Managing Weeds in Organic Crops
Organic crop production requires a high level of management to successfully culture a healthy and productive balance in the field environment, minimizing the number of weeds to the benefit of the crop production.
Eradicating well-established weeds is generally not the goal of organic producers. Rather, maintaining weeds at acceptable levels is achievable and harmonious with the philosophy of organic production.
A low level of weeds and certain species of weeds are not only tolerable, but also potentially useful. Weeds can be used as indicator plants revealing other production problems, such as soil structure, pH, or nutrient deficiencies. Understanding the relationship between current weed populations and the soil will help you practise the holistic approach required for successful organic farming.
To learn more about a variety of strategies to help you manage weeds, select an option below.
Effective Weed Management
Effective weed management depends on thoroughly understanding the biology and growth habit of the weed species. Is it an annual, winter annual, or perennial? At what soil temperatures does it germinate? Is it a strong or weak competitor? Does it propagate by seed production only? How long will the seeds remain viable in the soil? Does the weed favour growing in any specific soil condition? The interactions are complex. A practice that controls one weed may not work on another, or may actually cause it to increase. In most cases, a combination of a number of cultural and preventative practices will be required to effectively deal with the problem.
The focus of this element of weed management is to prevent the introduction of new weed species and prevent the establishment of new weeds in the seed bank, should a new weed be accidentally introduced. Many weeds have seeds or other reproductive parts that may remain viable in the soil for several years, or even decades. Because organic producers cannot take for granted that a new weed will be easily manageable once established, preventing the introduction of new weeds should be a cornerstone of organic weed management.
Prevention, also known as exclusion, begins with securing clean seed and feed sources. Every opportunity should be taken to prevent new weeds from being introduced to the farm. Care should be taken to select seed and feed sources that are free of weed species that are not currently present on the farm, particularly invasive or noxious weeds. Be aware of the sources of these inputs, and if necessary, visit the source and conduct testing to confirm weed content.
The contamination of seed is the single most important source of introduction of new species. Some seed cleaning facilities may not be equipped to do a good job of seed cleaning, and can be a source of new weed species if proper care is not taken in cleaning the equipment between batches. Elevators do not provide adequate cleaning for seed and, because of cross contamination from the seed of other producers, may result in a greater risk of introducing a new weed to your farm. Care should always be taken not to import new weed species when purchasing seed from outside the immediate farming area.
Prevention includes monitoring for new weed problems, which, in addition to the field itself, may develop along fence lines, shelterbelts, and road allowances or in other non-crop areas adjacent to cultivated fields. Pastures, silage towers or bunks, feed troughs and manure piles should also be monitored for the introduction of new weeds.
Proactively monitoring for the occurrence of new weed species allows producers an early opportunity to eradicate the problem before it becomes established. Eradication of an early weed infestation may be accomplished by pulling (for simple tap rooted plants), or digging up perennial plant colonies by the roots. Other techniques for small areas include solarization or mulching. Solarization involves laying clear plastic over an infested area to raise soil temperatures and kill all roots or seeds in the upper layer of soil. Mulching involves laying down an impermeable barrier to new plant growth in order to starve new shoots. Once infestations become larger, these methods are less practical. Spread of the weed can be minimized using intensive mowing and multi-species grazing.
Municipal weed inspectors are responsible for the enforcement of The Noxious Weeds Act and should be alerted to the occurrence of any new weeds to the area so that they may also monitor for other occurrences to prevent spread.
Sanitation, from the perspective of weed management, refers to the practice of minimizing the movement of weeds that are found on the farm to other parts of the farm. The primary activities of sanitation involve keeping equipment free of seed or other reproductive parts of weeds, and ensuring that field boundaries and other non-crop areas are kept free of weeds capable of reproducing. There is significant overlap between the activities carried out in both prevention and sanitation.
