production Practices

F A R M E R S

must develop his/her own
management plan based
on specific farm/field conOrganic farmers
ditions. A harrow or field
cultivator with a drag- or
select crop varieties
spring-tooth harrow attachthat compete well
ment can be used in the
spring to kill weeds before
with weeds.
planting. Cultivation then
is timed with the pulses of
weed seed germination
and growth. The initial
cultivation occurs when the weeds are at the most
vulnerable stage. Fields are rotary hoed at a slow
speed (5 mph) three to five days after planting to kill
weeds in their initial development or white-thread
stage. A harrow also can be used at this stage. One
week later, after plants have emerged, fields are
hoed again but at a slightly faster speed (7–9 mph).
To avoid killing soybean seedlings, it is critical that
soybeans are not hoed in the crook stage when the
soybean hypocotyl is just at the soil surface.
Soybeans also should not be hoed when plants are
greater than 8″ in height. For vegetable cropping
systems, various in-row weeding tool sets, including finger weeders, basket weeders, Bezzerides®
torsion weeders, Spyders®, Weed Badgers®, and
brush weeders, can be used alone or in combination
on a multiple component weeding frame (See Steel
in the Field (EDC 125), produced by the USDA

• Cultivar or variety selection. Organic farmers
select crop varieties that compete well with
weeds. Quick canopy-forming varieties assist
the crop competitiveness over weeds within
and between rows.
• Crop density. Planting at the maximum adapted
population will provide the crop an increased
competitive edge over weeds.
• Row spacing. Closer row spacing generally
leads to greater crop competition with weeds in
row middles. Some organic farmers have drilled
soybeans with rye and obtained excellent weed
control but lower yields.
• Seed grade. High germination rates are critical
for a rapid canopy.
• Sowing date. To provide a competitive advantage, warm season crops (e.g., corn, soybeans,
and dry beans) are planted when the soil is
adequately warmed (usually later than May 10)
to facilitate rapid germination. Warmer soils
allow quicker emergence and a more competitive crop without major losses in yield.

Physical Tactics for
Organic Weed Management
Physical controls are a key factor for weed management on all organic farms. Physical methods of
control include cultivation, propane flame burning,
and mulching.

Mechanical Cultivation

K. DELATE

Depending on the crop, cultivation offers the least
labor-intensive weed control method. Although
more than 95 percent of Iowa’s row crop acres are
treated with herbicides, cultivation remains a viable
weed management strategy, in conjunction with
other controls. Timely cultivation is critical in
organic weed management. While no prescription is
available for managing weeds in an organic system,
certain practices have been used successfully by
the majority of Iowa’s organic farmers. Each farmer

Row cultivators are used two to three times to control
weeds between rows.

Sustainable Agriculture and Research
when plants are small and turned
Education [SARE] Sustainable Agriculoutward on later passes to throw soil
ture Network [SAN]. (See page 8 for
around the base of the plant when they
ordering information.)
are taller.
Two to three row
Two to three row cultivations are
The Long-Term Agroecological
cultivations are
typical for Midwestern organic farmers.
Research (LTAR) projects funded by Iowa
The first cultivation occurs at a slow
State University’s Leopold Center for
typical for Midwestern
speed as soon as rotary hoeing ends. The
Sustainable Agriculture have compared
organic farmers.
second cultivation usually is completed
weed populations and management
at mid-season at a faster speed to throw
costs in organic and conventional grain
about 1″ of soil around the base of plants.
crops (Delate et al., 1999). In general,
The third cultivation is again performed
soybeans with a quicker closing canopy
at a slow speed (5 mph). Various attachments, such
had less weed pressure than organic corn. Soybean
as guidance systems, and plant protection devices,
yields were the same as conventional soybeans.
such as shields, can be added to cultivators (see
Once the fields were in their third year of rotation,
Steel in the Field).
corn weed pressure was diminished. Despite lower
The number of acres covered when cultivating
corn yields in one year, organic corn provided a
will depend on the size of the cultivator and the
greater return overall than conventional corn
speed of cultivation. A 6-row cultivator, at speeds of
due to organic premium prices. The results are
6–7 mph, can cover 100 acres in 11 hours.
summarized in Table 1.
Another study in Chariton, Iowa, compared
Cultivators with open-top shields and low-profile
weed populations and yields in organic systems
single sweeps throw less than 1″ of soil into the row
under different primary tillage methods (Table 2).
at this speed. Higher speeds may throw excess soil
Treatments included fall moldboard plowing, fall
onto the plant row and damage small crops. Disk
Kverneland® plowing, spring moldboard plowing,
hillers can be used to pull soil away from the plant
Table 1

