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Weed Management.

Weed management tools

Herbicides are one of the tools available to control weeds in orchards. Mulching, mowing and cultivation are also effective in managing weeds, especially in the year of planting. Each strategy has certain strengths and weaknesses as indicated in Table 4-14.

Table 4-14. Advantages and disadvantages of weed management tools

Cultivation/hoeing - effective, especially on small weeds - non-selective - controls all emerged growth - equipment readily available - may damage soil structure - may spread perennial weeds - may damage trees/roots - provides only short-term control
Mulching - effective if properly managed - non-selective - suppresses all emerging weeds - holds soil moisture as well - provides long-term control - availability of mulch - cost of mulch/application - attractive to rodents - may affect tree nutrition - must be free of weed seeds
Mowing - rescue treatment - quick suppression - equipment available - reduce seed spread - weeds may still compete - quick regrowth - several mowings required - may damage young trees
Herbicides - effective - easy to apply - can be selective - timely - require 2% soil organic matter - directed spray equipment - effects on pest complex - cost varies

Weed management strategies may combine several of these tools and it is important to be aware of the pros and cons of each. Mulching, cultivation and/or mowing may affect the performance of herbicides applied under trees, as well as using irrigation (Figure 4-183).


Figure 4-183. Weed "escapes" can occur around trickle irrigation emitters due to leaching of herbicides. Use spot treatments if escapes occur during the critical weed-free periods.

How herbicides work

Herbicides registered for orchards kill weeds in several different ways, but there are three broad categories of control methods.

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  • Burndown (contact) - herbicide is applied to existing vegetation and kills the top growth. Herbicides are directed under the trees to create a weed-free strip and/or applied to the row alleyways. Some burndown herbicides are systemic - they are absorbed into the plant and move to the growing point and/or the roots, and give longer-term control for perennial weeds. Some burndown herbicides are not systemic and target only above-ground green tissue. These contact herbicides control top growth, but do not give long-term control of perennials.

  • Soil residual (pre-emergent) - herbicide is applied to the soil surface before weed seedlings germinate. The chemical remains in the soil for several weeks to months, killing germinating seedlings. Generally, a rainfall of at least 12 mm is required to activate the herbicide, resulting in a longer period of control.

  • Post-emergent (selective) - these herbicides kill only certain weeds out of existing vegetation e.g. graminicides kill grasses only, or auxinic (hormone) herbicides kill only broadleaves. These are usually targeted for weed escapes or for problem weeds like thistles or annual grasses.

  • There are advantages and disadvantages to using these types of herbicides as indicated in Table 4-15. Advantage and disadvantages of herbicide control categories.

    Table 4-15. Advantages and disadvantages of herbicide control categories

    Category Advantages Disdvantages
    Burndown - controls all emerged weeds - inexpensive - widely available - can target critical weed-free period - may damage trees if absorbed (systemic) - early weeds compete with trees - perennial weeds are not killed - new seedlings germinate after application
    Soil residual - reliable control - effective for longer periods - can target critical weed-free period - cost effective - broad spectrum - longer window to apply - may cause tree injury - not safe on low organic matter soil - may have tree-age restriction - may leave residues after orchard removal - may require incorporation - applied before weed problems are known - may affect soil biology
    Selective - can target critical weed-free period - targets specific weeds - minimizes herbicide use - generally safe for trees - does not control a broad spectrum of weeds - often an extra application - additional expense - timing is critical

    Herbicides are grouped by modes of action by the Weed Science Society of America (WSSA), and are listed in the

    OMAFRA Publication 75, Guide to Weed Control.

    For each of the herbicide categories listed, there may be herbicides from several mode of action groups. For example, glyphosate (e.g. Roundup, Touchdown, Vantage) and paraquat (Gramoxone) are common herbicides used for burndown, but glyphosate is a Group 9 herbicide and paraquat is a Group 22 herbicide.