Attention should be directed to tillage and harvesting equipment. Equipment used for custom work should be closely scrutinized, as it has the highest likelihood of introducing new weed species. Equipment cleaning before leaving a field is important to prevent the deposit of weeds to the next field, as well as along the transport route. The spread or re-introduction of perennial weeds such as quack grass (Elymus repens) or Canada thistle (Cirsium arvense), as a result of rhizomes or roots on tillage equipment, is an example of how equipment cleaning could prevent future weed problems. Harvesting machinery, if not properly cleaned before leaving the field, can easily introduce new weed seeds to a field, as well as along the route between fields. Depending on the design of the machine, the header, rock trap, and grain tank are all major reservoirs of material from previous fields, as well as any catch points on the outside of the combine.
Manure is an important source of nutrients, particularly phosphorus, in organic cropping systems. Composting manure prior to field application is important in limiting the movement of weeds in manure. A large percentage of the weed seeds fed to animals in infested hay or straw will pass through the digestive tract unharmed. Manure from animals fed weedy forage should be composted for several weeks. During composting, the heating process that occurs in well managed compost will destroy many weed seeds. Proper composting involves frequent turning to aerate the pile and maintain high pile temperatures. Call the Agriculture Knowledge Centre at 1-866-457-2377 for details on composting manure.
An effective crop rotation is absolutely critical for weed, disease, insect and fertility management. Rotations will vary depending on the individual producer, crop/livestock mix, soil characteristics, as well as regional constraints such as moisture and fertility.
Research indicates that crop rotation can have a greater effect on weed species and densities than tillage practices. In a survey, 84 percent of the farmers indicated that a rotation, including forage crops, significantly reduced weed problems. Fifty percent noted a reduction in annual grasses, while 35 percent reported a reduction in broadleaf weeds.
A later study assessed weed populations in fields that had been taken from alfalfa production the previous year and compared those populations to fields under continuous cereal production. The fields that had been removed from alfalfa production had moderately lower levels of wild mustard, wild buckwheat, green foxtail, annual smartweed, and greatly lower populations of cleavers, wild oat, perennial sow thistle, Persian darnel, and Canada thistle.
At the same time, other weeds were found at higher levels after alfalfa, such as volunteer alfalfa, dandelion, and winter annual weeds such as stinkweed. Weeds with high seed dormancy levels, such as Lamb's quarters, did not differ between fields previously in alfalfa or cereals.
In designing a crop rotation for weed control, the overall key to success is diversity. The following principles or practices are commonly incorporated into successful rotations.
- Alternate competitive crops, such as alfalfa or barley, with less competitive crops, such as flax or lentil, in the rotation.
- Alternate between broadleaf and grass crops in the rotation. Weeds are more competitive with crops to which they are most closely related.
- Use perennial phases in crop rotations combined with mowing (cutting for hay) or intensive grazing to control perennials and to interrupt cycles of annual weeds in cultivated crops. Consider the growth stage of the weed, as well as the forage, when timing the cut.
- Vary seeding dates to prevent weeds from adapting to static management practices. Alternating winter and summer crops within the rotation is the ultimate in seeding date diversity.
- Where possible, use weed suppressing cover crops such as fall rye or sweetclover.
Within a selected cropping rotation, many additional agronomic or management decisions can be used to increase the competitive advantage of a crop. In many cases, the effectiveness of these strategies depends on regular field monitoring and adjusting management practices on an ongoing basis depending on crop, climatic and weed situations.
The timing of weed emergence relative to the crop is critical. For every day that wild oat (Avena fatua) emerges before a spring wheat or barley crop, yield losses increase by three percent. By the same logic, crop yield improves by three percent for each day wild oat emerges after the crop, until eventually the effect of weed competition on yield is insignificant when compared to plots kept free of weeds. Research indicates that there is a critical period following crop emergence when yields are reduced the greatest by weed growth. This varies between year, crop and weed species. In the case of wheat, this period is between two and four weeks after emergence. Attention to the following management factors can assure this critical period is as weed free as possible.
A) Crop Selection
Small grains, broadleaf and legume plants - Each of these plant groups have specific rooting habits, nutrient and moisture requirements. Within a rotation, the competitive ability of crops varies greatly. The most competitive crop is fall rye, followed closely by winter wheat, barley, oat, spring wheat, mustard, canola, peas, flax and, finally, low lying crops such as lentil, chickpea and dry beans. Most pulse crops, like lentil, are poor competitors against weeds. Canola offers poor competition to weeds in the seedling stage, but can be effective once it becomes established.
