Pest Management Guidelines
Weed Management in Organic Orchards
(Reviewed 12/07, updated 4/09)
Weed control in organically managed orchards requires special attention to preventing weed problems before they start. Any method that reduces the amount of weed seed in the orchard will diminish weed populations over time. One of the best ways to prevent weed problems is to control existing weeds before they go to seed.
The first step in developing a weed management program is to identify the weeds infesting the orchard or planting site. Become familiar with each weed's growth and reproductive habits in order to choose the most effective management options. See the weed photos linked to the weeds in the list of COMMON AND SCIENTIFIC NAMES OF WEEDS.
Transitioning mature, full-canopied trees to organic production will require less intensive weed management than starting a new organic orchard. Mature, shady orchards often have limited weed growth whereas weeds can more effectively compete with trees under full sunlight in newly planted orchards.
The season before trees are planted is a critical period for weed management so young trees can become established with reduced competition from weeds. Two methods of managing weeds at this time are cultivation and soil solarization.
Cultivation. Repeating several times a cycle of irrigation followed by cultivation to germinate and destroy young weeds can reduce the amount of weed seed in the orchard soil. Cultivation works well with summer annuals but not as well with perennial weeds such as nutsedge, field bindweed, bermudagrass, and johnsongrass. If the site is not already certified organic, herbicides can be used until the transition time to organic begins, which can be very helpful in ridding the area of these hard-to-control perennials. Or, if most of the weed seeds on the site are located in the surface 4 inches of soil, a soil-inverting plow can be used to bury them so deeply that they cannot germinate. Use a soil-inverting plow such as a Kverneland plow; a moldboard plow will not sufficiently invert the soil.
Soil solarization. Soil solarization can significantly reduce weed populations in the planned tree rows. Soil solarization traps the sun's energy beneath a layer of clear plastic, increasing the temperature in the top 6 inches of soil to levels lethal to many weed seedlings as well as vegetative structures of perennial weeds. In areas like the Central Valley, soil solarization can heat the soil to more than 130°F at a 3-inch depth. The effect of solarization diminishes at greater depths and it does not control perennials as well as annuals. Solarization does not control perennials as well as annuals. Seedlings of bermudagrass, johnsongrass, and field bindweed are controlled, but not the plants. Yellow nutsedge is partially controlled while purple nutsedge is not significantly affected.
Effective soil solarization begins with preparing a smooth seed bed so that the plastic can be placed as close as possible to the soil surface. Disc to break up clods and then smooth the soil. Remove any material that will puncture or raise the plastic sheets such as rocks and weeds.
Irrigate before or after applying the plastic because wet soil conducts heat better than dry soil. Cover the soil with plastic as soon as possible after irrigating. It is possible to irrigate after laying the plastic by installing the drip system or the microsprinkler line (with only the spaghetti tubing) before planting. Furrow irrigation under the plastic is another option. (If the entire site is irrigated, weed growth will occur in the untarped centers and will be difficult to control without disturbing the plastic.) After irrigation, allow the soil to dry somewhat to avoid compaction by heavy equipment.
Use clear plastic that is 1.5 to 2 mils thick and impregnated with UV inhibitors to prevent premature breakdown of the material. Contact plastic suppliers well in advance so they can formulate plastic tailored to your needs. Cover the planned tree row with plastic from 6 to 10 feet wide. The width depends on the middles management program planned for the orchard. Bury the sides of the plastic to create a seal on the soil; this also helps prevent the plastic from being blown away by wind. Machines that lay down the plastic are available to automate the process.
Black plastic suppresses weed seed germination but will not heat the soil sufficiently for solarization. Black plastic can be used as a mulch to suppress nutsedges or common purslane.
In the Central Valley, the plastic should be in place from June through August and can remain in place until planting begins. Solarization may not be as effective in cooler coastal areas. In these areas, apply plastic in August and September or May and June. Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface where they germinate.
In the nonbearing years, mulches can be used to help manage weeds in organic orchards. Once the trees are established, weeds in the middles of organic orchards are commonly managed with cover crops or mowing resident vegetation and weeds in the tree row are managed with a variety of strategies.
