1. Green Manures & The Weed Management Program

Green Manure Allelopathic

Green manures can smother weeds, can be allelopathic, and can be used to disrupt weed lifecycles. Photo of yellow sweetclover by Brenda Frick.

While green manures are primarily grown for their contributions to soil fertility, they can also play an important role in managing weeds.

Green manure legumes tend to be slow growing. However, once established, some green manures double as smother crops, out-competing weeds and depriving them of water, nutrients and sunlight. Green manures cover the soil surface, blocking light and altering the temperature and moisture at the soil surface in ways that can impede weed germination or interfere with weed growth. Some green manures are also allelopathic, containing or secreting compounds that can inhibit the germination of other plants.

Green manures can bring flexibility to a crop rotation by providing opportunities to change management practices and timing. Altering the green manure planting date can allow for additional weed control operations before planting and/or after termination. This can break weed patterns, by allowing specific management operations, such as seed bed preparation, seeding, in-crop weeding, and tillage, to occur at different times than they do in the rest of the rotation. Performing these operations at the same time of year every year leaves an open window of opportunity for weeds; weeds that perform well in between these management operations will thrive, and their populations will build. Changing the timing of operations “closes the windows” that are typically left open. This gives weeds fewer opportunities to adapt, and can prevent the build-up of large populations of specific weeds.

Green manures can also bring diversity to the rotation. Green manures can provide an opportunity to include longer-lived biennial and perennial legumes in a system otherwise dominated by annuals. Longer-lived green manure stands provide ground coverage and can compete with weeds at times that are typically more open to weed growth, particularly in the early spring and late fall. This can disrupt weed patterns.

Green manure termination can also provide a weed control opportunity. Terminating a green manure stand with tillage, mowing, rolling or grazing can not only terminate the green manure to allow the release of the nitrogen that it has accumulated, but can also kill weeds at the same time. Timing termination operations to maximize impacts on weeds can be a useful tool.

Green Manure Mulch

Mulch formed from a hairy vetch green manure. When thick enough, such mulches can smother weeds and disrupt weed emergence. Photo by Laura Telford.

Green manure mulches, formed when the green manure is terminated with mowing or the roller-crimper, can also suppress weeds. The mulch reduces light penetration and slows soil warming in the spring. Note that this can also slow crop emergence, especially of small-seeded crops. The mulch can also be a physical barrier to weed emergence. Mulches are often effective in managing small-seeded annual weeds, but may not be as effective against perennial weeds or large-seeded annuals such as wild oat. Mulch effectiveness depends on having sufficient green manure biomass left on the soil surface. In general, a mulch thickness of at least 2-3 inches or 6-7 tonnes of dry biomass per hectare is recommended, especially when no-till planting into the mulch. Lower biomass green manures, or green manures grown in dry regions, are not likely to adequately suppress weeds.

Be cautious of green manures that can become weeds. Volunteer plants can emerge in subsequent cash crops, and may have negative effects. Careful placement of these green manures in the crop rotation can alleviate these potential problems. For instance:

  • Seed from hairy vetch volunteers is difficult to clean from cereals. Hairy vetch can be expected to volunteer, as it can grow from hard seed not germinated when the green manure was planted or from seeds shed before the green manure is terminated. Do not use hairy vetch as a green manure before a cereal, especially one destined for milling.
  • Indianhead black lentils may contaminate a French green or red lentil cash crop. Do not use an Indianhead lentil green manure before a small-seeded lentil crop.
  • Field pea and faba bean green manures tend to be smaller-seeded varieties of these two crops. Volunteers from these small-seeded varieties may contaminate cash crop harvests of larger-seeded varieties of these crops.
  • Sweetclover volunteers can pollute alfalfa seed harvests. Do not grow sweetclover green manures if planning to grow alfalfa for seed.

Green manures provide fertility to the organic crop rotation. Weeds need soil fertility, just like cash crops. Some weeds thrive when soil fertility conditions are high, and so green manuring can support healthy weed populations, just as they support healthy crops. Green manures can be managed in ways that match nitrogen availability with crop needs through careful consideration of the crop rotation, termination timing and methods. This limits nitrogen availability for weeds. Refer to the Managing Green Manures resource for additional information.

It is important to also think of weeds in context. We have been conditioned to think of all weeds as detrimental, and indeed most can be. However, not every weed needs to be eliminated to produce successful crop harvests. Think about the economic impact of the weeds, versus the fact that they can also provide some services to the farm, including providing ground coverage, catching nutrients and supporting beneficial organisms. While it is important to make sure that weeds do not get out of hand, it is also important to think about the bigger picture.