    Be sure to know both the control category and herbicide group for herbicides used in the orchard. For example, a weed management strategy may require a burndown herbicide in the spring may use a Group 22 herbicide because a Group 9 herbicide was used the previous fall. Rotating herbicides with different modes of action is an important strategy to manage weed resistance.

    Many herbicides only control grasses or broadleaf weeds. For broad-spectrum control of weeds usually growing in orchards, apply a tank mix of two herbicides. Choose products with strengths on different weeds, e.g. Dual II Magnum (controls grasses well) is often tank mixed with Princep (controls broadleaf weeds well). If weed growth is present, include a burndown herbicide in the tank mix.

    Selective herbicides are useful for treatments on weed escapes, for specific problem weeds like grasses or vetch, or for reducing pressure from broadleaf weeds with fall treatments of 2,4-D. Waiting until weeds appear before using herbicides is a preferred approach in a truly integrated weed management, as long as practical considerations such as timeliness of application, resistance management and critical weed-free period have been considered.

    Issues with using herbicides

    Managing weed resistance

    There is concern that continuous use of the same herbicide selects for resistant weeds, and herbicide resistance has been identified in a growing number of weeds and locations around the world. In Ontario orchards, resistance to paraquat has been identified where repeated applications each year over several years was practiced.

    In orchard practice, it is common to use herbicides with different modes of action, often during the same growing season. For example, triazine herbicides (eg. Princep/simazine) have been widely used in orchards, creating concern of selecting for triazine-resistant pigweed (Figure 4-184) and lamb's-quarters. Corn growers had previously selected triazine-resistant weeds through continuous use of triazines alone. However, when orchard growers include other herbicide modes of action, e.g., Roundup, Ignite or Gramoxone, any resistant weeds are killed by these treatments. The escaped weeds are usually species that simazine does not control well e.g. pigweed and crabgrass. Only tests confirm if weeds are truly triazine-resistant. With more herbicides available to apple growers, choose different herbicide families and modes of action. Rotate herbicides within each year, and between years, in each orchard block.


    Figure 4-184. In many orchards, pigweed is a challenging weed, which often escapes after simazine is applied

    Contamination of surface and ground water

    Many herbicides are water soluble, and may leach through the soil profile, eventually reaching the ground water. Heavy rainfall or irrigation immediately after application contributes to this problem. Herbicide spills near wells are another potential source of ground water contamination. Avoid these situations, especially on low organic matter soils.

    Soil residual herbicides such as simazine bind tightly to soil particles and may be washed into surface water when soil erosion occurs. Practice erosion control if the orchard site is at risk of wind or water erosion.

    Remember herbicides lost through leaching or erosion are not available to control weeds in your orchard.

    Managing spray drift

    As spray applicators, growers are required by the Pesticides Act to ensure no pesticide lands off the intended target. There are two types of spray drift to avoid:

    • physical drift - when tiny droplets (less than 100 microns) are moved by wind and air currents off-target

    • vapour drift - when volatile products (e.g. 2.4-D) become a vapour and move off-target

    The Ontario Grower Pesticide Safety Course suggests 10 ways to reduce spray drift:

    1. use anti-drift technology - nozzle hoods, boom hoods, perforated screen, air-assist/curtain and shields (Figure 4-185

    2. use alternative application technology, e.g. wick wiper (Figure 4-186)

    3. where possible, don't spray - use cultivation, flamers, mulch biological control or mowers (Figure 4-187)

    4. read the label and follow directions for stage of growth, weather and other precautions

    5. watch the weather - and avoid spraying in winds over 10 km/h, temperatures above 25°C and low RH (<75%)

    6. use buffer strips, check label for required distance - if not listed, stay back at least one spray boom width

    7. use high water rates and larger nozzles - results in larger droplets and better spray coverage

    8. use adjuvants with caution - check label, may distort spray pattern

    9. lower the boom, may need wider angle nozzles

    10. choose the right nozzles - drift reducing styles are now available


    Figure 4-185. Mount shields to protect nozzles from wind and prevent drift - an added benefit is protecting low branches from herbicide


    Figure 4-186. Mount wick wipers to swipe weeds growing under trees without tree contact or herbicide drift