Tree-row management. During the nonbearing years, mulch may be used to control weeds in nonbearing orchards where harvest operations will not disperse it. Maintain the mulch layer throughout the year. In-row mulches of black plastic or a 4-inch layer of organic materials including compost, newspaper, straw, hay, and woods chips control weeds by preventing light penetration necessary for weed growth. Walnut leaves may be raked or blown from row middles into tree rows for mulch. To suppress perennial weeds, however, it may be necessary to apply up to 6 inches of mulch.
Once the trees are established, weeds in the tree row may be managed with in-row cultivation, cross discing, cross mowing, hand weeding, flaming, organically acceptable herbicides, or weeder geese. The choice of method depends in part on costs, tree spacing, the use of berms, orchard floor management practices, and the type of irrigation system. Weed fabrics are not practical for walnut orchards because harvest operations will disturb the fabric.
Weed fabrics. Weed fabrics can be used in nonbearing orchards and last 4 to 5 years. They are not practical for bearing orchards because harvest operations will disturb the fabric.
In-row cultivation. In-row cultivators are equipped with a sensor or trigger mechanism that pivots the cutting arm around the tree to avoid injury. Several companies make cultivation equipment; those that have performed well include equipment from Bezzerides, Kimco, and L&H Manufacturing.
Flaming. Flaming can effectively manage in-row weeds that are smaller than eight leaves. When flaming is used repeatedly, grasses will eventually become the dominant weeds because their growing points are close to the ground. Also, perennial weeds are suppressed, but not controlled with flaming. Protect the trunks of young trees from flamers to avoid injury to the cambium layer of the tree; also keep flamers away from the plastic irrigation tubing. Do not flame in orchards with a lot of dried vegetation in order to avoid fires that may injure trees and irrigations systems or spread out of control.
Herbicides. Several approved contact herbicides are available to use in organically certified orchards, but these materials are costly and tend to be less effective than synthetic herbicides. Check with the organic licensing organization to determine current status and any use restrictions for organically acceptable herbicides. As with any contact herbicides, good coverage is essential. Repeat applications are necessary to control newly emerged weeds. Add an organically acceptable surfactant to improve efficacy. Avoid spraying walnut foliage, as the materials will affect any green tissue.
Of the herbicides that are certified for use in organic orchards, only Matran EC, a clove oil product, is registered for use in California. This product has not been thoroughly evaluated for effectiveness in an orchard setting and further studies are needed.
Middles management. Consider planting a cover crop in the area between tree rows. Resident vegetation does not usually grow uniformly enough to compete well with newly invading weeds. In addition, resident vegetation includes weed species that continually colonize the tree row. Planted cover crops generally compete better with invasive weeds and thus reduce weed infestations in the orchards over time. Planted cover crops are either "green manures," reseeding annuals, or permanent sod. It is important to take into account the additional water needs of the cover crop so that it does not compete with trees for available water, or, in the case of dryland orchards, to eliminate the cover crop in spring in order to preserve available water for tree use.
Green manure cover crops. Green manure crops are seeded yearly and often include legumes in the mixture to maximize biomass and nitrogen. Generally they are mowed in spring and disced under by late April or early May to minimize water use.
Annual reseeded cover crops. To ensure success of an annual reseeded cover crop, plant it in walnut soon after harvest, before leaves fall and when rainfall or irrigation water is available to provide for germination and good seedling growth. Newly established cover crops may be seriously damaged by fall and winter orchard traffic during operations such as pruning, brush removal, chipping and shredding, and spraying. In orchards where these operations are planned, cover crops may be seeded in alternate middles and these operations carried out in the nonseeded middles. Or, plant cover crops in years when these operations are not planned for the orchard. In high elevation orchards, newly emerged cover crops may be killed or set back by late fall frosts.
If there is a potential for spring frost and the cover crop is tall, mow once before walnut bloom to minimize frost damage; the cover crop will regrow and flower later in the season. However, if mowing can be avoided, the cover crop will be most competitive. After most species in the cover crop have produced seed, mow or roll it using a ringroller. The ringroller will allow more seed production and also create a surface mulch that will shade the soil, preventing germination of weed seeds. Close mowing before harvest will break the cover crop into small pieces to accelerate decomposition.
Permanent sod. Regular mowing and additional irrigation and fertilizer are required in orchards where a permanent sod is maintained in the middles.
For more information on cover crops, see Cover Crops for Walnut Orchards, ANR Publication No. 21627.
UC IPM Pest Management Guidelines: Walnut
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