Within any specific crop, there may be varieties that differ in competitive ability. The use of varieties that achieve canopy closure early, or that have a more competitive architecture (taller or have more tillers) should be considered. Semi-dwarf varieties are significantly less competitive than taller lines. Research studies involving semi-dwarf winter wheat varieties resulted in a 14 to 30 percent greater yield reduction from downy brome (Bromus tectorum), than did taller cultivars. When selecting a variety that is competitive against weeds, we should look for the following traits: rapid early growth, many tillers, leafy and tall stature.
The competitive ability of a nurse crop when used in forage crop establishment should be considered. Some of the above factors that make a crop competitive against weeds may not be desirable when used as a nurse crop.
Introducing a green manure crop will further disrupt the weed cycle. The most common option is a pulse or legume forage crop. The forage crop will usually offer very good competition against weeds and tillage opportunities while the crop is incorporated into the soil. Removing the green manure crop early also allows a partial fallow to occur. A rotation of this nature would aid in controlling perennial weeds such as quack grass or Canada thistle (Cirsium arvense). There is also the option of cutting and spreading the green manure without actually incorporating it, thereby reducing soil degradation concerns.
The frequency that a crop is grown within a rotation can also affect the population of certain weed species. Weeds tend to be more successful when they are growing among closely related crops. A research study involving a two-year canola-fallow rotation had a stinkweed infestation of 190 plants/m2. Extending this rotation to a canola-barley-hay-fallow system reduced the density to 23 plants/m2.
B) Seeding Date
The time at which a crop is planted is probably the main factor determining the composition of weed flora. In the case of cereals, early seeding has consistently produced the most competitive crop with the highest yields. As a general rule, crops such as flax, pulses, and canola should not be seeded into cool soil. Under such conditions, the seedlings of these crops will be slow to develop and prone to disease. Since these crops are not good competitors, it is essential that they develop a strong uniform stand. Crops such as tame buckwheat and certain spice crops should be seeded considerably later than any of the above crops, to avoid the risk of late spring frosts.
Delayed seeding can be an effective weed control practice, offering tillage options either before seeding or after harvest. Tillage should begin early, in an attempt to warm the soil and stimulate weed growth. The number of tillage operations that can be performed will depend on the soil type, moisture conditions, erosion concerns and specific crop. In spite of reduced yields usually associated with moisture loss, wild oat control can be better than 80 percent. Early germinating broadleaf weeds such as wild buckwheat (Polygonum convolvulus), and certain mustard species, may also be controlled using this approach. In practicing delayed seeding, it is important that crop maturation, quality, and optimum yield are not unduly sacrificed, or put at risk.
Varying seeding dates will limit the ability of any one particular weed species to adapt and become problematic. Early seeded crops should be well established before species like green foxtail (Setaria viridis) emerge. Delayed seeding will allow the largest flush of wild oat (Avena fatua) to occur, providing an opportunity for tillage control before seeding. For optimum germination, green foxtail prefers higher soil temperatures than most cereals, and wild oat germinates best in cool soils.
Planting winter crops such as fall rye or winter wheat will offer competition to spring annuals since they generally out-compete these weed species. But these crops will become vulnerable to winter annuals like stinkweed (Thlaspi arvense), shepherd's-purse, or flixweed (Descurainia sophia). The fact that fall seeded crops are harvested early provides an opportunity to conduct a tillage program which will also help manage perennial weeds.
C) Seeding Rate
Increasing seeding rates 20-50 percent above normal can increase the competitive ability of some crops. If moisture conditions are adequate, higher seeding rates will also result in earlier maturity (two to three days), shorter plant height, reduced tillering, and possibly higher yields. Green feed and silage crops should be seeded at higher rates to increase crop competition and feed quality. Increased seeding rates should also be used where either post-seeding or post-emergence tillage is planned. This will help compensate for any damage caused by the in-crop tillage.
