Controlling weeds in your agroforestry planting

Agroforestry plantings require maintenance for survival and optimum growth. One important maintenance practice is controlling competing vegetation around the woody plants. Any plant that competes with the woody plant for moisture or nutrients is undesirable and considered a weed. This includes grasses, forbs, crops, or noxious weeds.

A two panel illustration showing stunted tree growth where there is no weed control, and tall healthy young trees where there is good weed control.
No weed control and Good weed control

Weeds should be controlled throughout the life of the agroforestry planting. This may require mechanical or chemical weed management until trees or shrubs are large enough to shade out weeds. Controlling weed competition is especially critical during the first three years after planting. Competing vegetation reduces water and nutrient availability, space and light thereby limiting plant growth and delaying the function of the planting.

Site preparation

Two rows of young trees where grass is trimmed and there are no tall weeds.
Controlling weeds in your agroforestry planting is critical for tree survival and establishment

Site preparation is essential for successful agroforestry planting establishment. Perennial weeds and grasses are much more difficult to control after the trees and shrubs have been planted. Focusing on removing perennial weeds before planting allows for the use of a wider range of tillage and herbicide control options.

Stake the rows and begin preparing the site one year before planting. Remove all grasses, forages and weeds with herbicides and/or mechanical cultivation.

Apply glyphosate at the planting site one or two weeks prior to tillage to kill the weed root system. This will enhance sod breakdown and make cultivation easier. Glyphosate is more effective if applied in the fall when the herbicide is more readily translocated to the weed roots. Spring site preparation may require higher glyphosate rates for effective perennial weed control.

If planning on installing plastic mulch, prepare the planting strip to a width of 2.5 metres (m); otherwise the strip only needs to be 1.2 m wide. Each strip should be cultivated and disced to a depth of 15 to 20 centimetres (cm) in the year prior to planting. When preparing the planting site, it is recommended to minimize crop residue present on the soil surface. The site should be free of sod clumps, and all weeds and large stones need to be removed. Following these steps will make tree planting and mulching much easier.

Non chemical weed control methods

After planting, weeds within the tree row can be controlled using mulches, specially designed mowers, tillers, or by hand hoeing.

Mechanical weed control

Cultivation is most effective just after a flush of weed growth when the weed seedlings are still small, less than 5.0 to 7.5 cm tall. Operate cultivation equipment slowly, especially in proximity to trees and shrubs during the establishment period.

Excessive tractor and equipment speed causes seedling disturbance and damage.

Controlling weeds within the tree row

A farmer uses a hoe to control weeds between tree plantings.
Hand hoeing for weed control

Weed control within the tree row is more difficult than between rows and may require specialized cultivation equipment, or the use of mulch, herbicide treatment, or a combination of treatments for optimum results.

Common within row tillage equipment includes hand hoes, shovels, line-trimmers, and specialized rotary tillers (such as the Weed Badger®).

Controlling weeds between the tree rows

Weed control between the rows is as important as within the row. Weeds growing between rows can have a significant negative effect on tree growth. One solution is to plant a noncompetitive turf grass, such as sheep fescue, between the rows. Mowing twice a year will help maintain the site.

Rows of tree seedlings covered in mulch.
Cultivation between rows of trees planted under plastic mulch

If grass is not planted, regular cultivation to kill weeds between the rows is another option. Tillage in the tree row should be shallow - no deeper than 5 cm - to avoid injuring tree roots.

Between row tillage equipment includes disc harrows, cultivators, and roto-tillers. When using tillage between tree rows with plastic mulch, remain at least 15 cm from the edge to avoid catching and tearing up the buried plastic.

Plastic mulch

Plastic mulch is a very effective method to control weeds within the tree row. Plastic mulch is non-perforated, contains a UV inhibitor and is applied with a specialized mulch applicator. Properly installed, weeds will be controlled for many years within the tree row.

Mulch is a very effective alternative to herbicides and mechanical weed control. It is a one time expense that offers significant returns over the long run.

