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COG Organic Field Crop Handbook

1.7 Weed Management


During the transition period from conventional to organic agriculture, weeds are often one of the biggest problems encountered by farmers. With careful management, these problems can be overcome. A well-designed crop rotation and soil-improvement program which increases and maintains the biological activity of the soil will ensure a vigorous crop and reduced competition from weeds.

Weeds are only a problem if they reduce crop yields or cause harvesting problems. The organic farmer does not expect entirely clean fields but sees the farm as an ecological system that has a diversity of plants, where the crop is the dominant species. The techniques used to control weeds focus on giving the crop a head start rather than eliminating all species of weeds.

An understanding of the life cycle of problem weeds, including the depth of seed germination, time of emergence, growing habits and the vigor of its rhizome, will help the farmer decide what control measures are appropriate and when to use them.



1. Prevention is better than cure

Prevention is always easier than trying to find a cure. Improving soil conditions on your farm is the first step in eliminating weed problems. Weeds may tolerate compaction and drainage problems better than many crops. As a result, weeds are more competitive and problems more severe. Similarly, high levels of soluble nutrients can stimulate greater weed growth. Maintaining favorable soil conditions for microbial activity is the preferable "first line of defence" against weeds. A biologically-active soil with good drainage will improve the vigor of the crop and reduce weed problems. If the crop gets off to a good start and forms a dense canopy, it can usually compete successfully with weeds during the growing season.


Crop rotation

Crop rotation is the key to both soil improvement and effective weed control. The rotation can include specific crops to control problem weeds in particular fields. Crops suffer the most from weeds that share similar growth patterns. For example, Shepherd’s-purse thrives in fields of winter wheat because its life cycle is identical to that of the grain. By rotating crops with different nutrient and management requirements in the same field, weed life cycles are disrupted. Cool season canopy crops are alternated with warm-season row crops which can provide more cultivation opportunities. Clover or forage crops discourage some weeds by shading and lessen the reproductive success of others because they are cut for hay before the seeds ripen. Inclusion of competitive smother crops or allelopathic crops in the rotation provides further control options.


Smother crops and cover crops

Buckwheat is an example of a smother crop that grows rapidly and forms a dense canopy which slows weed establishment. Fall-planted cover crops or over-wintering production crops will greatly reduce weeds in the next growing season. If the soil is left bare, weeds will grow. Farmers have observed that cover crops incorporated in the spring provide better weed control than those that are winter-killed. This may be because the cover crop has a mulching effect or because of toxins -- an allelopathic effect -- produced by the cover crops.


Allelopathic crops

Some plants, like rye and sunflowers, produce natural, chemical toxins which inhibit the growth of other plant species. This phenomenon is known as allelopathy and can be used to assist in weed control. For example, rye used as a fall cover crop can be effective in suppressing quack grass. The inhibitory effect on weeds diminishes when rye is disked into the soil rather than left on the surface. Sunflowers, and small grains such as oats and wheat, release toxins either as root exudate or from decaying plant materials. Sunflowers seem to maintain their weed-suppressing ability when incorporated into the soil.


[photo 7.1: Sunflowers]

Nurse crops

Nurse crops, such as oats and barley, used in the year of establishing forages help prevent weeds from establishing before the forage becomes dominant.


Crop selection

Crops can be selected for their ability to compete or adapt to weeds. Fast-growing canopy crops and tall crops with extra leaf area are more competitive than vegetables. Barley is more tolerant of weed competition than wheat, and wheat can tolerate weeds better than oats. Select vigorous varieties; some farmers have found that taller varieties, such as Rodney oats, compete more readily with weeds than the newer, shorter-stemmed varieties.