2. Green Manure Options for Weed Management

When weed management is one of the goals for your green manure, there are options that can help to improve your odds of reducing weed pressures for later stages of the rotation.

2.1 Green Manure Choice

2.1.1 Choose Competitive Green Manures
Green Manure Biomass

Choose high biomass, competitive green manures, like hairy vetch, to suppress weed growth. Photo by Laura Telford.

The legume species grown as green manures on the Prairies vary in their competitiveness. Sweetclover and hairy vetch are among the most competitive once established, while lentils and chickling vetch tend to be less competitive. When planting into fields with high weed pressures, consider planting the most competitive of the green manures that are well adapted to your growing conditions. See Choosing a Green Manure resource for guidance.  If using a less competitive green manure, consider strategies that can give the green manure a leg-up over weeds, such as intercropping with a cerealincreasing seeding rates, or performing additional weed control operations.

2.1.2. Choose Green Manures That Interfere with Weed Patterns

Choose green manures that have lifecycles that differ from the other crops in your rotation, or that provide flexibility in management.

Integrating longer-lived biennial or perennial green manures in annual grain rotations provides early and late season ground cover and competition at times that are typically windows for weed growth. Biennial or perennial green manures have a competitive advantage over spring-germinating weeds, and can also effectively suppress some perennial weeds. For example, including perennial alfalfa has been demonstrated to reduce Canada thistle populations, especially when the alfalfa stand is cut for forage. Perennial green manures also tend to suppress annual weeds, which are typically triggered to germinate by the soil disturbance associated with establishing an annual crop. Yellow sweetclover green manures have been observed to reduce populations of common weeds such as lamb’s quarters, flixweed, kochia, Russian thistle, dandelion, downy brome and perennial sowthistle within the green manure and/or for the following crop. Increasing the frequency of perennial green manure stands in the rotation can, however, lead to an increase in perennial weed populations, such as dandelions and quack grass. Similarly, repeatedly growing biennial green manures can promote the growth of perennial weeds, such as Canada thistle.

Including annual legume green manures can allow for flexibility in the timing of planting and weed control operations. In annual grain crop rotations, the same operations are typically performed in the same windows of time year after year. Weeds that perform well in between these management operations will thrive, and their populations will build. By altering the timing of operations like seed bed preparation, seeding, in-crop weeding, and tillage, the typical openings for weeds are closed. This gives weeds fewer opportunities to adapt, and can prevent the build-up of large populations of specific weeds.

Green Manure Lifecycle 1

Choose green manures with lifecycles that differ from your major crops, like perennial alfalfa, to disrupt weed cycles. Photo by Laura Telford.

Including winter annual legume green manures in the rotation can also interfere with weed patterns.  Hairy vetch is a winter annual that, when planted in the late summer, will overwinter under many Prairie conditions. This provides late fall and early spring ground cover, and early spring growth and competition against weeds. Alternatively, hairy vetch can be planted in the spring. This spring-planted hairy vetch will typically winterkill in Prairie climates. But, even spring-planted hairy vetch will continue growing late into the fall, producing biomass, accumulating nitrogen and providing few opportunities for weeds. When this hairy vetch biomass is left on the soil surface, it can also suppress weeds in the spring. Both of these strategies remove windows of opportunity for fall and spring weed growth in annual cropping systems.

Choose green manures that can be managed in ways that reduce weed pressures, both within the green manure and in the overall rotation. Consider green manures that can tolerate post-emergence weed control, like pea. Also consider green manures that can be terminated at a time that will prevent weed seed set. Annual green manures need only be grown for about six to nine weeks out of the four month growing season, and so can afford opportunities for pre-plant tillage to reduce annual weed pressures and/or post-termination tillage to reduce Canada thistle. Green manures that can leave high-residue mulches can also help to reduce weed pressures. See the Green Manure Management for Weed Suppression section for more information.

You can also think outside the box, and consider novel green manures, green manure mixes or cocktail mixes. Warm-season legumes can be used as green manures, and will quickly emerge and grow in July heat and then winterkill, leaving crop residues that can then be worked in the spring.  Cocktail mixes include warm and cool season legumes and cereals, broadleaves and brassicas; these mixes can provide coverage throughout the growing season, and the mixture of crops optimizes resource use, limiting opportunities for weeds.

2.1.3. Choose Allelopathic Green Manures

Green Manure Allelopathic 2

Choose allelopathic green manures, like sweetclover, to interfere with weed germination. Photo by Andy Hammermeister.

