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. Controlling existing weeds before they go to seed is one of the best ways to prevent weed problems.
The first step in developing a weed management program is to identify the weeds infesting the orchard or planting site. Becoming familiar with each weed's growth and reproductive habits will help you 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 while weeds in newly planted orchards more effectively compete with trees.
Weed Management Before Planting
The season before trees are planted is a critical period for weed management. It is important that young trees become established with little or no competition from weeds. Weeds can be effectively managed at this time by cultivation and soil solarization.
Cultivation followed by irrigation to germinate new weeds, followed by cultivation again to kill weed seedlings,is an especially effective weed control method to use before planting trees. Frequent cultivation lowers weed seed populations in the soil, thus reducing weed growth. At least two cycles of cultivation and irrigation, followed by a shallow cultivation are needed for a marked reduction in weed seedlings. This method is not effective for the control of established perennial weeds.
If the site is not already certified organic, herbicides can be very helpful in ridding the area of hard-to-control perennials and can be used until the transition time to organic begins. Cultivation when the soil is very dry is an effective method to control perennial grasses such as bermudagrass and johnsongrass. Cultivation cuts the rhizomes into small pieces so they can dry out. Rework the soil frequently with a spring tooth harrow to pull new rhizomes to the surface to dry out. If the soil is irrigated, or rain occurs before total control of the perennial plant is achieved, the rhizome pieces will begin to grow and the effectiveness of this practice is greatly reduced. Working the soil when wet can increase the population of perennial weeds, because each piece of cut rhizome can root and develop into a new plant.
Field bindweed growth can be reduced for up to 2 years by deep plowing or using a reclamation blade (a large V-shaped blade) to cut the roots at a depth of 16 to 18 inches in dry soil. Nutsedge infestations can be reduced by deep plowing with large moldboard plows that bury the nutlets to a depth of at least 12 inches. Seedlings of many perennials can be controlled by repeated cultivation.
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. This process can increase the temperature in the top foot of soil to levels lethal to many weed seedlings and vegetative structures of perennial weeds. Solarization will not control perennial weeds as well as it does annuals. Seedlings of bermudagrass, johnsongrass, and field bindweed are controlled, but mature plants are not. Yellow nutsedge is partially controlled while purple nutsedge is not significantly affected.
Preparing a smooth seed bed so that the plastic can lie as close as possible to the soil surface is imperative for effective soil solarization and can be accomplished by first discing the soil to break up clods and then rolling or rotavating to smooth the soil. Remove any materials such as rocks, sticks, or weeds that will puncture or raise the plastic sheets.
Irrigate before or after applying the plastic. Wet soils hold and conduct heat better than dry soil. Seeds that have begun the process of germination by imbibing water are easier to kill than hard seeds. Cover the soil with plastic as soon as possible after irrigating, allowing the soil to dry somewhat to avoid compaction by heavy equipment. It is possible to irrigate after laying the plastic by installing the drip system or the microsprinkler line, using only the spaghetti tubing, before planting. Another option is to furrow irrigation under the plastic. If the entire site is irrigated, weed growth will occur in the untarped centers during the solarization process and will be difficult to control without disturbing the plastic.
Use clear plastic that is 1.5 to 2 mils thick and impregnated with UV inhibitors to prevent premature breakdown. Contact plastic suppliers well in advance so they can formulate plastic tailored to your needs. Cover the area designated as the tree row with plastic from 6- to 10-feet wide, depending on the middles management program you have chosen for the orchard. Bury the plastic on all sides to create a seal on the soil and help prevent the plastic from being blown away by wind. Machines are available that lay down the plastic and automate this otherwise labor-intensive process.
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.
Black plastic suppresses weed seed germination but will not heat the soil sufficiently for solarization. Plastic mulches may not be successful in suppressing species like nutsedge. All plastic should be removed before planting.