A healthy tree seedling surrounded by plastic mulch.
Green ash planted under plastic mulch

The plastic-covered area within the tree row allows trees to maximize their growth potential and get a head start due to elimination of weed competition. In addition, soil moisture loss due to surface evaporation is greatly reduced within the tree row. Under the plastic mulch, the soil warms sooner and stays warm longer during the growing season promoting and stimulating tree growth and root development.

Trees under plastic mulch can experience a 25 per cent increase in growth over trees planted without mulch.

Normal rainfall will provide enough moisture for growing trees through lateral flow of water migrating from soil adjacent to the plastic-covered soil. All tree species may require supplemental watering during extended dry periods regardless of whether mulch has been used or not.

Although the plastic is quite tough, it should be inspected periodically throughout the growing season for holes and to ensure the edges are secured. Large rips or holes can be repaired by first using sod staples to close the gap, second, stabilizing with rocks, and third, covering with soil. Ensure the holes around the base of the trees are large enough to accommodate the growing trees as plastic rubbing trees can damage the bark. To prevent damage from livestock, fence off the agroforestry planting area.

Spraying herbicides or mowing is recommended for controlling weeds adjacent to the edge of the mulch. Care should be taken to prevent herbicide drift when spraying weeds near the trees. Stay at least 15 cm from the edge when using tillage equipment alongside the plastic mulch, since the buried edges may get caught and ripped.

Organic mulch

A farmer’s hands pulling straw mulch around a seedling.
Pulling straw mulch away from the tree seedling base

Organic mulches consist of loose materials such as sawdust, slough hay or wood chips. These mulches control weeds and conserve soil moisture. They are typically more difficult to apply than plastic mulch, can be difficult to obtain in large quantities and application is often laborious, although no specialized equipment is required.

A 10 cm layer of organic mulch is needed to provide adequate weed control. In most cases, soil temperature is cooler under organic mulches. On nutrient poor soils, decomposing organic mulches may cause nitrogen deficiency which will be evident from pale green leaves and slower than expected growth. If this occurs, apply nitrogen fertilizer in the spring or early summer.

Sawdust is a by-product of the forestry industry. Sawdust forms a dense mat on the soil's surface that prevents weeds from germinating and growing by blocking sunlight. They are applied over seedlings by hand or mechanically with a modified manure spreader or bunk feeder. Do not apply more than 10 cm of sawdust as this will restrict soil aeration. After a period of time, the sawdust will consolidate and a crust will form on the surface.

A farmer rakes sawdust from a small tractor over a row of new plantings.
Applying sawdust over a newly planted tree row

Wood chips are usually waste products from the forestry industry. Chips may also be available from landscapers or tree care companies from chipped tree trimmings. The chips are normally applied by hand. To reduce rodent damage, keep the chips approximately 10 cm from the base of the tree.

Do not use a depth greater than 10 cm as this will reduce root development and the amount of air available to the soil. Occasionally, wood chips will need to be replenished. Composted wood chips make good mulch, especially when it contains a mixture of leaves, bark and wood.

Fresh wood chips may be used around established trees and shrubs. But around young plants, avoid using uncomposted wood chips that have been piled without exposure to oxygen, as anaerobic conditions may develop leading to the production of toxic alcohols and organic acids.

Chemical weed control methods

Safety precautions should be taken to protect you, others and the environment from careless or improper use of herbicides. Always use protective equipment and consult the product label for instructions on proper use, storage, handling and disposal of herbicides. Avoid spray drift to non-target plants.

Pre-planting

For pre-plant pre-emergence treatment, two options exist: the application of trifluralin or a mixture of trifluralin plus metribuzin. Both treatments require tillage for incorporation and can be applied in the fall or in the spring, prior to planting. The treatment chosen is dependent on the tree species being planted.

Trifluralin

A tractor pulling an herbicide sprayer over a field.
Herbicide application for pre-planting weed control

Trifluralin inhibits cell division and cell formation and is absorbed by the emerging shoot as it passes through the treated soil. In grasses, it is the coleoptile and coleoptilar node that are the primary absorption sites; in broadleaf plants, it is the emerging stem segment between the seed and the soil surface that is affected.