Allelopathic plants produce compounds that interfere with the growth and germination of other plants. These compounds are released into the soil, either through the plant’s roots or during plant decomposition.

Sweetclover green manures, particularly yellow varieties with high coumarin levels (such as Yukon) are allelopathic. Hairy vetch is an annual legume green manure with some allelopathic properties. These green manure options are highly competitive after their slow establishment, and can help to alleviate weed problems both in the green manure and in the following crop. Alternatively, fall rye is a rapid-growing allelopathic cover crop that can be planted alongside a legume in a green manure mix or after legume green manure termination to suppress weeds. Note that allelopathic green manures do have the potential to also suppress growth of following crops. This effect tends to be most evident on small-seeded crops, and can be reduced by allowing a period between green manure termination and planting the next crop.

When growing allelopathic green manures, removing biomass as hay can reduce the allelopathic effect. On the other hand, leaving terminated sweetclover residues on the soil surface can reduce the germination of winter annual weeds.

3. Green Manure Management for Weed Suppression

There are a number of management practices that can be used to manage weeds within the green manure crop. Many of these management practices require some forethought, as they are steps that can be taken before and during planting. Combining management practices, including choosing competitive crops or cultivars, increasing plant densities, and implementing weed control measures can result in drastic reductions in weed populations.

3.1 General Green Manure Management

3.1.1 Green Manure Choice

As outlined in the Green Manure Options for Weed Management section, species choice can be important when using the green manure as a weed management tool as well as a fertility source. Choose competitive green manures, green manures with allelopathy, and green manures that diversify your rotation.

3.1.2 Use Mixed-Crop Green Manures

Intercropping legume green manures with a cereal can result in a more competitive green manure stand. Cereals are quick-growing and vigourous early in the season, while legume green manures are slower to establish. Cereals can provide competition against weeds early in the season that can improve the overall competitiveness of the green manure stand. Cereals will also take up available soil nitrogen, limiting the amount available to support weed growth. This nitrogen uptake also pushes the legume in the mix to establish a relationship with Rhizobia for nitrogen fixation, and may improve the efficiency of this nitrogen fixation. Together, an upright cereal and vining or climbing legume can more effectively capture sunlight, water and nutrients, leaving few opportunities for weeds.

Green Manure Mixed Crop

Use green manure mixes, like this mix of hairy vetch with a cereal, to improve the competitive ability of the green manure stand. Photo by Laura Telford.

Mixed crops of peas and oats, as well as hairy vetch and barley, have been shown to be good competitors against weeds. In both instances, the mixed green manures have been found to have lower weed populations than sole crops of the legume component of the mix.

Nurse crops are also a good choice when establishing a biennial or perennial green manure. Underseeding or relay seeding the green manure legume with a cereal crop not only allows a cash crop harvest in the green manure establishment year, but also supports the establishment of the green manure. The cereal nurse crop competes against weeds, limiting weed pressure during the slow establishment phase of the green manure. It is typically suggested that the seeding rate of the nurse crop be reduced to allow the green manure to establish well.

For more information on green manure mixes, see Managing Green Manures and the Green Manure Profiles.

3.1.3 Be Prepared to be Flexible

One of the many benefits of green manures is the flexibility they can bring. Adapt your management to your circumstances to gain the most benefit from the green manure crop, both in terms of fertility and weed management. Be prepared to alter planting or termination timing, to perform additional weed management operations, and to use green manures when windows of opportunity arise. Be aware that early or late termination will alter nitrogen availability from the green manure, while late termination also risks seed set that can lead to volunteers in the next crop and also increases water use. See the Termination section of Managing Green Manures for more information.

3.1.4 Manage the Green Manure with Care

Green Manure Care

A well-managed green manure stand can provide few opportunities for weeds while producing a dense stand that will provide the nitrogen needed to fuel a rotation. Photo by Brenda Frick.

Investing in the growth of a high quality green manure stand will reduce the opportunity for weeds. Use good seed, do not skimp on seeding rates, prepare a good seed bed, inoculate, and monitor the green manure crop. In short, treat your green manure with the same care you provide to your cash crops. They are, after all, an essential part of the crop rotation. When used wisely and managed to maximize their growth, green manures have the potential to provide the nitrogen needed to fuel your rotation while also aiding in the management of weeds.  See Managing Green Manures for details.

3.2 Steps Taken at Green Manure Seeding for Weed Management

3.2.1 Use High Quality Seed

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Use high quality, inoculated legume seed, planted at a high seeding rate into a well prepared seed bed to give green manures a leg up on weeds. Photo by Brenda Frick.