Costs and Effectiveness of Weed Control Measures in Organic vs. Conventional Systems
System

Weed Populations

Management Costs

Yields

Returns

Organic soybeans:
Southwest Iowa

Statistically the same
as conventional

Less than
conventional
($103/acre)

Statistically the same
as conventional
(48 bu/acre)

368% greater
than conventional

Organic corn:
Southwest Iowa

Statistically equal
to conventional
in 2 out of 3 years

Similar to
conventional
($172/acre)

Statistically similar
in 2 out of 3 years
(average: 130 bu/acre)

228% greater
than conventional

Table 2

Effect of Tillage on Weed Management and Yields
System

Weed Populations

Yields

Organic soybeans:
Southeast Iowa

Statistically greater grasses in Rotavator® in Year 1;
no statistical differences in Year 2

No statistical differences in Year 1;
spring-plowed greater in Year 2
(59 bu/acre)

Organic corn:
Southeast Iowa

Statistically greater broadleaves in spring-plowed;
statistically greater grasses in Rotavator®

Spring-plowed greater (227 bu/acre),
but no statistical differences between
spring and fall moldboard plowed

Weed Management
F O R

O R G A N I C

K. DELATE

and fall and spring Rotavator® plowing. Kverneland®
plows are reported to disturb less of the soil profile
and are very effective on CRP land. Rotavators® are
equally effective on CRP land and provide a more
friable seed bed. Results of this two-year study
(soybeans—two years and corn—one year only)
showed greater yields in spring-plowed fields overall. Weed populations were greater in the
Rotavator® treatment over two seasons and three
weed-scouting periods.

F A R M E R S

Propane flame burners can be used on organic farms to
control weeds between and within rows.

Propane Flame Burning
Many organic farmers
have included propane
Many organic farmers
(LP) flame-burners as an
additional tool in their
have included
weed management toolpropane (LP)
box. Flaming is used
particularly during times
flame-burners as
of high field moisture
an additional tool
when tillage with large
machinery is not feasible.
in their weed
In drier weather, flaming is
management toolbox.
used in conjunction with
cultivation. Flame weeding
of corn can be accomplished when corn is less
than 2″ in height and
when corn is 8″ to 1.5′ in height. Other growth
stages are considered potentially damaging to the
corn meristem (growing point).
According to several Iowa farmers, soybeans
can be flamed prior to emergence and at the 1′ to
1.5′ height stage. Caution must be used in flaming
soybeans, however. Mixed results have been
reported, including damaged soybeans and
decreased yields. Because the growing terminal of
corn is more protected within the whorl, corn can
withstand greater damage from flaming, compared
to soybeans with their exposed growing points.
Several flame burners are commercially available,
including complete units and individual burner kits
that can be attached to tractor tool bars. Flamers

are usually run at 38–40 psi with a tractor speed of
4 mph, but specific rates will be based on the
crop stage, weed species present, and the manufacturer’s recommendations.

Mulching
Many organic horticultural operations rely on manual labor and a combination of mulching/cultivation
for adequate weed control. Natural mulches and
synthetic mulches, including polyethylene film and
polypropylene landscape fabric, are used in organic
operations. Synthetic mulches have proven their
durability in numerous vegetable production
systems. According to certified organic regulations
in Iowa, however (IDALS, 2000), synthetic mulches
must remain intact during the growing season and
must be removed from the field at the end of the
season. Despite the labor-saving aspect of synthetic
mulches, most organic growers in Iowa prefer
natural mulches for weed control. Natural mulches
are used to regulate surface soil temperatures
and moisture, improve soil quality, and suppress
weed activity. Organic mulches add organic matter
to the soil during decomposition, thus increasing
nutrients, soil porosity, water holding capacity,
microbial populations, and cation exchange capacity.
Straw mulch is commonly used in organic horticultural operations, such as garlic, strawberry, and
herb farms, to control weeds and protect against
harsh environments.

