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Weed Control

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Orchard drive rows (alleyways) between tree rows are usually covered with grass, or some other cover crop that is easily mowed. Ground cover is important in tree fruit orchards to inhibit soil loss and erosion, provide traction for tractors and other equipment, and to help with water infiltration and drainage in the soil. Weeds, on the other hand, are defined as “plants out of place.” Weed seeds blow in on the wind, come through irrigation water, and are carried by birds and other animals. Perennial weed species regrow each year from below-ground parts. Tree fruit growers need to continually manage weeds in orchards, especially in the “weed-strip,” the three to eight feet wide band under the trees, in the tree row. Because the weed-strip is difficult to mow (e.g., low-hanging branches, tree trunks, trellis posts and irrigation systems), other weed management methods are generally used in that region.

There are many benefits to controlling weeds in orchards year-round:

  1. Weeds compete directly with fruit trees for needed water and nutrients during the growing season. This is particularly so for young trees. Keeping the weed-strip mostly clear of weeds can save the grower between 50,000 and 100,000 gallons of water per acre per year.
  2. Weeds can interfere with irrigation of the trees by blocking the sprinkler pattern, causing uneven or inefficient irrigation, or by plugging sprinklers. Micro-sprinklers are even more susceptible than other styles of sprinklers because they are often low to the ground. This is not a problem with drip irrigation.
  3. Serious weed competition can cause young trees to have stunted growth, reduced fruit size and yield. This can result in significant economic losses.
  4. Certain insect pests of tree fruit that live in host-plant weeds can multiply there and migrate up into the trees causing direct damage to the fruit resulting in significant economic losses.
  5. Tree-damaging rodents (see links below) like to hide and overwinter in the habitat created by weed cover and while there, they feed on tree bark and roots and cause damage. Rodent pests (meadow voles, mice, etc.) will be deprived of  habitat next to trees if weeds are controlled, especially in the fall.
  6. Significant weed cover under trees also makes worker and machine access difficult (e.g., dangerous for ladder work).

Weed Control Strategies

There are many weed control strategies used by tree fruit growers, depending on the types of weeds, area to be controlled, availability and feasibility of labor, and whether the site is under conventional or organic production practices. There are often annual and perennial weeds growing in the orchard simultaneously, in different stages of growth. Soil types, site location, and irrigation, are all variables that contribute to different weed pressures for different orchard locations. Some weeds introduce themselves to certain areas of an orchard, and grow in patches, and others may cover the entire orchard floor. These are some reasons why one method or product used will not control all of the weeds at a site. Successful weed management in orchards requires a year-round system combining different strategies. The first step in a viable weed control program is to correctly identify the types of weeds that are present. By knowing the weed and its life cycle, one can determine the accurate rate and timing of herbicides or other treatments to obtain the most reliable results.

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Conventional Weed Management

Growers not using organic practices have a full spectrum of weed control options available, including a wide choice of chemical controls. Conventional growers can use both pre-emergent and post-emergent herbicides to eliminate weeds. Organic growers are limited to a few pre-emergent, non-selective contact materials (see discussion below). Herbicides act in different ways. Pre-emergent products are directed at controlling the germinating weed seed before it emerges from the ground, and are best at preventing annual weeds. Post-emergent products are directed at controlling weeds that are emerged from the ground and are easily visible. An herbicide may be selective or non-selective. Post-emergent products can also be divided into contact or systemic herbicides.

Contact or foliage-applied herbicides will kill the above-ground parts of weeds that are present, which will kill many weeds. Soil-active herbicides may provide long-term or seasonal control of developing weeds. These are incorporated into or applied to the soil surface. Systemic herbicides can move from the foliage to the roots and eliminate more difficult to control weeds, particularly perennials that have hardy root systems that produce new top growth. Some of these herbicides can also travel to the tree roots, so it is important to use caution and always follow product label guidelines to avoid tree injury. (For additional information see the WSU Crop Protection Guide Weed Control recommendations). Newly planted fruit trees with green bark are very sensitive to herbicide damage, and require protection from contact with herbicides, as do some mature stone fruit trees. Tubes or milk cartons around the trunk or painting the bark white can protect the sensitive bark. Herbicide treatments in the fall often give the best results for perennial weeds, as the herbicide moves to the roots along with the sugars the plant is moving to store for next year. Weeds are tougher in the spring, and harder to eliminate. Orchard site is a factor in determining treatment choices, as certain herbicides may be de-activated in soils with more organic matter or clay and thus not work as well as expected.

Herbicide Resistance Management

Weeds can develop chemical resistance in the same general manner as insects, mites and disease organisms. Herbicide resistance comes from using the same material or materials with the same mode of action repeatedly without switching to another product or method. Always read the label for instructions on herbicide mode of action rotation. Mixtures of herbices with different modes of action may also be recommended in some cases to help lower resistance potential. It is important, however, when mixing herbicides that they have different modes of action, and that each of the materials is effective against the target weed species (reference: WSU Crop Protection Guide). The most certain way to avoid herbicide resistance is to use non-chemical methods for reducing the weed population. Non-chemical methods may include: the use of soil cultivation to control emerged plants and bury non-germinated seed; irrigation and water management; mowing; weed fabric or mulch; and thermal control (e.g., flaming). A description of non-chemical weed control methods is listed below in the Organic Weed Management section.