Ensure green manure seed is high quality and does not have a high weed presence. Weedy seed can bring new weeds on to your farm, or could reinforce the existing weed population. If using seed produced on farm and saved, be sure that it is cleaned before planting.

Old or low quality seed may not germinate well and may have low vigour, providing openings where weeds can establish. If in you are unsure of the quality of your seed, test the germination. Increase seeding rates when germination is low, or consider replacing the seed. See Managing Green Manures for details.

3.2.2 Increase the Seeding Rate

Green manures should be planted at a rate no lower than the recommended seeding rate (see Managing Green Manures or the Green Manure Profiles for specific recommendations). Planting at a lower rate produces a lower density stand that will be less competitive against weeds. Be sure to choose a green manure that you can seed affordably at the proper seeding rate. If germination rates are low, increase the seeding rate to compensate (see Managing Green Manures for details).

When planting a green manure into a field known to have high weed pressure, consider increasing the seeding rate above the recommended seeding rate. Planting a green manure crop at a high seeding rate increases the density of plants in the green manure stand. This limits space and resource availability for weeds. Higher seeding rates can also compensate for possible losses that may result if pre- or post-emergent weed control is required. Increasing seeding rates by 1.5 to 2 times is often recommended.

For instance, a Saskatchewan study that found increasing seeding rates of Indianhead lentil, chickling vetch and field pea resulted in higher biomass green manure stands with lower weed populations. This is reinforced by another Saskatchewan study, in which seeding lentil at one and half times to double its recommended seeding rate with narrow row spacing was found to improve lentil yield when harvested as a pulse crop. This could also be applied to lentils seeded as a green manure.

For large-seeded annual legumes such as pea, typical seeding rate recommendations target a green manure stand of 50 plants per m2. Under weedy conditions, seeding rates for large-seeded annuals should target a density of up to 78-150 plants per m2 to counter weeds while remaining economical. Smaller-seeded green manures such as Indianhead lentil are typically planted with a target plant density of approximately 110 plants per m2. Under weedy conditions, small-seeded annuals should be seeded at a rate that results in 220-300 plants per m2. In both cases, this suggests that seeding rates should be increased from 1.5 to 3 times the recommended rate, with typical recommendations suggesting that seeding rates are up to doubled when weed pressures are high.

3.2.3 Seed Into a Well Prepared Seed Bed

A well prepared seed bed gives any crop a good start. Well prepared soils help to provide good seed-to-soil contact that will promote good germination and establishment. Seed bed preparation also provides weed control. Plant immediately after the final seed bed preparation operation, so that weeds will not have an opportunity to germinate before the green manure legume does. See the Managing Green Manures resource for further information.

3.2.4 Consider Fertility

Green Manure Fertility

Inoculating green manures puts the legume roots in close contact with the Rhizobia that fix nitrogen. Photo by Brenda Frick.

Be sure to inoculate legume green manures with the appropriate Rhizobia bacteria. This will help to ensure that the legumes quickly and efficiently partner with their nitrogen-fixing bacteria, improving nitrogen fixation. Legumes do not need supplemental nitrogen fertility when well-nodulated. Limiting nitrogen in the system pushes the legume towards partnering with their nitrogen-fixing Rhizobia. This means that the legume, but not the weeds, is adequately supplied with the nitrogen required to support vigourous crop growth. Be sure to check with your certification body to ensure the inoculant formulation is permitted. See Managing Green Manures for more information about proper inoculation of legumes.

Legumes do require other nutrients to perform at their best. Making sure soils are well supplied with macro and micronutrients required for healthy plant growth (including phosphorus, potassium, sulfur, magnesium, copper, zinc, iron, and molybdenum) will help to support a competitive green manure. If green manures typically do not perform well on your farm, consider testing the soil and the plant tissue for nutrient levels.

Some farmers are experimenting with applying manures in the green manure year, in an effort to boost soil phosphorus levels. Applying the manure in the green manure can allow an opportunity to control any flushes of weeds that capitalize on the newly available soil phosphorus.

3.2.5 Think About Planting Patterns

Consider planting the green manure with a narrow row spacing, or broadcast seeding small-seeded legumes. Green manure seeding can also be performed in two operations, so that rows are cross-planted, or planted at 45 or 90 degree angles.  Cross-planting can be a good option when mixed crop green manures are used, where one crop can be seeded over the other crop at an angle.  All of these techniques help to provide better soil coverage, which will speed canopy closure and provide fewer openings for weeds.