Mulch can be made from small grain, soybean
straw, or baled corn stalks. A Bale Chopper®, pulled
by a tractor along a raised vegetable bed, can be
used to shred small square straw bales for rapid
mulch application. Other methods of straw mulch
application include hand-rolling round hay bales
across vegetable beds before planting. Wood chips,
shredded newspaper, and other plant-based residues can be used as mulches, typically in tree crops.
Caution must be taken to select materials free
of synthetic substances, such as preservatives in
wood or synthetic dyes in paper products. Your
organic certification could be jeopardized if
synthetic chemicals are introduced through mulch
materials. Mulch made from materials grown on
organic farms prevents any possibility of pesticide
residues from conventionally grown materials.
Wood chips should not be applied to a rapidly
growing crop, such as an annual plant, to avoid
competition with the crop’s nitrogen source as the
wood chips degrade.
Other mulches include living and mowed
residues of cover crops (SAN, 1998). Under cover
crop residue mulches, weed emergence is decreased
by reductions in both light transmittance and daily
soil temperatures. Hairy vetch (Vicia villosa Roth)
and rye (Secale cereale L.) residue mulches are
used in many organic operations. Mulches should
be maintained at a depth of 6 inches to prevent
stimulation of weed growth. Results with living
mulches have ranged from unwanted competition
with the crop and delaying fruit set, to having no
effect on yields. Much of the effectiveness of living
mulches depends on the availability of field
moisture for both crop and mulch.
Many horticultural crops, such as medicinal and
culinary herbs, produce greater yields when
mulched, rather than cultivated for weed management. Two natural fiber mulches that are currently
in the research stage for organic horticultural crop
production are a flax straw mat and a wool mat.
These mulches may prove effective for organic
operations, provided the mats are approved for the
certified organic label.

Effects on Soil Quality

Organic farmers are concerned about the effect of
multiple tillage passes on soil quality, including
erosion. In our studies to date, no differences in soil
quality (physical and chemical properties) were
observed among the different tillage treatments. In
research at the Neely-Kinyon Farm in Southwest
Iowa, after one growing season under organic management, microbial biomass carbon was 228 percent
greater in the organic system, maggroaggregate
stability was 15 percent greater, organic carbon was
6 percent greater, particulate organic matter carbon
was 8 percent greater, and N mineralization potential was 7 percent greater. Nitrate-N was 44 percent
greater in the conventional system, as reflected in
the excess corn stalk nitrate detected in the first
year. Potential effects on soil quality will continue to
be monitored, but practices employed by organic
farmers, such as adding organic matter through crop
rotations, cover crops, green manures, manure and
compost applications, and other biological fertilizers, will help mitigate the erosive effects of tillage.

Conclusion

Integrated weed manageTrial and error will
ment on organic farms requires extensive planning
govern many decisions
and management. Crop
in the first years of
rotations are the basis for
successful organic farming
organic farming.
and are necessary for
breaking weed, insect, and
disease cycles. Cultivation
must be completed with
properly set equipment under soil conditions that
are not conducive to compaction. Monitoring weed
growth stages also is critical in determining ideal
cultivation times. Trial and error will govern many
decisions in the first years of organic farming. Learning from other organic farmers and following advice
from Iowa State University may help in the transition.

Weed Management
F O R

O R G A N I C

F A R M E R S

References
Bowman, G. (ed.). 1997. Steel in the Field—A farmer’s

IDALS (Iowa Department of Agriculture and Land

guide to weed management tools. Sustainable

Stewardship). 2000. Iowa Organic Certification and

Agriculture Network, USDA, National Agriculture

Organic Standards. Des Moines, IA.

Library, Beltsville, MD.

Rynk, R. 1992. On-Farm Composting Handbook. NRAES-

Delate, K., and C. Cambardella, 2000. Integrating organic

54. Northeast Regional Agricultural Engineering

soybeans following CRP land. USDA-SARE Annual

Service. Ithaca, NY.

Report, N.C. SARE, University of Nebraska, Lincoln, NE.

Sustainable Agriculture Network (SAN). 1998. Managing

Delate, K., C. Cambardella, K. Taylor, and B. Burcham.

cover crops profitably. 2nd Edition. Sustainable

1999. Comparison of organic and conventional rota-

Agriculture Network, USDA National Agriculture

tions at the Neely-Kinyon Long-Term Agroecological

Library, Beltsville, MD.

Research (LTAR) site: First year results. Leopold
Center for Sustainable Agriculture Annual Report,
Iowa State University, Ames, IA.

LEOPOLD CENTER

This research project was partially funded by
the Leopold Center for Sustainable Agriculture
at Iowa State University and the USDA-SARE
(Sustainable Agriculture Research & Education)
Program.

Publications mentioned in this booklet can be ordered by
contacting any ISU Extension county office or the ISU
Extension Distribution Center at (515) 294-5247. There is a
charge, plus shipping and handling, for some publications.

Prepared by Kathleen Delate and Robert Hartzler,
Iowa State University.

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