Weed Management after Planting
Mulches can be used to help manage weeds in organic orchards in the years before the orchard bears fruit (nonbearing). The mulch works by blocking light preventing weed germination and growth. Many materials can be used as mulch: municipal yard waste, wood chips, straw, hay, sawdust, newspaper, and others. To be effective, mulches must block all light to the weeds, and materials vary in the depth necessary to accomplish this. A good rule is: the larger or looser that the mulch pieces are, the deeper the mulch should be. Organic mulches must be maintained in a layer at least 4 inches thick.
Once the trees are established, weeds in the middles of organic orchards are commonly managed with cover crops or mowing resident vegetation while weeds in the tree row can be managed with a variety of strategies. Weeds in the tree row may be managed with in-row cultivation, cross discing, cross mowing, hand hoeing, flaming, organically acceptable herbicides, mulches, or animals. The choice depends on cost, tree spacing, the use of berms, 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. 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. Furrow-irrigated orchards are amenable to in-row cultivation. 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.
Flaming can effectively manage in-row weeds that are smaller than eight leaves. Grasses will eventually become the dominant weeds when flaming is used repeatedly because their growing points are at or below ground level. Flaming will suppress but not control perennial weeds. To avoid injury to the cambium layer of the tree, protect the trunks of young trees from flamers. Keep flamers away from plastic irrigation tubing. Do not flame in orchards that have a lot of dried vegetation to avoid fires that may injure trees, irrigations systems, or spread and get out of control.
Organically certified herbicides are available for use; these materials are generally costly and tend to be much less effective than conventional herbicides. Make sure that the herbicides used are accepted by your organic licensing organization and are registered, or exempted by your state pesticide regulator. As with any contact herbicides, good coverage is essential. Repeat applications will be necessary to control newly emerged weeds. An organically acceptable surfactant can be used to improve efficacy. Avoid spraying nectarine foliage because the herbicides will affect any green tissue.
Sheep can used for weed control in organic orchards and can be very effective in controlling weeds. Their effectiveness depends on several factors, among them the amount of feed available (cover crop and weeds), and the density (number per acre) of sheep used. Goats are grazers and are often used to control the brush around orchards. If goats are used in the orchard, they must be carefully managed to avoid damage to the trees. Geese can also be used to manage grass weeds in orchards. Geese prefer grass species and will eat other weeds and crops only after the grasses are gone. They have a particular preference for the rhizomes of two especially troublesome orchard weeds: johnsongrass and bermudagrass. If confined, will dig the rhizomes up and eat them. Generally, four geese per acre are needed. Consult the following Web site for further information on geese: http://www.metzerfarms.com/UsingWeederGeese.cfm. In most cases all animals used in orchards will require some form of protection from predators (dogs, coyotes, etc.). Consult local statutes to determine if or when animals must be removed from the orchard before harvest.
Management between tree rows
Consider planting a cover crop in the area between tree rows. Resident vegetation does not usually produce uniform growth. Planted cover crops generally compete better with invasive weeds and thus reduce weed infestations in the orchards over time. Resident vegetation often includes weed species that grow into the tree row. Care must be taken to adjust watering regimes to account for the increased use of water by the cover crop so that it does not compete with trees for available water. Cover crops in dryland orchards should be plowed under in spring before the trees become water stressed.
An annual cover crop that reseeds itself will compete against weeds and reduce the potential for weed problems in the future. Plant the cover crops in nectarine soon after harvest, before leaves fall and rainfall, or irrigation water, is available to provide for good germination and 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. Alternatively, cover crops can be planted in years when these operations are not planned for the orchard.
Mow tall cover crops once before bloom to minimize frost damage; the cover crop will regrow and flower later in the season. If mowing can be avoided, the cover crop will be most competitive. An exception is subclover cover crop, which will compete with taller weeds if mowed before mid-March. The cover crop should be mowed or ringrolled after most species have produced seed. Ringrolling will create a surface mulch that will shade the soil, preventing many weed seeds from germinating.