University of California • Division of Agriculture and Natural Resources
Publication 7250
Weed management in organic vegetable production sys-
tems must involve the use of many techniques and strate-
gies, all with the goal of achieving economically accept-
able weed control and crop yields. Weeds can always be
pulled or cut, but the question is simply how much time
and money can a grower expend to reduce weed pres-
sure. The more a grower is able to reduce weed pressure,
the more economical it is to produce crops.
Ideally, growers would like to achieve a level of zero
weeds on the farm. In practice, this may not be achiev-
able, but any reduction in weeds and in the amount of
weed seed or perennial propagules reaching the soil
will make subsequent weed control operations less
expensive. An understanding of what resources weeds
require and why weeds are present in the first place is
useful when you begin to formulate a control strategy.
For weeds to grow, they must have access to water,
nutrients, and light. The first or biggest plant to occupy
a site has a competitive advantage over later plants. The
cultural practices used in vegetable production (for
instance, using transplants, pre-emergent flaming of
weeds, pre-germination of weeds) often provide oppor-
tunities for the crop to gain that advantage. The goal is
for the crop to outcompete the weeds, reducing the
availability of resources to the weeds. If you can give
the crop a competitive advantage through organically
acceptable techniques, subsequent hand weeding oper-
ations and costs can be minimized. The following are
common techniques available to organic growers to
manage weeds in vegetable production operations.
CULTURAL PRACTICES
Water Management
Effective water management is key to controlling
weeds in a vegetable operation. There are a number of
ways that careful irrigation management can help you
reduce weed pressure on your crops:
• Pre-germination of weeds. In pre-germination, irriga-
tion or rainfall germinates weed seeds just before the
cash crop is planted. The newly germinated weeds
can be killed by light cultivation or flaming. Pre-ger-
mination should occur as close as possible to the date
of planting to ensure that changes in weather condi-
tions do not have an opportunity to change the spec-
trum of weeds (cool vs. warm season) in the field.
• Planting to moisture. Another technique similar to pre-
germination is planting to moisture. After weeds are
killed by cultivation, the top 2 to 3 inches of soil are
allowed to dry and form a dust mulch. At planting,
the dust mulch is pushed away and large-seeded
vegetables such as corn or beans can be planted into
the zone of soil moisture. These seeds can germinate,
grow, and provide partial shading of the soil surface
without supplemental irrigations that would other-
wise provide for an early flush of weeds.
• Buried drip irrigation. Drip tape buried below the sur-
face of the planting bed can provide moisture to the
crop and minimize the amount of moisture that is
available to weeds closer to the surface. If properly
managed, this technique can provide significant
weed control during periods without rain.
Crop Competition
Crops that grow vigorously can often outcompete
weeds. Weeds grow best where competition is sparse;
for instance, between rows or in gaps in a crop stand.
Crops that are well adapted to their planted areas are
often better competitors since they will tend to occupy a
site rapidly. If you increase the density of the crop by
decreasing the in-row spacing or by reducing the space
Specific information on organic vegetable production practices in California is scarce, and growers need sound information
to guide their management decisions. The Organic Vegetable Production in California Series is made up of publications
written by Farm Advisors and Specialists from the University of California’s Division of Agriculture and Natural
Resources. Each publication addresses a key aspect of organic production practices applicable to all vegetable crops.
WEED MANAGEMENT
FOR ORGANIC CROPS
RICHARD SMITH, UC Cooperative Extension Farm Advisor, Monterey
and Santa Cruz Counties; W. THOMAS LANINI, UCCE Weed Ecologist,
UC Davis; MARK GASKELL, UCCE Farm Advisor, Santa Barbara and
San Luis Obispo Counties; JEFF MITCHELL, UCCE Vegetable Crops
Specialist, Kearney Agricultural Center, Parlier; STEVEN T. KOIKE,
UCCE Farm Advisor, Monterey and Santa Cruz Counties; and
CALVIN FOUCHE, UCCE Farm Advisor, San Joaquin County
VEGETABLE
RESEARCH AND
INFORMATION
CENTER
Organic
Vegetable
Production in
California
Series
Small Farm
Program
vric.ucdavis.eduwww.sfc.ucdavis.edu
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between rows you will improve the crop’s competitive-
ness. A close-planted crop will close the canopy more
rapidly, reducing the weeds’ ability to compete. Some
crops (including tomato, bean, and sweet corn) com-
pete effectively with weeds if given an early competi-
tive advantage, while others (including onion and gar-
lic) never establish a competitive canopy. The use of
transplants give the crop an advantage over the weeds
because transplants enter the field larger and more
developed that the weeds. With help from subsequent
cultivation or hand weeding operations, a transplanted
crop can develop a full canopy and crowd out weeds.