Crop management

Producing vigorous competitive crops is the soundest, most economical method of control. It is important to match the fertility of the soil with the crop needs. A common mistake leading to problems with annual weeds, such as pigweed and lamb's quarters, is overfertilizing. Nitrogen management is important. If there are high levels of soluble N when growing grain legumes, the balance is tipped in favor of the weeds and a lot of cultivation is necessary to keep weeds from reducing bean yields. If a field is being seeded down, conditions should give the advantage to the underseeding in order to control weeds. Use of composted manure rather than raw manure will reduce weeds, but heavy applications of young compost will also create problems as it is relatively high in soluble nutrients. Heating during the composting process will kill many of the weed seeds.


[Illustration - line drawing: Lamb's quarters]

The degree of competition between the crop and the weeds can also be altered by manipulating a number of other factors:


Time of seeding: Small grains and small seeded legumes should be seeded early in the spring so that they can become established and successfully compete with weeds, that germinate later. If germination coincides with the first flush of weeds there will be intense competition. For other crops such as beans, planting should be delayed until soil temperatures favour rapid germination. This also allows time for tillage operations to eliminate cool-weather weeds.


Row spacing: Row width is reduced so that the leaf canopy closes in fast and shades out weeds. However, closer spacing between rows should be balanced with leaving rows wide enough for cultivation.


Seeding rate: Organic farmers recommend a high seeding rate for a denser stand to shade out weeds and leave fewer empty spaces where weeds can get established. For example, in Ontario, soybeans are sown 21-29 seeds/metre (6-9 seeds/foot) for a high density stand in 55-71 cm (22-28 inch) rows. Another example is the seeding rate for oats. If you have weed problems, sow oats at 2-3 bu/acre, rather than 1.5-2 bu/acre. Use the lower rate if you are underseeding the oats with a forage crop. Always sow clean seed.

Other preventative measures include the mowing of fence lines, roadsides, waste areas and adjacent pastures to prevent weeds seeding and spreading into adjacent fields. Timing is important. Mow when weeds are most vulnerable; annuals at first flower, and perennials when reserves are lowest between full leaf and flowering. Maintain a close watch over the fields during the growing season -- removal of a few plants by hand will require little effort and prevent trouble in the future. Annual weeds produce large amounts of seed and the seedbank of small-seeded weeds may be measured in thousands per square metre of soil. For example, pigweed (Amaranthus retroflexus) produces 110,000 seeds per plant.

It is advisable to clean all tillage and harvesting machinery before moving from one area to another.



2. Mechanical methods of weed control

[Photos: 7.2, Blind harrowing; 7.3, Rotary hoe; 7.4, Weeder harrow; 7.5, Mid-mounted cultivator; and 7.6, Inter-row cultivation -- all on one page, here or after next para if possible]

Weeds have the greatest impact on yields in the early stages of crop establishment. If the weeds are controlled during this period, the crops have time to produce enough biomass to compete for nutrients and sunlight.

A variety of tillage options are available to the farmer. The exact timing of the operations will be determined by the weather and the condition of the soil, but it is important to watch the fields closely and work when the weeds are small. Crop injury and poor weed control are a result of cultivating at the wrong time or with poorly-adjusted equipment. Excessive tillage should be avoided as it may dry out the soil, accelerate losses of organic matter or cause compaction. Light tillage operations to control weeds in standing crops have a minimal negative impact on the soil. The mechanical methods for weed control in field crops are summarized in Table 5.


[Table 5]

Pre-plant tillage / false seed bed

Tillage promotes weed germination by exposing dormant seeds to oxygen. Therefore, tillage immediately prior to planting creates tremendous weed pressure on the crop. However, the crop will have the advantage if the weeds are allowed to germinate and then killed by a second tillage operation immediately before seeding. Use a rod weeder or light harrows which stir up only the top inch of soil and do not bring up more weed seeds. If weed pressure if high, the operation can be repeated before seeding row crops or buckwheat. This technique is also effective when used in early fall to germinate next year's weeds which will then be winter-killed; planting a cover crop may be more beneficial.