Under certain environmental conditions, higher seeding rates may increase the disease incidence and result in higher lodging losses.
D) Row Spacing
In general, narrow row spacing in cereal, pulse and most oilseed crops offers the best competitive advantage against weeds. From a crop-weed perspective, any system that places seeds equal distances apart in all directions is the ideal arrangement. This allows plants an equal opportunity to compete for water and nutrients. In addition, there is less open space for weeds to grow. Less competitive crops, such as lentil, will tend to benefit most from narrow rows.
Some crops, such as dry beans, corn, potatoes and vegetable crops, have been traditionally grown in wide rows in order to facilitate weed control through tillage after the crop has emerged. This is generally referred to as row-cropping. Row spacing can range from 40 to 90 cm (15 to 36 in.) and can be done by blocking off runs in a typical seed drill to achieve the desired spacing, or with the use of specialized precision row planters. See In-Crop Tillage below for information on row-crop tillage.
E) Seeding Depth
Optimum seeding depth will vary between crops. This will depend on seed size, soil type and moisture conditions. Seeding depth should not be deeper than required to achieve a quick and even emergence, ensuring optimum competition. Pre-seeding tillage should be as shallow as possible, since deep tillage will result in excessive soil moisture loss, and make seeding at a uniform depth difficult. If the soil is loose, consideration may be given to packing prior to seeding. This will firm the soil and help bring moisture closer to the soil surface.
F) Plant Nutrition
The level of soil fertility changes the relative competitiveness between the crop and weed. It is important that an imbalance of any nutrient does not occur. A balance of nutrients assures optimum crop emergence and healthy plants. At higher levels, or an imbalance favouring higher levels of nitrogen, weeds like wild oat are generally more competitive than a crop. This has been confirmed for wheat, barley and flax. A similar situation can be stated for numerous broadleaf weed species.
Soil available nitrogen has been shown to stimulate the germination of wild oat, green foxtail and possibly other weed species. Conversely, nitrogen may discourage individual species such as horsetail (Equisetum arvense) which is only prevalent on low fertility areas. Barnyard grass (Echinochola crus-galli) may be stimulated to grow where high levels of phosphate occur.
An imbalance of nutrients can readily occur where large applications of manure have been applied. Soil testing should be conducted on a regular basis to monitor nutrient levels, which will help in making appropriate management decisions.
Mechanical Control by Tillage
Soil cultivation has exerted a considerable influence on the evolution of weed flora. This has resulted in an increase in both perennial weeds, which rely on fragmentation and dispersal for successful regeneration, and for annual species that can produce dormant seeds of a potentially long life span. Survival of these annual species depends on incorporation into a buried seed bank. Subsequent germination of this seed is dependent on cultivations that return this seed to, or near, the soil surface.
The type and frequency of cultivation will influence the composition and density of weed species. For certain weeds, such as wild oat, timing of tillage is more important than the type of cultivation. In other situations, like Canada thistle, the type of implement, timing and frequency are all important.
In general, reduced cultivation will discourage the growth of broadleaf plants and encourage grass species such as wild oat and green foxtail.
A) Pre-Seeding Tillage
The objectives of this practice are to prepare a seed bed so planting can occur at a uniform depth and to control weeds that germinate prior to seeding. Although generally discouraged because of moisture and erosion concerns, if a number of tillage operations are planned prior to seeding, the first operation should be the deepest, with each successive one shallower. The first operation should be to aerate and warm the soil, or in many cases, to expose small seeds to light in order to stimulate weed growth. The following operations should destroy weed growth while conserving as much soil moisture as possible. The loss of soil moisture due to tillage can be a critical problem, hindering successful crop establishment. Delayed seeding can also cause difficulties in areas where short growing seasons exist. It is important to balance these negative aspects against the benefit in weed control.
B) Post-Harvest Tillage
Tillage following harvest is effective in controlling winter annual and biennial species. Tillage should be shallow (less than 10 cm) to avoid burying weed seeds in the soil.
A certain amount of control of perennial weeds, such as quack grass and Canada thistle, can be achieved, but this is weather dependent. Successful control depends largely on having freezing temperatures shortly after the tillage operation.