    Figure 4-187. Use mowers with wing arms to cut weeds around tree trunks - a sensor kicks out the mower arm around the tree to prevent mechanical injury

    Effect of herbicides on non-target pests

    Herbicides can affect organisms in orchards, not just weeds:

    • some herbicides are toxic to soil fauna like earthworms (e.g. linuron (Lorox)

    • some insect pests thrive on weeds, control these weeds to reduce fruit damage, such as:

      • tarnished plant bugs thrive on creeping Charlie and chickweed - control these weeds to reduce injury

      • twospotted spider mites overwinter on the ground cover and build up populations on these weeds in the spring - remove host weeds early in the season to reduce problems

      • caution - where tarnished plant bug or twospotted spider mites are established on the orchard ground cover, avoid killing the weeds with herbicide and forcing pests into trees
    • controlling dandelions in orchard sod (e.g. with 2,4-D) may affect bee activity during pollination time - honeybees appear to prefer apple blossoms to dandelions but bees may use dandelions to maintain hive strength in spring if flying conditions are poor during apple bloom

    Crop safety

    Herbicides can cause injury to apple trees if used improperly. Read the label to determine restrictions on soil type or age of tree.

    • Soil organic matter - generally soils with at least 2% organic matter (OM) are required to use soil residual herbicides. For low organic matter soils (OM <2%), choose Devrinol because its crop safety is not dependent on OM. Mulch is also a good choice for low OM soils. A soil OM of 3% is recommended to avoid crop injury when using Sinbar, because it tends to be less bound to soil particles.

    • Age of trees - some herbicides are only registered for new trees. For Treflan, this restriction is because the product must be incorporated. For other products, there has not been research to prove there is no residue in the fruit. Some herbicides are only used on older trees. For example, linuron is restricted to some tree species more than 10 years old because the original research was only done on trees 10 years old. In some cases, there is a need to wait until the trees have a larger root system or thicker bark to avoid tree damage.

    • Age of trees - some herbicides are only registered for new trees. For Treflan, this restriction is because the product must be incorporated. For other products, there has not been research to prove there is no residue in the fruit. Some herbicides are only used on older trees. For example, linuron is restricted to some tree species more than 10 years old because the original research was only done on trees 10 years old. In some cases, there is a need to wait until the trees have a larger root system or thicker bark to avoid tree damage.

      For orchards scheduled for removal in the next three years, avoid soil-applied herbicides with a long carryover period. Do not use simazine (Princep, Simadex), Devrinol and Sinbar for several years before orchards are removed.

      Soil incorporation requirement

      Devrinol and Treflan require incorporation into the soil to prevent sunlight from breaking them down. For Treflan, cultivation with tillage equipment is required, restricting its use to newly planted trees. For Devrinol, incorporation by rainfall or irrigation must take place within two days in the summer or seven days in the late fall.

      Strategies for integrated weed management (IWM)

      Integrated weed management, like integrated pest management, is a multidisciplinary approach to controlling weeds in the orchard using chemical, biological and cultural techniques. The following strategies (and combinations) are used successfully in Ontario apple orchards before planting, in the planting year and in established orchards.

      Before planting the orchard

      Managing weeds in orchards prior to tree planting reduces weed problems later in the orchard life. Using the pre-plant year to build organic matter with a green manure crop (Figure 4-188) also gives several opportunities to reduce weed pressure.


      Figure 4-188. A green manure crop, like this Sudan-sorghum cross, can be incorporated in the fall prior to planting to suppress weeds and improve soil tilth

      Begin with a general burndown treatment (e.g. glyphosate) after weed growth has begun. Plant the green manure crop, choosing one that grows thickly and suppresses annual weeds. If a grassy cover like cereal or Sudan-sorghum is used, use a selective herbicide like 2,4-D to control broadleaf perennial weeds like thistles. As well, use wiper applications with Roundup targeted when the weed is most susceptible on problem weed patches - Canada thistle at early flower bud, milkweed at flower bud, bindweed at full flower, quack grass actively growing with at least three to four new leaves on each shoot. Where perennial weeds are present, choose the higher rate of glyphosate as listed on the label.