Ridge tillage

Ridge tillage can be used effectively to control weeds and to reduce tillage operations. Ridges are built at last cultivation in a corn crop, or later in the summer or early fall after a green manure or forage stand. They can be lightly seeded with an annual cover crop. In the spring the planter sweep will remove the top inch of soil together with any crop residue and weeds from the ridge, leaving a weed-free seed bed (see the Ontario Ministry of Agriculture and Food's factsheet # 88-081 on ridge tillage).


Pre-emergent tillage or blind harrowing

Blind harrowing is done at the critical moment after the crop has been planted, has sprouted but has not yet emerged. The field is harrowed to kill the weeds which have already sprouted. The crop will emerge shortly afterwards, having gained a competitive edge on the next generation of weeds. If the crop leaf has already appeared it will not be able to break through if it is covered by soil. A planting depth of 5 cm (2 inches) is recommended if blind harrowing is to be used. This technique is effective in controlling broadleaf annual weeds in cereal crops. When used with row crops, it gives the crop a lead on emerging weeds until the plant is large enough to withstand other mechanical controls. A rod weeder or any type of harrow can be used, but do not use this method if the crop has been underseeded with grass or legumes. If necessary, underseeding can be delayed until after harrowing.


Rotary hoe

A rotary hoe is often used for pre-emergent tillage in crops such as corn and soybeans, as well as for control of weeds once the crop has emerged. It is commonly used about 5-7 days after planting and again 7-10 days later. The action of the hoe lifts and mixes the soil, uprooting small weeds less than 2.5 cm (1 inch) tall. Weeds are killed more easily when they are in the white stage before they break through the soil surface. Speeds of 10-20 km/hr are required, and best results are obtained if the soil is dry and the operation is carried out in late morning or afternoon. If the soil is moist, the hoe may transplant seedlings. The rotary hoe is also used to break up soil crust before the crop emerges.


Weeder harrow

The weeder or flex-tine harrow is used once the crop has emerged, at the 4th leaf stage in cereals that are not underseeded and in row crops such as corn and soybeans that are 15 cm (6 inches) high. The springy tines of the harrow are gentle enough not to harm the crop while uprooting or covering the smaller weeds.


Inter-row cultivation

Inter-row cultivation, also called scuffling, of row crops uproots small weeds and cuts off larger weeds once the crop is about 10 cm high. Again, the emphasis should be on cultivating before the weeds become a problem -- small weeds are easiest to control. Straight rows and a uniform crop stand will make the job easier. Various types of equipment can be used; front- or mid-mounted cultivators allow more precision. Often equipment needs can be met by retooling machinery already on hand. It is important to maintain a good working edge or point on all the soil engaging parts of the cultivator. Use the appropriate equipment for the crop at the correct speed, depth and spacing. Cultivating too deeply or too closely to the row can result in root pruning. Two or three cultivations early in the season will probably be all that is required. The first pass is critical and should leave adequate soil adjacent to the row which can be moved into the row to smother or bury weeds there without damaging the crop. Once plants are about knee high, cultivation will likely damage the crop.

For maximum weed control in row crops, use of the rotary hoe or weeder harrow should precede inter-row cultivation.


Flame weeding

Flame weeding is gaining in popularity for spot weeding and in-row weeding and it can be combined with cultivating between the rows. Gas pressure and ground speed are used to control the heat at which the weeds will be killed without damaging the crop. Staggered burners direct high heat to the base of the crop for about one-tenth of a second. The heat expands the water in the weeds, bursting the cell walls. In three or four days, they are brown and dead. The best results are obtained on hot dry days when the weeds are small. Flaming can also be used before planting, after seed bed preparation has stimulated the germination of weed seeds, to give a 60-70 per cent reduction in weeds.



3. Are weeds always a problem?

The presence of weeds should not automatically be judged as damaging. At a certain point, weeds affect yield and harvestability and increase the cost of handling and storage of grain, but below that point they can have a beneficial effect. The tolerance level of weeds depends on the crop and on how the soil nutrients are being managed.

