Weed control in organically managed orchards requires special attention to prevent 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 correctly identify the diversity of 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 out as a new organic orchard. Mature, shady orchards often have limited weed growth whereas weeds can more effectively compete with trees in newly planted orchards where there is more sunlight available to the weeds.
Weed Management Before Planting
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.
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 reducing the number 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 deeply so 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 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 foot of soil to levels lethal to many weed seedlings as well as vegetative structures of perennial weeds. However, 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, sticks, and weeds.
Irrigate before or after applying the plastic because wet soil conducts heat better than dry soil. Cover the soil with clear 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 optimum time for solarization is June through August. Solarization may not be as effective in cooler coastal areas. In these areas, apply plastic when fog cover is less likely. This is usually in August and September or May and June. In all cases, the plastic should be in place for a minimum of 6 weeks and can remain in place until planting begins. Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface where they germinate.
Weed Management after Planting
In the non-bearing years, mulches can be used to help manage weeds in organic orchards. Since organic mulches may reduce the soil temperature slightly, it is often better to apply these materials when the trees have been in the ground for at least one full year to avoid the possibility of reduced tree growth. Once the trees are established, weeds in the middles of organic orchards are commonly managed with cover crops or mowing resident vegetation while the weeds in the tree row can be managed with a variety of strategies.
During the non-bearing years, mulch may be used to control weeds in the orchards. Maintain the mulch layer throughout the year. In-row mulches of black plastic weed block or a 4-inch layer of organic materials including compost, newspaper, straw, hay, and wood chips control weeds by preventing light penetration necessary for weed growth.
Once the trees are established, weeds in the tree row may be managed with shallow in-row cultivation, cross-discing, cross-mowing, hand hoeing, flaming, organically acceptable herbicides, mulches, 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.
In-row cultivators are equipped with a sensor or trigger mechanism that pivots the cutting arm around the tree to avoid injury. In-row cultivation is most successful when weeds are small. Several companies make cultivation equipment; those that have performed well include equipment from Bezzerides, Kimco, and L & H Manufacturing.
- Sprinkler-irrigated orchards require extra precautions to ensure proper operation of the trigger mechanism on the cultivator so that it moves away from the sprinkler head in the same way as it does for the tree.
- Microsprinkler irrigation lines and emitters can be protected from damage by suspending the surface lines, with the microsprinklers positioned upside down, in the trees or on stakes.
- Drip lines may be buried to avoid damage.
- Furrow-irrigated orchards are amenable to in-row cultivation.
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 and not readily killed with flaming. 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.
- To prevent damage to irrigation equipment, microsprinkler irrigation lines and emitters can be suspended in the trees or on stakes with the emitters positioned upside down, and drip lines may be buried.
Check with the organic licensing organization to determine current status and any use restrictions for organically acceptable herbicides. Good coverage for contact herbicides is essential. Repeat applications are necessary to control newly emerged weeds. Add an organically acceptable surfactant to improve efficacy. Avoid spraying tree foliage as the materials will affect any green tissue.
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: https://www.metzerfarms.com/GooseBreedComparisonNew.cfm?.
Management between tree rows
Consider planting a cover crop in the area between tree rows that is compatible with the irrigation system. With surface irrigation, cover crops are not recommended because they impede water flow. Summer cover crops work best with a sprinkler system, whereas winter cover crops work with most systems, but sprinklers help with full seeding.
Resident vegetation does not usually grow uniformly enough to compete well with newly invading weeds. In addition, resident vegetation often 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. 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.
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 soon after harvest, before leaves fall from the orchard trees and when rainfall or irrigation water is available to provide for germination and good seedling growth. Don't use clover cover crops (red, subclovers) in cherries as they can harbor X-disease.
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.
Once cover crops are established, sheep can be grazed in orchards during winter months. 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 ring-roller. The ring-roller will allow more seed production and also create a surface mulch that will shade the soil, preventing germination of weed seeds. For more information on cover crops, see Covercrops for California Agriculture.