Reducing the Weed Seed Bank
Practices that reduce the production of weed seed also
reduce weed pressure and can help keep weeding costs
down over time. In an ideal situation, no weed would
be allowed to go to seed. Any that do go to seed can
aggravate weed problems for many years to come. As
an example, common purslane seed has been shown to
remain viable for over 20 years in the soil, and black
mustard seed survives for over 40 years. The longevity
of weed seed, together with the large numbers of seed
produced by individual plants (100,000 per plant for
large purslane or barnyardgrass plants), can lead to the
long-term build-up of enormous seed banks in the soil.
If you make it a policy to remove weeds prior to seed
production, you can reduce weed pressure in subse-
quent seasons.
Careful weed management during the season is
important, but it must be followed up with off-season
weed control as well. Short-season crops such as lettuce
can provide opportunities for frequent cultivations and
a rapid turnover of crops on the land, thus reducing
some weeds’ ability to mature and set seed. Highly
competitive cover crops can also smother weeds. If you
carry weeds with seed out of the field for disposal, you
can also significantly reduce the seed bank. Each of
these techniques can help growers minimize weed
problems, and that translates to lower hoeing bills.
CULTIVATION
Cultivation is probably the most widely used weed
control method in organic vegetable operations.
Mechanical cultivation uproots or buries weeds. Burial
works best on small weeds, while larger weeds are bet-
ter controlled by destruction of the root-shoot connec-
tion or by slicing, cutting, or turning the soil to elimi-
nate the root system’s contact with the soil. Cultivation
is effective against almost all weeds, with the exception
of certain parasitic forms such as dodder. Effective cul-
tivation must precisely and accurately target weed-
growth areas, and so requires good land preparation
and bed shaping. Shallow cultivation usually is best,
since it brings fewer weed seeds to the surface. Level
beds allow more precise depth of tillage. Cultivation
requires relatively dry soil; subsequent irrigations
should be delayed long enough to prevent the weeds
from re-rooting. In addition, cultivations should be car-
ried out early enough in the growth cycle to kill weeds
such as burning nettle and purslane that set seed early
in the growth cycle.
The goal of cultivation is to cut out weeds as close to
the seed row as possible without disturbing the crop. In
most cases, precision cultivation can take care of the
weeds on over 80 percent of the bed. The remaining
weeds must be removed from the seed row by hand or
using other mechanical means. Here are some common
cultivation implements:
• Various knives, L-shaped and crescent-shaped beet
hoes, and sweeps can be used to cut and uproot
weeds on bed tops within 1 to 3 inches of the crop
row. These can sometimes be combined with
reversed-disc hillers that cut vining weeds such as
field bindweed and move soil away from the crop
row. Disc hillers are often reversed as crops get larger
so they will throw soil around the base of the crop
plant to bury weeds. Rolling cultivators (Lillistons)
have become common cultivating implements for a
number of crops. A rolling cultivator’s primary pur-
pose is to uproot weeds, but it can also be adjusted to
throw soil and bury weeds in the crop row.
• A new generation of cultivators has been developed
to remove weeds from between the seed rows, and in
some situations from the seed row itself. Spring-tine
cultivators, torsion Bezzerides cultivators, Budding
in-row weeders, and brush hoes all can be adjusted
to take out weeds between seed rows or close to the
seed row. Some of these cultivators can remove
weeds from the seed row itself in fields planted to
tough-stemmed crops like cotton. Computer-guided
cultivators that can distinguish the crop from weeds
are under development and may soon be able to
remove weeds selectively from within the seed row.
• Cultivation implements are often mounted on sleds
for accurate, close cultivation in row crops. Guide
wheels, cone wheels, and other devices are also used,
but in general these are less precise than sleds.