Weeds do have a role to play. They can collect and hold leachable nutrients, which will be made available to following crops when the weeds decompose. They cover and protect the soil, conserve moisture, increase the organic matter content and bring up minerals from the subsoil. They also harbor beneficial insects.

Many of the above-mentioned benefits can also be achieved with cover crops which are more efficient than weeds and do not self-seed. It may be a mistake to let weeds grow unless they are very carefully managed.



4. Weeds as indicators of soil conditions

Weed type and concentration are responses to the condition of the soil. Some people believe they can be used as indicators of what's wrong or what's right with the soil. Increasing numbers of any weed or distinct groups of weeds are usually a bad symptom. In years of drought, weeds appear that are rarely seen under normal conditions. Some observations are reported below; however this is an area where more research is needed to examine the relationships between weed species, nutrient balances and trace mineral availability.

• Quack grass (variously known as twitch grass, couch grass or scutch grass) -- soil compaction at the soil surface, a hardpan or crusty surface; calcium deficiency.

• Milkweed and thistles -- deep soil compaction, with milkweed favoring moist land and thistles growing in drier conditions with low humus levels.

• Chickweed -- incomplete decomposition of organic matter. (Researchers in Nova Scotia reported that chickweed predominated in soils when there were problems with phytotoxicity, that is when plant residues produced substances which inhibited growth of the following crop.)

• Ragweed -- "complexed" or unavailable potassium.

• Silvery cinquefoil -- dry shallow soil.

• Docks and sorrels -- waterlogged or poorly drained acidic soil.

• Lamb’s quarters, pigweed, nettles -- tilled or cultivated soil with high fertility or humus, unless weeds are pale and stunted, then low fertility.

• Hawkweeds and knapweed -- acidic soil.



5. Solutions to some weed problems

Canada thistle is usually controlled by establishing a vigorous, dense stand of alfalfa or red clover. The forage is clipped three times in the growing season which reduces the rhizome sugars of the Canada thistle to the point where the plant is critically weakened. If there is a very serious problem, the land is left fallow for a cleaning phase. The Canada thistle is allowed to regrow, and is then cultivated or mowed down, up to four or five times.


[Illustration: Canada thistle]

Wild oats are an annual weed whose seeds can remain dormant in the soil for many years. Often a problem in fields of small grains, wild oats can be controlled by having row crops and/or forages in the rotation before seeding to small grains and by blind harrowing spring-planted cereals. Other control techniques include undersowing cereals or delaying planting so that the early germinating wild oats are killed by tillage before seeding.


Milkweed is best controlled with a rotation of alfalfa or red clover, or spring planted buckwheat.


Quack grass is controlled by exhausting its root reserves by timely cultivation. Following harvest, the infested area should be worked, stimulating the rhizomes to new growth. Cultivation is repeated as the plant shows 10 cm of new growth. Other strategies are to cultivate in the spring and plant a green manure crop of buckwheat, followed by fall rye, or to interplant rye grass into the row crop.

Most farmers successfully control weeds using a combination of techniques but emphasis should be placed on improving soil conditions rather than relying on mechanical means.



Further reading

Patriquin, D.G., "Weed Control in Organic Farming Systems" in Altieri M. & M. Leibman (eds.), Weed Management in Agroecosystems: Ecolocial Approaches, CRC Press, Boca Raton, Fla., 1989

Andres L. & Zettel T., "Principles of Preventive Weed Control", Ecological Farmers Association of Ontario Newsletter, Nov. 1988, pp. 8-12

Cocannouer, J.A., Weeds: Guardians of the Soil, Devin-Adair, New York, NY, 1950

"Weed Control Guide", New Farm. Vol. 12, No. 3, March/April 1990

Pfeiffer, E., Weeds and What they Tell, Tri-fold Book Service, Toronto, Ont., 1943

Walters, C., Weeds: Control without Poisons, Acres USA, Kansas City, MO, 1991,320 pp.


Copyright 1992 Canadian Organic Growers. Inc

Reprinted with permission. All rights reserved.


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