3.2.6 Seed Early

Most legumes are quite cold-hardy early in their lifecycles. Planting early in the season can allow the crop to emerge before the weeds, giving the crop a head start. An early-seeded stand can also provide a good opportunity for pre-emergent weed control, as the weeds will tend to be small. Note that germination of the legume green manure will be slower in colder soil than in warmer soils.

3.2.7 Seed Late

As an alternative to seeding early, take extra time for weed control measures before planting. Seed bed preparation often involves tillage. This soil disturbance can stimulate weed seed germination. Crops that emerge before the weeds tend to have a competitive edge, while those that emerge at the same time or after weeds are at a competitive disadvantage. Waiting for the first flush of weeds to emerge after the seed bed has been prepared, then tilling and planting can give the crop a leg up over weeds. This is often referred to as a stale seed bed. This option is most effective on winter annual weeds and early emerging weeds like lamb’s quarters, wild mustard, wild oats, stinkweed and knotweed.

3.3 Weed Management After Planting the Green Manure

3.3.1 Pre-Emergence Weed Control

Pre-emergence weed control, performed after planting but before the green manure emerges, can help to give the green manure an advantage over weeds. Weeds that emerge ahead of the green manure are likely to thrive if not controlled. If weed pressures are low, pre-emergence weeding may not be required.  But, if you see an abundance of weeds emerging ahead of your green manure after planting, consider a pre-emergence operation.

Harrows, rotary hoes, and rod-weeders are typically used for pre-emergence weeding. This equipment is run shallowly, and can up-root small weed seedlings. Harrows work well in some conditions, but may not be practical or successful when there is a lot of residue from a previous crop on the field, or if the field is rocky. Rotary hoes work well when there is surface residue on soils, while rod-weeders are effective in dry conditions.

Seed the green manure at a high rate and at the deeper end of the recommended planting depth if you anticipate the need for pre-emergence harrowing. Seed into moisture. Run the harrowing equipment at a shallow depth to minimize disturbance to the green manure and to limit stimulating germination of deeper weed seeds. Some stand losses can result from pre-emergence weed control.

Pre-emergence weed control is best performed when soils are dry and weeds are small. Pre-emergence harrowing is most effective when weeds are in the white thread stage – the weeds will look like white threads when you dig through the soil, with very small roots and shoots that have not yet fully emerged. Wet conditions and compacted soils hinder the success of pre-emergence weed control. Hard packed soils will require more aggressive operations than do more loosely packed soils. Be sure to carefully inspect fields to make sure your crop has not yet emerged before harrowing.

3.3.2 Post-Emergence Harrowing for Weed Control

Some green manure legumes, such as field pea, lentils and faba bean, can tolerate post-emergence harrowing. Others, such as hairy vetch, cannot. Field peas are particularly resilient to post-emergence harrowing. Be more cautious with lentils or faba beans.  In general, smaller-seeded perennial and biennial legumes are less tolerant of post-emergence harrowing than the annual legumes, which tend to have larger seeds that are planted deeper in the soil. These larger-seeded plants are less likely to be uprooted by the harrows, and can better tolerate being covered with soil.

If you have planted a green manure crop that is tolerant of post-emergence weed control and weed pressures are high, you may wish to consider harrowing or rod-weeding after the crop has emerged. Post-emergence harrowing is most effective on small-seeded, shallow-rooted weeds. Consider pre-emergence weeding if the weeds are present but small before the crop emerges, or post-emergence weeding if this is the case after the crop has emerged. Both pre- and post-emergence weed control are best performed when weeds are at the white thread stage – when they have a small shoot and root, but have not yet fully emerged.

Note that some green manure stand losses can be expected when using post-emergence harrowing or rod-weeding. When harrowed at the proper time (see below), losses of pea and lentil stands can be expected to be in the order of 20-30%.  If you anticipate the possibility of post-emergence harrowing, increase the green manure seeding rate, and plant at the deeper end of the range of recommended planting depths.

Post-emergence harrowing should be done in the early stages of green manure growth. Faba bean can typically be harrowed when it is 2-6” in height. Field peas and lentils should be harrowed when they are seedlings of no more than 4” in height. Losses will be minimized when harrowing is performed at the proper stage.

Harrowing should be done on hot, sunny days when uprooted weeds will quickly dry out and die. Harrows should be set so that disturbance is low. Trials suggest that tine harrows be set at a backwards 45° angle, and rotary harrows at a 45° angle.

Harrowing will be less practical and successful when soils are compacted, when there is a lot of residue on the soil surface, when weeds are deep in the soil, and when fields are wet. Post-emergence weed control can also be performed with a rod-weeder or rotary hoe. If using the rotary hoe, more surface residue can be tolerated, but timing is critical – weeds must be in the very early emergence stages for control. Post-emergence rod-weeding is best performed when soils are dry and loose, with the crop planted well below the rod depth. Rod-weeding can better handle larger weeds than the other post-emergence options.