C) Summerfallow Tillage
Tillage operations should be as shallow as possible to avoid bringing new weed seeds to the soil surface, in addition to preventing moisture loss. The initial operation should always be the deepest, with subsequent ones progressively shallower. Tillage is most effective when the soil surface is dry and air temperature is high. Tilling small seedlings when the soil surface is moist will usually produce poor results, as many of the seedlings are transplanted, rather than being killed.
Summerfallow tillage is particularly effective for the control of perennials such as Canada thistle, quack grass and sow thistle (Sonchus arvensis). If these weeds occur in patches, they should be worked separately to avoid spreading the weed over the entire field, as well as to avoid excessive and unnecessary tillage.
In addition, weeds like Canada thistle are best left undisturbed until they reach the green bud or early flower stage. The plant is weakest at this stage of growth, and thus most vulnerable to tillage. Once tillage begins, it should continue each time Canada thistle reaches the bud stage until August 1, then afterward each time the plant reaches a height of about 7.5 cm, until freeze up. This approach will starve the root system and prevent it from forming any food reserves. It will enter winter in a very weakened state, and many will not survive.
For field bindweed, experiments conducted in Kansas by Phillips and Timmons between 1936 and 1943 found that repeated tillage 12 days after the emergence of new shoots resulted in control of established plants within two fallow seasons. Furthermore, their experiments also showed that this strategy required the fewest number of tillage operations over that two year period to completely eliminate the perennial root.
Quackgrass problems should be handled in a somewhat different manner. Tillage to control this weed depends on physically damaging the root system. In dry years, a cultivator with narrow spikes will be effective, as this will drag roots and rhizomes to the surface where they will dry out and die. In wet years or areas, the first tillage operation should be with a disc implement that cuts the rhizomes into small pieces. Each of these smaller sections of rhizome will try to establish a new plant, which in turn has to be destroyed by subsequent tillage. New plant growth should not be allowed to get taller than 7.5 cm before being tilled.
In either of the above cases, tillage should not be deeper than required to do an effective job. Shallow tillage will concentrate the rhizomes on or near the soil surface, resulting in a more uniform emergence and better control from future tillage operations.
Extensive tillage should always be approached with caution. It can create serious soil erosion problems, particularly if it is done on a large area. Where soil erosion is a problem, tillage should be integrated with other methods to ensure the soil is protected.
Weeds that emerge ahead of the crop can reduce yields significantly more than those that emerge following the crop. Depth of seeding should be at the deeper end of the recommended range to ensure crop plants are well anchored at a uniform depth.
A) Post-Seeding Tillage
Post-seeding tillage may be done by harrowing or using a rod-weeder. Best results will be obtained on a warm, sunny day when the soil surface is dry. It is important to ensure that the crop has not sprouted into the tillage zone. Experiments conducted at the Agriculture and Agri-Food Canada Research Farm at Scott, SK suggest that a rod-weeder works very well when used about five to seven days after seeding peas in mid-May. An early spring tillage operation should take place about 10 to 12 days prior to seeding.
B) Post-Emergence Tillage
Post-emergence tillage may be done on weeds that emerge shortly after the crop can be controlled by a harrowing operation. Small seeded species such as green foxtail, Lamb's-Quarters (Chenopodium album) and redroot pigweed (Amaranthus retroflexus), which usually emerge from shallow depths, can be well controlled. The effectiveness of this strategy is dependent on many factors: competitive ability of the crop, the seeding depth, crop stage, weather and soil conditions, and implement and tractor speed. A certain amount of crop damage is likely to occur; therefore, seeding rates should be increased to compensate for the damage. Wild oat can germinate from substantially deeper in the soil. Under these circumstances, control may not be as satisfactory. A field inspection of weeds and the depth from which they are emerging should be done prior to any post-emergent tillage.
This is also dependent on soil surface conditions being dry, and the weather warm and windy. It is generally recommended that the harrowing operation should be in the direction of the rows. However, recent research indicates that the direction is not as important as once thought.
This technique should only be considered for use during the following stages of crop development:
- Wheat - From the two-to four-leaf stage.