Various implements can be attached to these guid-
ance setups to remove weeds.
Even the best cultivators will not eliminate all
weeds, so some hand weeding is often necessary. It is
easier to remove weeds by hand while they are small.
The proper timing of cultivations depends on the speed
of weed growth: in spring a two- to three-week period
Weed Management for Organic Crops • 2
is about right; in the fall or winter, longer periods
between cultivations may be appropriate. The practice
and experience of the grower are important factors in
effective cultivation.
Weeds that compete with the crop early in the crop
cycle may be more damaging to crop yield than weeds
that establish later in the season. Late-season weeding
may disturb the crop’s root system or knock off flowers
or fruit, which may reduce yields. Obviously, late sea-
son cultivations to reduce weed seed production must
be weighed against the potential for yield loss.
FLAMERS
Flamers are useful for weed control. Propane-fueled
models are the most common. Flaming does not burn
weeds to ashes; rather, the flame rapidly raises the
temperature of the weeds to more than 130°F; The sud-
den increase in temperature causes the plants’ cell sap
to expand, rupturing the cell walls. For greatest flam-
ing efficiency, weeds must have fewer than two true
leaves. Grasses are difficult to impossible to kill by
flaming because the growing point is protected under-
ground. After flaming, weeds that have been killed
rapidly change from a glossy appearance to a duller
appearance.
Flaming can be used prior to crop emergence in
slow-germinating vegetables such as peppers, carrots,
onions, and parsley. In addition, flaming can be used
postemergence on crops such as young onion and gar-
lic or as a directed treatment to the base of tougher
crops (such as sweet corn) when they are 12 or more
inches tall. Postemergence flaming does adversely
impact the yield of the crop, so its use must be weighed
against the potential damage the weeds might cause.
Typically, flaming can be applied at a speed of 3 to 5
mph through fields, although this depends on the heat
output of the unit being used. Best results are obtained
under windless conditions, as winds can prevent the
heat from reaching the target weeds. The efficiency of
flaming is greatly reduced if moisture from dew or rain
is present on the plants. Early morning and early
evening are the best times to observe the flame patterns
and adjust the equipment.
STERILIZATION
Soil sterilization in organic agriculture involves the use
of heat or naturally generated biocides to kill weeds.
Heat is applied as steam or by soil solarization. In
steam sterilization, the steam is injected into the soil to
kill weed seeds. The large quantities of fuel and water
required by this technique make it an expensive choice,
so its use is limited to small acreages of high-value hor-
ticultural crops or landscaping. Ozone is a naturally
occurring biocide that is being researched for use as a
soil sterilant. The ozone is generated mechanically and
then injected into the soil. Ozone injection shows
promise as a weed-reduction tool, but it is unclear at
this time whether this technique will be considered an
organically acceptable practice.
Soil solarization involves placing a clear plastic
mulch over a tilled, moist soil to allow the solar energy
to heat the soil and kill germinating weed seeds. To be
most effective, solarization should be performed during
summer and fall periods of maximum solar radiation
exposure. These are the steps involved in solarization:
1. Prepare the area as if for planting, making certain
that the area is as level as possible.
2. Irrigate to field capacity and then place the clear plas-
tic mulch over the area, covering the edges of the
plastic with soil to hold them in place. Irrigation and
pressing the plastic down for a tight fit against the
soil reduces the amount of airspace and ensures good
transfer of heat. Any tears in the plastic should be
promptly repaired or they will allow heat to escape.
3. Allow the plastic to remain in place for a minimum
of 4 weeks (length of treatment depends on the
amount of solar radiation).
4. Remove the plastic mulch and immediately plant the
crop. You can remove the plastic or use it as a plastic
mulch for the subsequent crop by burning holes in
the film and transplanting into those holes.
Solarization generally yields successful results in the
Central and Imperial Valleys, but is less reliable in
coastal valleys.
MULCHES
Mulching is another weed control method. A mulch
blocks light, preventing weed germination and growth.
The materials that can be used as mulches are varied,
and include plastics and organic materials such as
municipal yard waste, wood chips, straw, hay, saw-
dust, and newspaper. To be effective, a mulch needs to
block all light to the weeds, and some mulch materials
require a thicker application layer that others to accom-
plish this.