3.3.3 Mowing for Weed Control

Mowing perennial or biennial green manure stands can help to reduce populations of problematic perennial weeds in later stages of the rotation. Mow the green manure when the perennial weeds flower, and mow again each time new flower stalks are seen. These repeated mowings can help to deplete energy stores in the weed roots, weakening or even killing the weeds. The same can also happen to the green manure, so be sure to stop mowing with enough time before freeze-up to allow the green manure to regrow and store enough reserves to overwinter, unless this is the final year of the green manure stand, in which case this weakening can help to ease termination.

Note that removing the mowed green manure biomass, which is often taken for hay, reduces the nitrogen contributions of the green manure stand.  If the stand is allowed time to recover and regrow before termination, nitrogen contributions may not be greatly reduced. However, removing hay also removes other nutrients, including phosphorus. This can deplete soils, unless manure is returned to the field.

There can also be opportunities for mowing annual legume green manures. These annuals will generally not recover when mowed after they have flowered.  Mowing well before flowering can allow for regrowth. Alternately, high-set mowers, swathers, or trimmers may be able to cut off tall weed flower stalks without damaging a lower-growing green manure.

While these tactics are not likely to improve the green manure stand, they can help to limit weed proliferation into later phases of the rotation.

3.3.4. Grazing for Weed Control

Allowing livestock to graze green manure stands can also help to reduce weed populations, if grazing is well-managed. Perennial green manure stands can be repeatedly grazed. Grazing can also be used to terminate an annual green manure stand. Like mowing, this tactic is not likely to improve the green manure stand, but can help to limit weed proliferation into later phases of the rotation.

Green Manure Grazing 2

Mob grazing livestock will often eat the green manure and weeds in the stand. Photo by Brenda Frick.

Livestock find many weeds palatable, and are likely to graze weeds along with green manure plants. Sheep and goats are particularly amenable to grazing weeds. Cattle often favour grassy plants and will selectively graze, and so are not as good at controlling weeds. Mob grazing cattle will encourage less selective feeding, and will also trample uneaten plants. Goats will happily eat woodier plants and thistles. Sheep will also eat thistles, tend to cut plants close to the ground, and will also eat leafy spurge.

Grazing before weeds set seeds will limit contributions to the weed seedbank, and hence future weed populations. Like mowing, repeated grazing can also help to deplete root energy reserves of perennial weeds, weakening the plants.

Note that grazing will speed the cycling of nutrients – see Managing Green Manures for more information.

3.4 Weed Management at Green Manure Termination and After

3.4.1 Green Manure Termination

Green Manure Termination

Green manure termination will also terminate weeds and can prevent weeds from adding to the weed seed bank. Photo by Laura Telford.

Termination of the green manure will also terminate weeds in the green manure. If managed well, termination by mowing, tillage, grazing, or roller-crimping can all effectively kill most annual weeds. However, weeds that have not yet flowered will bounce back after a pass of the roller-crimper and weeds that grazing animals avoid will continue to grow.

Deciding when to terminate can be a balance between achieving maximum nitrogen contributions from the green manure, conserving soil water, and ensuring weeds do not go to seed. For maximum nitrogen contributions, terminate when the green manure is in flower. To conserve soil moisture, terminate earlier. When weed populations are high, or could be particularly problematic if seeds from this year’s weeds germinate and contaminate the following crop, the green manure is best terminated before the weeds flower.

3.4.2 Terminate Early if Weed Pressure is High

A sacrifice in the amount of nitrogen that the green manure will contribute might be the price to be paid for preventing weeds from going to seed. In a study in the Dark Brown soil zone in Saskatchewan, weeds were found to accumulate more additional biomass than annual green manures when termination was delayed from early bud to full bloom. This suggests that when weed pressure is high, early termination results in the green manure contributing a higher proportion of the terminated biomass.

In years with very high weed pressure, or when the green manure legume makes up less than 25-50% of the stand, a consideration could be made to terminate the green manure very early. Then, another green manure can be sown.