- Barley - From the two-to four-leaf stage (before tillering). Barley is more sensitive than wheat.
- Oat - Oat is more susceptible than wheat or barley to damage. Post-emergent harrowing is not recommended.
- Sunflower - Safe up to the six-leaf stage. Harrow across the rows. Adjust the seeding rate to make up for the approximately 5,000 plants/acre that will be lost each time the crop is harrowed.
- Fababean - Harrow across the rows when the crop is between five to 15 cm tall and the plants are dry.
- Lentil, Field Pea - Harrow in the seedling stage (no more than 10 cm tall). The crop must be dry to minimize the spread of disease. Use tine harrows only. Lentil is more sensitive to damage than pea.
Some crops, such as dry beans, corn, potatoes, and vegetable crops, have been traditionally grown in wide rows in order to facilitate weed control through tillage, after the crop has emerged. This has also been evaluated in the more traditional prairie crops such as wheat, barley, flax and peas. While inter-row tillage does a good job of controlling weeds between the rows, weeds such as wild oat and wild mustard growing in the rows merely compensated to produce just as much biomass and seed as areas that were not tilled.
In limited research, fenugreek and chickpea have shown some positive response to row-cropping methods, but spice crops such as coriander, cumin, and caraway were not as successful. As tillage can stimulate weed seeds to germinate, several passes through the field may be necessary to manage the resulting flushes. Seeding and tillage passes also need to be highly accurate to prevent damage to the crop.
It is important in row-crop systems for tillage equipment to match seeding equipment widths or for tillage implements to be a simple fraction (i.e. 1/2, 1/3, or 1/4) of the seeder so that rows remain constant. Row-crop tillage equipment can be quite specialized, with shrouds to protect the crop and sensors to make fine adjustments to the path of the implement, so that tillage can be completed as close to the row as possible without damage to the crop.
Reducing the number of weed seeds that are returned to the soil before and during the harvesting operation will have a significant impact on weed pressure in future years.
Many combine harvester models can be fitted with a chaff collection system that captures the material that is too small to be collected in the grain, and is normally blown out the back of the combine as waste. Small lightweight material usually contains a high number of weed seeds. A chaff collector captures this material and either blows it into a trailing wagon or deposits it on top of a straw swath for baling later. Swathing enhances this process by cutting many weeds before they have shed seed and allowing them to dry so that seed is collected in the chaff rather than shattering to the ground prior to harvest, or at the cutter-bar.
Another system of harvest that removes weed seeds with the crop is the McLeod Harvester, which is a system similar to the old reaper and thresher system of the early 20th century. The McLeod Harvester removes grain, chaff and other small light material in bulk, and transports it to a stationary separating unit, where the grain is separated from the rest. The straw is left on the field.
Chaff from either the chaff collecting system or the McLeod Harvester system may be composted or fed to livestock. Grinding feed will reduce the viability of larger seeds, but small, hard coated seeds will remain intact. Therefore, it is important that manure from livestock be composted to reduce the viability of those weeds, or feed from these sources may be cooked at boiling temperatures before feeding to reduce weed seed viability.
Some Australian producers have adopted a management system whereby chaff, containing weed seeds from the entire width of the swath, is deposited in a narrow band behind the combine. By following exactly the same harvest paths year after year, with the assistance of GPS systems, the seed bank is restricted to those narrow bands through the field.
Within these chaff deposition bands, weeds compete with each other and the crop, while the seed bank declines in the soil between the bands. As a result, the crop needs to compete with fewer and fewer weeds in the area outside the chaff bands. With a high number of weeds in a small fraction of the field and a low number of weeds in the larger portion of the field, yield is better overall than it would be if weed populations were spread evenly across the entire field.
Other Control Methods
Methods such as mowing and burning may be used in specific situations. Mowing can be effective for perennial weed control. Success depends on repeated mowing to deplete the plant's food reserves. For annual and biennial weeds, mowing should occur before the first flowers open to prevent any seed production. Some annual and biennial weeds will not recover after one cut. However, others may survive and continue to flower below the cut line. One strategy to correct this is to cut the plant as high as possible and still remove flowers in the first and any subsequent cuts, lowering the height of the mower with each successive cut.