Trifluralin's main effect is on root development.

For effective weed control, trifluralin must be incorporated into the upper layer of soil to prevent volatilization and photodegradation. It is therefore difficult to use trifluralin around existing trees; consequently this herbicide is recommended for weed control as part of site preparation prior to planting.

Within twenty-four hours of application, trifluralin must be incorporated to a depth of 8.0 cm. This is best accomplished by using a disc type implement or field cultivator with mounted harrows (not a chisel plow or deep tillage cultivator). A rotovator also works well.

Trifluralin controls some broadleaf species like chickweed, cow cockle, goosefoot, knotweed, lamb's quarters, pigweed species, purslane, Russian thistle, and wild buckwheat. It also controls some grasses such as, barnyard grass, bromegrass, foxtail species, Persian darnel, and wild oats.

Tree species for which trifluralin is registered include caragana, green ash and Scots pine. Do not plant unrooted willow or poplar cuttings into treated soil- they will not root. Tank mixing with metribuzin results in more registered species and additional weeds controlled.

Metribuzin

A tractor tilling a field.
Incorporation of pre-emergent herbicide

Metribuzin is readily absorbed by plant roots and moves upward into the shoots and leaves. In susceptible plants, photosynthesis is inhibited and cells become leaky which then dry up and disintegrate.

Susceptible weed seedlings emerge through the treated soil, but become chlorotic before turning brown and drying up. This process usually takes about two to five days. This usually occurs before the weeds are large enough to be noticeable.

The weed species controlled by metribuzin include barnyard grass, green foxtail, lamb's quarters, Persian darnel, prostrate pigweed, purslane, redroot pigweed, Russian thistle, shepherd's purse, stinkweed, wild buckwheat, wild mustard, wild oats, and witch grass.

The herbicide is registered for use prior to planting caragana, choke cherry, green ash, lilac, rooted poplar and sea buckthorn.

It is recommended that metribuzin be tank mixed with trifluralin and applied prior to planting. This tank mix should be incorporated into the soil to an 8.0 cm depth within twenty-four hours of application.

For incorporation, a disc type implement or field cultivator with mounted harrows should be used (not a chisel plow or deep tillage cultivator). If available, a rotovator also works well.

After planting / Established agroforestry plantings

Clopyralid

Clopyralid is a selective herbicide used for broadleaf weeds, especially clover and thistle. Application of clopyralid has varying control dependent on the application rate. Control ranges from top growth of thistle to season long control with suppression into the following year.

Apply after all thistles have emerged and when the majority are in the rosette to pre-bud stage.

Dichlobenil

A tractor pulling an herbicide applicator.
Granular herbicide applicator

Following soil application, susceptible weed seeds do not emerge as dichlobenil inhibits germination and cell division. It is readily absorbed by roots and leaves. Since this product is a granular herbicide that is soil applied, foliar uptake is not a factor in the tolerance of tree and shrub species when properly applied.

Dichlobenil is a volatile herbicide, which can be lost in the form of vapours following application to warm, moist soil. Therefore, it is recommended that it be applied to established agroforestry plantings in late fall; if application is delayed until spring, it should be incorporated with shallow tillage.

If dichlobenil volatizes following spring application, trees and shrubs may be significantly damaged if the vapours are absorbed by the opening buds or new leaves.

Several weed species are controlled by dichlobenil, including annual bluegrass, artemisia, bindweed, blue aster, Canada thistle, chickweed, common groundsel, crabgrass, dandelion, evening primrose, field horsetail, foxtail species, goosefoot species, Kochia, lamb's quarters, mustard species, pigweed species, plantain, prostrate knotweed, purslane, quack grass, ragweed, sheep sorrel, shepherd's purse, smartweed, sow-thistle, spurge, vetch, wild barley, wild buckwheat, wild carrot, witch grass, yellow rocket, and yellow wood-sorrel.

Dichlobenil is registered for use with the following tree species: caragana, crabapple, green ash, lilac, maple, poplar, rose, and willow. Application is not recommended until the trees are at least one year old.