3.4.3 Fall Weed Control

Green manures are grown for a shorter duration than most cash crops. This can provide time after green manure termination for weed control operations. Timing tillage for the fall can help to manage weeds. This can be accomplished either by timing green manure termination for the fall, or with additional tillage operations done solely for weed control after the green manure has been terminated. Weeds that overwinter, in particular perennials and winter annuals, are particularly sensitive to fall tillage. Other annual weeds, and particularly volunteer crops, may be prompted to germinate with fall tillage as well. These germinated weeds will then winterkill, helping to deplete the annual weed seedbank.  Be aware that fall tillage will leave soils exposed over the winter, limiting snow capture and leaving soils vulnerable to erosion. Fall tillage can also bury weed seeds, adding to the weed seedbank and future weed populations; this can be limited by using only shallow tillage.

3.4.4 Leave Mulches on The Surface

Leaving the plant material from a terminated green manure on the surface is an alternative to immediately incorporating the green manure. Green manure mulches can be generated when green manures are terminated with the roller-crimper, mowing, wide blade cultivation or are winterkilled.

Green Manure Mulch 2

Leaving green manure residues on the soil surface can limit weed germination. Photo by Laura Telford.

Green manure mulches provide ground cover that can interfere with cues used by weeds to trigger germination. Mulches reduce light penetration, reduce soil temperature, and limit fluctuations in soil temperature. By disrupting these cues, weed germination is suppressed.

Green manure mulches can be left in place for a short time period, can be left over winter and tilled in the spring, or can be left as a mulch for no-till planting the next crop. If no-till planting, a spring tillage operation – which will often trigger weed germination – is avoided. However, ensure that mulches are sufficiently thick, requiring a biomass of 6-8 tonnes per hectare, to give good weed control when no-till planting into the mulch.

3.4.5 Cropping After the Green Manure

Green Manure Crop

Weeds need soil fertility, just like cash crops. Some weeds thrive when soil fertility conditions are high, and so green manuring can support healthy weed populations just as they support healthy crops. To limit the ability of weeds to use the flush of fertility released from a green manure, be sure to follow the green manure crop with a heavy-nitrogen using cash crop, such as high protein wheat or hemp. Also consider when nutrients will be released from the decaying green manure residues, and try to time this release with crop planting. Otherwise, these valuable nutrients will serve to build weed populations or risk being lost from the system. See Managing Green Manures for more information on how green manure termination timings and methods impact nitrogen fertility.

5. Digging Deeper (Additional Resources)

Supporting References:

Baird, J.M., F.L. Walley and S.J. Shirtliffe. 2009. Optimal seeding rate for organic production of field pea in the northern Great Plains. Canadian Journal of Plant Science 89: 455-464.

Baird, J.M., F.L. Walley and S.J. Shirtliffe. 2009. Optimal seeding rate for organic production of lentil in the northern Great Plains. Canadian Journal of Plant Science 89: 1089-1097.

Benaragama, D. and S.J. Shirtliffe. 2013. Integrating Cultural and Mechanical Methods for Additive Weed Control in Organic Systems. Agronomy Journal 105: 1728-1734.

Blackshaw, R.E., L.J. Molnar and J.R. Moyer. 2010. Sweet clover termination effects on weeds, soil water, soil nitrogen, and succeeding wheat yield. Agronomy Journal 102: 634-641.

Blackshaw, R.E., J.R. Moyer, R.C. Doram and A.L. Boswell. 2001. Yellow sweetclover, green manure, and its residues effectively suppress weeds during fallow. Weed Science 49: 406-413.

Cicek, H., J.R. Thiessen Martens, K.C. Bamford and M.H. Entz. 2015. Forage potential of six leguminous green manures and effect of grazing on following grain crops. Renewable Agriculture and Food Systems 30: 503-514.

Clark, A. (Ed). 2012. Managing Cover Crops Profitably – Third Edition. Sustainable Agriculture Research and Education. Available at http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition [Link verified 14 April 2016].

Frick, B. (Ed). 2013. Organic Farming on the Prairies, Second edition. Saskatchewan Organic Directorate.

Frick, B. 1998. Weed Management for Organic Producers: Literature Search. For the Saskatchewan Organic Directorate and the Agri-Food Innovation Fund, Special Crop Spoke Program. Available at http://saskorganics.org/wp-content/uploads/2016/05/Weed_Management_for_Organic_producers.pdf [Link verified 16 June 2016].

Frick, B. and E. Johnson. 2002. Crop husbandry for weed management (9-13). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/extension/ext_weed_crop_husbandry.asp [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Crop rotations for organic systems (9-14). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.organicagcentre.ca/extension/ext_weed_rotations.asp  [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Growing a competitive crop – first step in weed control (9-1). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/extension/ext_weed_competitive.asp [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Livestock and other beneficial organisms for weed management. Research Report 2002, Agri-Food Innovation Fund. Available at http://www.organicagcentre.ca/extension/ext_weed_livestock.asp [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Post-emergence field pea harrowing – rotary or tine? (9-8). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/researchdatabase/res_emergpea_rota.asp [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Soil fertility affects weed and crop competition (9-15). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.organicagcentre.ca/extension/ext_weed_fertility.asp [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Summerfallow as a weed management strategy – pros and cons (9-3). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/extension/ext_weed_summerfallow.asp [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Tillage for weed control – an introduction (9-5). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info /extension/ext_weed_tillage.asp  [Link verified 2 June 2016].