In limited research, cutting weed material above short stature crops did not prevent yield losses from weeds in the year of clipping, but did reduce the production of seed and subsequent emergence of weeds in following years. The timing of clipping should be after weeds have flowered, but prior to seed set. Some indeterminate weeds may continue to flower below the cut line and produce significant amounts of seed.
Flaming or burning is controversial as to its effectiveness. Only weed seeds present in the windrow, and on the immediate soil surface below the windrow, are affected by burning. In the case of wild oat, it has been suggested that 15 to 45 percent of these seeds will be destroyed.
Flaming, burning or other radiant heat sources kill very few (if any) weed seeds beneath the soil surface. For soil stewardship and the preservation of organic matter, burning should only be practised on windrowed straw or on gathered weed material from patches within a field.
The success of this practice depends on generating high enough temperatures over a sustained period to destroy the weed seeds. Stubble fires may develop high temperatures, but only for a short time, usually not long enough to kill most weed seeds. Burning, however, may break the seed dormancy in certain weeds, causing them to grow. Depending on the situation, this may be beneficial.
Weeds are responsible for reducing crop growth by two main mechanisms. The first mechanism, which we are most familiar with, involves competition for resources such as moisture, nutrients and light. The second mechanism, allelopathy, involves the release of toxins into the environment. Allelopathy may be defined as any negative or positive plant response that occurs from chemicals produced by another plant. Because of all the factors involved, it is a complex area of study. Reduced crop growth due to allelopathy has been suggested for 90 different weed species.
Quackgrass shoots and rhizomes reduce emergence and growth of alfalfa and cause chlorotic and stunted growth of tame oat and barley. In the case of alfalfa, growth reductions as high as 85 percent have been reported. In numerous legumes, quack grass residue has been reported to reduce root nodulation. This same type of allelopathic interference occurs within and between crop species, and may be used to retard weed growth.
Crop residues from fall rye (Secale cereale) that have been incorporated into the soil have been shown to inhibit a number of weed species, including wild oat. This may be used to an advantage in a summerfallow situation. Fall rye would be planted in the fall and incorporated the following spring for weed control. In a similar fashion, using sweetclover (Melitotus alba) as a green manure crop is not only beneficial to the soil, but is allelopathic to a number of weeds.
Surface residues of immature cereals appear to be particularly inhibitory to the emergence of annuals, including lamb's quarters, purslane (Portulaca oleracea) and redroot pigweed. Preliminary research indicates that canola residues may also have allelopathic properties towards certain broadleaf and grassy weeds.
Biological Weed Control
Classical biological control involves utilizing a parasite or predator (such as a microorganism or insect) from the weed's place of origin to regulate a weed population. Classical biological control has good potential, but at present it has been most successful in rangeland, where even a small reduction in weed vigour can increase forage production. Weeds such as nodding thistle (Carduus nutans),leafy spurge (Euphorbia esula), and scentless chamomile (Matricaria perforatum) are being controlled with good, moderate and good success respectively.
Management of weeds in organic systems is a long and complex process, requiring a high level of management. To ensure the success of the organic farm, it is critical to know what types of weeds are present and understand effective management techniques. It is also important to realize there may be some weeds that cannot be managed or tolerated in an organic system, and these should be avoided. Deep rooted, persistent perennial weeds, such as leafy spurge, field bindweed, Russian knapweed or toadflax, may make transition to organic production expensive or prohibitive.
Transition to organic production is a challenging time, as both the manager and the field find a new balance. Crops in transition must be marketed in the regular commodity markets, and yield losses from weeds during transition can have a significant economic impact. Therefore, transition to organic production should be approached in stages, with a small portion of the land being integrated into organic production each year. Forage crops make good transition crops, since they require less management and are effective in reducing annual weed seeds in the soil.
An organic system that is diverse and well managed can be highly productive and in harmony with land around it. The intent of this document is to emphasize how the integration of management and cropping diversity is the key to successful organic production - in other words, harmonizing the land and the producer.