Agricultural pest management
Weed Management in Organic Orchards
(Reviewed 4/10, updated 4/10)
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.
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 scheduled 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 common purslane or nutsedges (nutsedge will grow through clear plastic).
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.
Once the trees are established, weeds in the tree row may be managed with in-row cultivation, cross discing, cross mowing, hand hoeing, flaming, organically acceptable herbicides (provides suppression of certain weed species), mulches, or weeder geese. The choice of method depends in part on costs, tree spacing, use of berms, orchard floor management practices, and type of irrigation system.
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. Some approved contact herbicides are available to use in organically certified orchards. These materials are costly and tend to be much 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 peach foliage as the materials will affect any green tissue.
Weeder geese. Geese are occasionally used in orchards. They feed mainly on grasses but will turn to other weeds once the grasses are gone. If confined, they will eat johnsongrass and bermudagrass rhizomes, which are difficult to manage in organic systems. Young geese are best because they eat larger quantities of food, although having at least one older goose helps to protect the younger birds. Generally about 4 geese per acre are needed. Provide geese with drinking water and shade. Protect them from dogs and other predators; portable fencing works well. Consult the following Web site for further information on geese: http://www.metzerfarms.com/UsingWeederGeese.cfm.
Management Between Tree Rows
An annual cover crop that reseeds itself will compete against weeds and reduce the potential for problems in the future. To ensure success, plant the cover crops in peach 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, 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.
If there is a potential for frost and the cover crop is tall, mow once before 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, except for a subclover cover crop, which will compete with taller weeds if mowed before mid-March. 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.
UC IPM Pest Management Guidelines: Peach
J. A. Roncoroni, UC Cooperative Extension Napa County
Acknowledgment for contributions to Weeds:K. J. Hembree, UC Cooperative Extension Fresno County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
A. Shrestha, UC IPM Program, Kearney Agricultural Center, Parlier