Frick, B. and E. Johnson. 2002. Using allelopathic and cover crops to suppress weeds (9-2). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/extension/ext_weed_allelopathic.asp [Link verified 2 June 2016].

Halde, C. and M.H. Entz. 2014. Flax (Linum usitatissimum L.) production system performance under organic rotational no-till and two organic tilled systems in a cool subhumid continental climate. Soil and Tillage Research 143: 145-154.

Halde, C., R.H. Gulden and M.H. Entz. 2014. Selecting cover crop mulches for organic rotational no-till systems in Manitoba, Canada. Agronomy Journal 106: 1193-1204.

Johnson, E. 2002. Post-emergence harrowing for weed control (9-7). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/ResearchDatabase/res_emerg_weedctrl.asp [Link verified 2 June 2016].

Johnson, E. 2002. Field pea – manage seeding for best pre-emergence weed control (9-17). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/ResearchDatabase/res_fieldpea_preemergweed.asp [Link verified 2 June 2016].

Johnson, E. 2002. Selectivity – and important concept in mechanical weed control (9-18). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/ResearchDatabase/res_select_weedctrl.asp [Link verified 2 June 2016].

Johnson, E. 2002. Organic lentil production – optimum seeding rate, row spacing (9-20). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/ResearchDatabase/res_lentil_seedspace.asp [Link verified 2 June 2016].

Johnson, E. 2002. Clipping weeds above a crop canopy (9-21). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/ResearchDatabase/res_clipweed_canopy.asp [Link verified 2 June 2016].

Johnson, E. 2002. Weed control – false seed bed technique (9-23). Research Report 2002, Agri-Food Innovation Fund. Available at http://www.oacc.info/ResearchDatabase/res_weedctrl_falseseedbed.asp [Link verified 2 June 2016].

Lawley, Y.E. and S.J. Shirtliffe. 2004. Optimum seeding rates for three annual green manure crops: the effect of weed competition. Proceedings of Soils and Crops Conference, 2004.

Mohler, C.L. and M. Liebman. 1987. Weed productivity and composition in sole crops and intercrops of barley and field pea. Journal of Applied Ecology 24: 685-699.

Moyer, J.R., R.E. Blackshaw and H.C. Huang. 2007. Effect of sweetclover cultivars and management practices on following weed infestations and wheat yield. Canadian Journal of Plant Science 87: 973-983.

Schonbeck, M. 2015 How Cover Crops Suppress Weeds. eOrganic. Available at http://articles.extension.org/pages/18524/how-cover-crops-suppress-weeds [Link verified 3 June 2016].

Schonbeck, M. 2011. Plant and Manage Cover Crops for Maximum Weed Suppression. eOrganic. Available at http://articles.extension.org/pages/18525/plant-and-manage-cover-crops-for-maximum-weed-suppression [Link verified 3 June 2016].

Shirtliffe, S.J. and J.D. Knight. 2006. Saskatchewan Organic On-Farm Research: Part II: Soil Fertility and Weed Management. Available at http://organic.usask.ca/Soil%20Fertility%20and%20weed%20management.pdf [Link verified 3 June 2016].

Thiessen Martens, J., B. Irvine, M. Entz and D. Derksen. Date Unknown. Weed Management Options Which Reduce Pesticide Use. Available at http://www.umanitoba.ca/outreach/naturalagriculture/weed/files/home/home_e.htm [Link verified 17 June 2016].

Vaisman, I., M.H. Entz, K.C. Bamford and I. Cushon. 2014. Green manure species respond differently to blade rolling. Canadian Journal of Plant Science 94: 1507-1511.

Vaisman, I., M.H. Entz, D.N. Flaten and R.H. Gulden. 2011. Blade roller-green manure interactions on nitrogen dynamics, weeds and organic wheat. Agronomy Journal 103: 879-889.

White, R.H., A.D. Worsham and U. Blum. 1989. Allelopathic Potential of Legume Debris and Aqueous Extracts. Weed Science 37: 674-679.

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The Green Manure Toolkit was Developed by the Organic Agriculture Centre of Canada.