Plastic mulches vary in thickness from 1.5 mil to
about 4 mils. The most common color for weed-control
plastic is black, since it completely blocks light. More
recently, a clear, infrared-transmitting (IRT) plastic has
been introduced. The IRT plastic blocks certain wave-
lengths of light but allows others to pass, and that heats
the soil better for early-season crop growth. Plastic
mulches are generally placed on the beds and their
edges covered with dirt to keep them from blowing
Weed Management for Organic Crops • 3
Weed Management for Organic Crops • 4
away. Drip irrigation is needed to get moisture to the
crop under the plastic mulch. Certain weeds, including
nutsedge, are able to penetrate the plastic and so are not
completely controlled by plastic mulches. Other weeds
can grow in the openings provided for crops. Further
problems with plastic mulches include difficulties keep-
ing them in place under windy conditions, disposal
after the crop is harvested (they are not as yet recy-
clable), and their cost (including the cost of needed drip
irrigation).
Organic mulches such as municipal yard waste,
straw, hay, and wood chips must be maintained in a
layer 4 or more inches thick in order to block out light.
Organic mulches break down over time, and the origi-
nal thickness typically reduces by 60 percent after one
year. Coarse green waste works better as a mulch.
Organic mulches are mostly used for permanent crops,
landscaping, and noncrop areas, although they are also
very effective for transplanted vegetables.
Organic mulches can be grown in place. Plants used
to produce organic mulches include cereals, clovers,
vetches, and fava beans. These mulches (or living
mulches, as they are sometimes termed) must die or be
killed before or shortly after crop planting in order to
avoid excessive competition with the crop. Living
mulches were developed in the eastern United States,
but are currently being tested on various fruiting veg-
etables in California (see UC ANR Publication 7248, Soil
Management and Soil Quality for Organic Crops).
BENEFICIAL ORGANISMS
Weeds are subject to disease and insect attacks just as
crops are. Most biological control of weeds occurs in
range or noncrop areas. As a result, biological control
has little relevance for vegetable growers.
Geese have been used for weed control in tree, vine,
and certain row crops. Most types of geese will graze
weeds, but Chinese weeder geese are considered the
best for row crops. Chinese weeder geese are smaller
than other types and tend to walk around delicate crop
plants rather than over them. Geese prefer grass
species and will eat other weeds and crops only if they
are hungry and all of the grasses are gone. If confined,
geese will even dig up and eat Johnsongrass and
bermudagrass rhizomes. You must take care to avoid
placing geese near any grass crops such as corn,
sorghum, or small grains, as this is their preferred
food. Fruiting vegetables, such as tomatoes when they
begin to color, might also be vulnerable, so geese
would have to be removed from tomato fields at cer-
tain times. Geese require drinking water, shade during
hot weather, and protection from dogs and other
predators.
CHEMICAL CONTROL
Herbicides are chemicals that kill or suppress plants by
affecting their physiological processes. Only a limited
number of herbicides are organically acceptable, and
these include contact materials such as acetic acid (vine-
gar), citric acid, and solutions of sodium nitrate, as well
as a preemergent material, corn gluten. Herbicides can
be used for selective weed control by manipulating the
timing of application or placement of material, or by
exploiting differences in the chemical tolerances of the
crop and the target weed. Weeds that emerge before the
crop can be killed with contact herbicides (acetic acid,
etc.). These herbicides kill plants that have emerged,
but have no residual activity on those that emerge later.
Corn gluten is a preemergence material that is applied
to the soil to suppress weeds as they germinate.
Currently, the efficacy of these organically acceptable
herbicides is marginal at best.
OTHER PUBLICATIONS IN THIS SERIES
Organic Certification, Farm Production Planning, and
Marketing, UC ANR Publication 7247
Soil Management and Soil Quality for Organic Crops,
UC ANR Publication 7248
Soil Fertility Management for Organic Crops,
UC ANR Publication 7249
Insect Pest Management for Organic Crops,
UC ANR Publication 7251
Plant Disease Management for Organic Crops,
UC ANR Publication 7252
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