Apply to established agroforestry plantings in early spring or late fall prior to annual weed emergence, or after cultivation has removed existing weeds.

Linuron

Linuron interferes with the photosynthetic process in susceptible plants, resulting in their death. The best times for treatment are late fall or early spring when the trees are dormant, few weeds present and those weed species that are present are susceptible to linuron.

Used as a pre-emergent, linuron must be incorporated into the germination zone of the soil with rainfall or sprinkler irrigation within 10 days of application. Without this moisture for incorporation, linuron effectiveness is reduced.

Post-emergence application must be conducted when the weeds are very small. If application takes place during the growing season, spray must be directed away to avoid injury to the trees. Do not disturb the soil after linuron application.

Linuron can be applied in the fall of the same year the trees are planted. Annual application in either spring or late fall will reduce the need for other weed control measures during the growing season.

The weed species controlled include, common chickweed, common groundsel, goosefoot species, green smartweed, lady's thumb, lamb's-quarters, mustard species, pigweed species, prostrate knotweed, purslane, ragweed, shepherd's purse, stinkweed, and wild buckwheat.

Linuron is registered to be used with the following: caragana, choke cherry, green ash, maple, pine, poplar, spruce, and willow.

Simazine

Simazine is absorbed through plant roots and translocated to the shoots where it interferes with photosynthesis, resulting in eventual death. Very limited foliar absorption occurs following post-emergence application.

Applied as a pre-emergent, simazine works similar to linuron. However, it is more persistent, providing a longer period of control. Apply to established plantings in the fall or early spring, to a weed free soil surface with limited trash cover. Weeds present at the time of application will not be controlled.

The weed species controlled include barnyard grass, chickweed, common groundsel, green foxtail, lady's thumb, lamb's-quarters, mustard species, nightshade species, pigweed species, purslane, ragweed, shepherd's purse, smartweed, speedwell species, wild buckwheat, wild oats, and witch grass.

Simazine can be used for weed control with the following tree species: caragana, dogwood, green ash, maple, rose, and spruce. Application is not recommended until the trees have been established for at least one year.

After emergence, grass weeds only

Fluazifop-p-butyl

Fluazifop-p-butyl is a selective post-emergence herbicide used for control of most annual and perennial weeds. It is absorbed primarily by leaves, and then is translocated to roots and rhizomes.

Apply when grasses are actively growing, and annual grasses are in the two to five leaf stage and quack grass is in the three to five leaf stage. Till pre-application to break up rhizomes and to improve control of quack grass.

Do not cultivate for five days after applying fluazifop-p-butyl. Expect lower control when weeds are stressed due to drought, excess moisture and humidity, low temperature and/or very low relative humidity. Regrowth by tillering may occur if application is made under the above conditions. Since there is no residual activity, a new flush of weeds may emerge after the first flush has been controlled.

This herbicide is registered for weed control in tree and shrub species that may be used for agroforestry plantings. Fluazifop-p-butly can be applied as an over-the-top spray to caragana, lilac, maple, pine, poplar, red elder, rose, snowberry, spruce, and willow.

If applying to poplar, only one application per year is recommended. Apply as a directed spray around dogwood to avoid spray contact with the foliage.

Sethoxydim

Sethoxydim is a contact and systemic herbicide. It is rapidly absorbed by plant leaves. Once absorbed, it stops weed growth by blocking the production of new membranes. Leaves become chlorotic and then dry up. This process takes one to three weeks.

Most effective weed control is achieved when grasses are actively growing. Weeds stressed by drought, flooding, hot or prolonged cool temperatures and poor fertility are more difficult to control. Apply it once per growing season by ground as a directed spray.

Grassy weeds controlled by sethoxydim include barnyard grass foxtail barley, green foxtail, Persian darnel, quack grass, volunteer cereals (barley, oats, wheat), wild oats, and witch grass.

Sethoxydim is registered for use with the following tree species: buffaloberry, caragana, green ash, lilac, maple, pine, sea buckthorn, and spruce.

