Agricultural pest management
Weed Management in Organic Orchards
(Reviewed 10/14, updated 10/14)
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 numbers over time. One of the best ways to prevent weed problems is to control existing weeds before they go to seed. It is usually best to use conventional tactics, including synthetic herbicides, for one or two years after planting; this helps reduce the weed seed bank and weed numbers, and makes weed control by organically-approved means less expensive later. However, this approach will require three more years of not using synthetic herbicides in the orchard for it to be certified as organic.
It is essential 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 a cycle of irrigation followed by cultivation several times 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 deeply so that they cannot germinate. Use a soil-inverting plow such as a Kverneland plow; a standard moldboard plow will not sufficiently invert the soil.
Soil solarization can significantly reduce weed numbers 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 and 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. Also weed seeds absorb the moisture and germinate, making them more easily killed by solarization. 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 common purslane. For nutsedge suppression, the plastic should be laid with some slack to it, so the nutsedge's sharp growing points cannot penetrate it.
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 either May through June or August through September to avoid summer fog. Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface where they can 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 woven fabric 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 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 and not readily killed with flaming. Also, perennial weeds are suppressed, but not controlled with flaming.
Herbicides. Contact herbicides that are allowed for use in an organic orchard include d-limonene (Avenger) and clove oil (Matran II, Burnout II). 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. In most cases, a spray volume of at least 60 gallons per acre will be required when using these products. Repeat applications are necessary to control newly emerged weeds. Add an organically acceptable surfactant to improve efficacy. Avoid spraying tree foliage to prevent injury to green tissue. Broadcast application of herbicides is usually not economical. However, organic herbicides are useful for spot treatments, particularly to control weeds in mulches, because this will help to preserve the mulch and increase its useful life span.
Before using any animals, check federal, state, and local food safety regulations and comply with them. Consult the University of Idaho and University of Missouri websites for further information on grazing animals.
Weeder geese. Geese can be used to manage grass weeds in orchards. Geese prefer grass species and will only eat other weeds and crops after the grasses are gone. If confined, they will even dig up and eat johnsongrass and bermudagrass rhizomes, which they prefer. These grasses are otherwise 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 four 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 Metzer Farms website for further information on geese.
Other animals: Sheep and goats are sometimes used in organic orchards as well. Sheep will effectively remove all weeds down to ground level. Goats are browsers and must be carefully managed to avoid damage, especially to young trees. Both sheep and goats are generally used during the time when trees are dormant and the chance of grazing damage is minimal.
Management between tree rows
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 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.
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 ringroller. The ringroller 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, UC ANR Publication 21471.
UC IPM Pest Management Guidelines: Apricot
K. J. Hembree, UC Cooperative Extension, Fresno County