Herbicide Resources:

Organic Weed Management

Organic growers can use a variety of weed management practices in tree fruit orchards that don’t involve the use of synthetic herbicides. However, they are often time consuming and may be less effective. Non-organic growers may also employ non-chemical weed control methods to prevent issues with herbicide resistance and to follow good integrated fruit production practices. Organic weed management practices are listed below as well as on our Organic Production page on this website and on the WSU Organic and Integrated Tree Fruit Production Orchard Floor Management webpage.

Cover Crops
Cover crops can be planted in the drive row and under trees to improve soil fertility and suppress weeds. When the cover crops are mowed, they increase the organic matter in the soil. Cover crops also increase soil microbial activity and improve water infiltration and storage. The presence of cover crops will require more water and nutrition than a bare weed-strip to ensure that the trees are getting enough nutrition. And they may increase the risk of rodent damage by providing habitat. Cover crops can be mowed, and blown or raked into the tree row as mulch. Some cover crops are annuals and will die on their own at the end of the life cycle. Others are perennials and can persist for a number of years.

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Mulches
Mulches can be used for weed suppression, sometimes in combination with other barrier materials. Wood chip mulch can effectively suppress weeds, but require a large volume of material that can be costly to spread even if the wood chips are locally available for little or no cost. Certain mulches can provide multiple benefits, such as good weed control (1-3 years), improved soil quality, water conservation, increased tree growth and fruit yield, and/or suppression of parasitic nematodes. Information on mulching can be found here or viewed in this video. Side discharge mowers can blow grass from the alleyway under the trees for a cost-effective mulch, but may not deliver enough material for complete weed control.

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Weed Fabric
Various materials are available for use as weed fabrics in the tree row, that control weeds by depriving them of light. Water and soluble nutrients pass through them to get to the tree roots. Most fabrics are black, but one product is black on the bottom and white on top, and thus serves the dual purpose of controlling weeds and increasing light in the lower tree canopy. Materials are made to last as long at 10-15 years. A common grower practice is to use a strip of weed fabric on each side of the tree (the width of half of the “weed strip”) and pin it together between the trunks. Some growers will open the fabric over winter to disrupt rodent habitat (this is a drawback of the fabric) and possibly apply soil amendments to the tree row then. The fabric is then closed the next spring when weeds begin to emerge. The weed fabric can also influence tree performance by changing the temperature dynamics in the soil and under the tree.

Cultural Practices
Manual cultivation, mechanical weeders and tillage under the trees in the weed-strip area can reduce weed pressure but it can also dry and degrade the soil, and damage the fine feeder roots near the soil surface. However, vertebrate pest problems may be reduced when the soil is mechanically disturbed. Follow guidelines on soil depth and spacing from the tree to protect tree roots. Several mechanical weeding machines are available, with different strengths and weaknesses. For example, some are less disruptive to soil and shallow roots; others are better for uprooting well-established grassy weeds; and certain designs are simpler with few moving parts or hydraulics.

Hoeing, hand pulling and even grazing animals or geese may also be used for weed control in small-scale organic operations. Hoeing and hand pulling weeds are time consuming and require a labor force, and grazing requires close supervision so that the animals don’t damage the trees. The WSU Integrated Weed Control Project page has information on biological weed control agents and revegetation. Additional information about biological weed control agents can be found on the PNW Weed Management Handbook webpage.

Flame Weeding
Flame weeding damages the weeds by disrupting the protective leaf surface, causing them to wilt and die. Most flame weeders are mounted to tractors or pulled by an ATV. Propane burners are aimed at the surface of weeds in the tree row, while the tractor or ATV drives along. Shields on the burners prevent heat from damaging the tree trunks, but heat damage can occur if the burners are too close to trees, for too long. Fire danger when conditions are too dry or windy is a concern. There are other thermal weeders available that do not have open flame, but they are more expensive to operate. Because flaming does not kill the crown or root system, for some weeds, the flaming treatment needs to be repeated throughout the growing season to deplete the plant’s food reserves.

Organic Herbicides
Organically approved herbicides are available but may not provide effective control of grasses and perennial weeds that grow under trees. Some products are approved for organic use, but only in non-crop areas. Growers using organic herbicides generally need to complement them with other control methods. Some approved products include acetic acid, clove oil, corn gluten meal, D-liminene, lemongrass oil, pine oil and fatty acid soaps. (Always check with your certifier before using an organic herbicide to ensure its compliance. Most organic herbicides are non-selective materials, that are applied to actively growing weeds, and work by disrupting the protective leaf surface, which leads to desiccation and wilting.

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