After weed emergence, directed spray only

Amitrole

A small, specialized herbicide spryer on a lawn.
Specialized spray equipment provides safer, cost effective and accurate spraying

Amitrole is a non-selective systemic herbicide that inhibits chlorophyll production. It is applied as a directed spray to weed foliage. Prevent spray drift from contacting trunks or foliage of the trees and shrubs. Amitrole has residual activity for two to four weeks in moist, warm soil.

Apply to actively growing plants. Good coverage is essential. If weeds are mature, it is advisable to cut them and then spray the regrowth. Do not disturb treated plants for at least two weeks after application. For control of quack grass and Canada thistle, apply in spring or late fall to actively growing plants 15-20 cm tall; then plough or disc ten to fourteen days following application.

Amitrole can be tank mixed with glyphosate for improved control of some weeds, including dandelion.

Glyphosate

Glyphosate's ability to move throughout the plant, following absorption, enables it to control perennial weeds as well as annuals. This characteristic makes it useful for site preparation as well as maintenance. It can be used to clean up perennial weeds such as Canada thistle and quack grass.

When spraying around trees, glyphosate application should be restricted to a directed or shielded spray as contact with foliage or green, thin immature bark will result in significant tree damage or death of deciduous trees.

The only glyphosate formulations that should be used contain isopropyl amine salt, also known as glyphosate IPA salt.

Conifers, such as spruce and pine, are an exception to this - they are sensitive to glyphosate only during the period of active growth (April to August). Colorado spruce can be sprayed after July 15, while white spruce and Scots pine should not be sprayed before September 1.

Glyphosate is recommended for use around spruce trees, but there are certain products, such as the newer formulations, that can cause severe damage.

Paraquat

Paraquat is a contact non-selective herbicide that is rapidly absorbed. Under normal conditions, it remains in the leaf tissue of treated plants. It is non-selective, and will damage or kill plants if improper application occurs. Upon absorption by the plant leaves, a chain reaction begins where cell membranes are destroyed, resulting in wilting, browning and death of the leaves and other green tissue.

For best weed control, paraquat should be applied when weeds are less than 5.0 cm in height or diameter. Large, well established annual weeds may generate new top growth.

With established perennial weeds, paraquat must be applied repeatedly to provide control, eventually depleting stored energy in the weed's root system, resulting in death. Spray should cover weed foliage thoroughly. Repeat application is required to control regrowth. It may be tank mixed with linuron and simazine products for residual control.

When applying paraquat along agroforestry plantings, spray must not come in contact with tree foliage, green buds or green immature bark. This can be achieved by a directed spray and/or the some form of shielded spray equipment. Do not use on small conifers unless they are protected from the spray.

Conifer needles sprayed accidentally with paraquat will be lost permanently.

Herbicide application equipment

A small tractor pulls a U-boom sprayer over young trees.
U-boom sprayer

Field sprayers can be adapted to apply herbicides in shelterbelts; however, a special spray boom or a spray gun will be needed. Special spray booms are not commercially available but can be fabricated in your own shop or by a commercial fabricator.

Both sides of a newly planted tree row can be sprayed simultaneously by straddling the row with this boom. For trees over 120 cm tall, herbicide can be applied along one side with only one nozzle in use. A spray gun, attached to a field sprayer, can be used to apply directed sprays in agroforestry plantings. This equipment is better suited to foliar than soil applied herbicides because it is very difficult to accurately control the herbicide application rate.

It is particularly useful for spraying spotty infestations of weeds. Backpack sprayers are practical for use in small plantings and for spot treatment of weed infestations. Careful application is required to avoid spray drift. Granular herbicides are more suitable than liquid sprays when debris or dead vegetation is present. Granules can fall through to the soil surface without being intercepted by dead vegetation.

Hand-operated applicators are available for spreading granular herbicides. Tractor mounted spreaders are also available, but they are usually not practical for use in the typical agroforestry planting. Uniform application and correct calibration are important considerations when using granular formulations.

Herbicide product information

All label instructions must be followed carefully. Consult the herbicide label for detailed information on the weeds controlled and personal safety when handling herbicides. Contact your local herbicide retailer for further product information availability, and current prices.

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