      Establish sod ground cover the year before the orchard is planted. Early fall is the preferred time to plant the grass, and once established, the future tree row can be killed with a burndown herbicide (Figure 4-189).


      Figure 4-189. Orchard sod can be established in the year prior to tree planting - sod strips, where trees are to be planted, are killed out with a burndown herbicide

      First year (new plantings)

      After planting the orchard in the spring, choose one of the residual herbicides registered for new plantings (Figure 4-190). Extend the weed-free band beyond the drip line of the tree, and maintain it weed-free during the critical period. Most soil-applied residual herbicides give 8-12 weeks of weed control.


      Figure 4-190. Properly maintained herbicide strips, just wider than the tree's drip line, eliminate weed competition

      Install plastic mulch (Figure 4-191) and plant trees directly through holes punched in the plastic. Black mulch is recommended to keep weeds from growing under the plastic, and soil moisture is conserved with the mulch. The life of the mulch varies depending on the material, e.g. thin plastics last one year and landscape fabrics may last up to five years.


      Figure 4-191. Installing black plastic mulch at planting will control weeds, conserve moisture and warm the soil

      There is little shading in young orchards and weed escapes are likely to begin in early summer. Use either a selective herbicide (e.g. Venture or Poast) for grass, or cultivate or hoe to keep tree rows clean until late July. Some small weeds growing under the trees by late summer slow tree growth and help trees harden off for winter. Tillage tools like the Weed Badger (rotational finger weeders) are also effective if done every three to four weeks. Many implements have a sensor that kicks equipment out around the trees to avoid damage. Avoid late tillage in the fall to prevent late tree growth and winter injury.

      Established orchards

      Early season

      Some weeds, like chickweed, usually begin growth early in the season before bud break. As an orchard matures, patches of perennial weeds begin to establish and are usually growing by bud break. Use one of these strategies for weed control.

      • Wait for weeds to emerge and use a burndown treatment, followed by mulching and/or a residual herbicide.

      • Apply mulch and/or one of the residual herbicides before weed growth begins.Control weed escapes that begin before the critical period of fruit bud initiation occurs (generally in early June when terminal buds set). Herbicide choice depends on weeds are present and how early in the growing season they appear.

      Either strategy is acceptable if done in a timely manner. Remember the goal is to prevent weed competition from bud break through terminal bud set and fruit bud initiation.


      Some growers use organic mulch under the trees to suppress weeds and conserve moisture. Avoid weed seeds in the mulch, and monitor nutrient levels in the trees as the mulch breaks down. Figure 4-192 shows some mulching effect from clippings mown from the sod row middles.


      Figure 4-192. Blowing grass clippings under trees with a modified mower creates an efficient and cheap mulch

      There is some concern with using translocated herbicides like glyphosate and amitrole after June, because trees are moving stored reserves to their roots and may move the herbicide there too. Use these herbicides carefully at this time with directional shielded sprays or wick wipers (Figure 4-193). This timing is useful for controlling perennial weeds like thistles or bindweed that are most susceptible at early bloom to systemic herbicides.


      Figure 4-193. Translocated systemic herbicides can be safely applied near trees using a wick wiper - either the hockey stick model (left) or the rope wick (right) have been effective, be sure to time it for the sensitive stage of the weed


      Fall applications of 2,4-D or glyphosate directed away from the trees are useful for controlling perennial weeds. Wait until useful fruit is removed from the orchard floor before applying. Destroy flowers of weed escapes before weed seeds are allowed to spread. Mowers with swing arms that can reach under the trees are effective, especially if weeds are small and lush. Plan a weed management strategy in advance. Focus on controlling weeds during the critical weed-free period, and integrating all the tools of weed management to maximize yields in apple trees. Use information from weed scouting to adjust the strategy as required. Be sure to make notes of results and adjust the strategy for the following season as needed.

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