Weed Management in Organic Orchards

Pay special attention in organic pear orchards to prevent 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.

As in all orchards, 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 or use the University of California Weed Research and Information Center's Weed Identification Tool.

Transitioning mature, full-canopied trees to organic production will require less intensive weed management than starting a new organic orchard. Mature trees have a well-developed root system and are able to compete well against weeds for water and nutrients. Also many mature pear orchards are irrigated using full-coverage undertree sprinklers that can deliver adequate water to meet the water-use needs of the tree.

The season before planting 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 a cycle of irrigation followed by cultivation to germinate and kill 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 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. If the site is not yet certified organic, herbicides can be used until the transition time to organic begins; this can be very helpful to reduce the amount of hard-to-control perennial weeds in the planting area.

Soil solarization

Soil solarization can significantly reduce weed populations when used before or shortly after planting trees. 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.

How much of the soil surface should be covered depends on the kind of weeds and weed numbers. If perennial weeds are present, completely cover the area to reduce reinfestation; weeds in the untreated soil can reinfest treated areas during cultivation or furrow irrigation. Strip coverage uses less plastic and the plastic sheet edges are not joined together, so it is often more practical and less expensive. However, weeds may eventually reinfest treated areas, hindering long-term control of weeds.

Not all weeds are controlled by solarization. Most annual weed seedlings are controlled, but purslane and crabgrass may be more difficult to control. Also, solarization does not control perennials as well as annuals. Seedlings of johnsongrass and field bindweed are controlled, and bermudagrass and johnsongrass rhizomes may be controlled if they are not deep in the soil. However, solarization alone is not effective for the control of field bindweed rhizomes. Yellow nutsedge is partially controlled while purple nutsedge is not significantly affected. Avoid cultivating solarized soil deeper than 3 inches to prevent bringing viable weed seeds to the surface where they can germinate.

To solarize:

1. Contact plastic suppliers well in advance so they can formulate plastic tailored to your needs. Check with your certifying agent before choosing or purchasing solarization material to make sure it complies with organic requirements. It is also a good idea to check with your landfill for disposal and recycling requirements.
2. Effective soil solarization begins with preparing a smooth seedbed 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 material such as rocks and weeds that will puncture or raise the plastic sheets.
3. 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 (allow 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 before planting. Furrow irrigation under the plastic is another option, but solarization will be less effective in the furrows because of the gap between the plastic and the soil.
4. Use clear plastic that is 1.5 to 2 mils thick and impregnated with UV inhibitors to prevent premature breakdown of the material. 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 weeds not controlled by solarization.
5. For strip solarization, cover the planned tree row with plastic from 4 to 8 feet wide depending 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.
6. In the Central Valley and North Coast, 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 where summer fog is a concern; in these areas, apply plastic in August and September or May and June.

For more information on soil solarization, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds.

Once the trees are established, weeds in the tree row can be managed with a variety of strategies. The middles of organic orchards are commonly managed with cover crops or by mowing resident vegetation. For more information on orchard floor management, see Chapter 13 in the Pear Production and Handling Manual.

Tree-row management

Various types of mulch may be used to control weeds in nonbearing orchards. Maintain the mulch layer throughout the year. In-row mulches of fabric mulch or a 4- to 6-inch layer of organic materials, including newspaper and wood chips, control weeds by preventing light penetration necessary for weed growth. Mulch also has the benefit of conserving moisture. However, if possible, make sure the mulch is not placed against the trunk to prevent crown rot. Mulch may allow populations of gophers or voles (meadow mice) to build up because their runways cannot be seen.

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, mulches, or weeder geese. The choice of method depends in part on costs, tree spacing, the use of berms, orchard floor management practices, risk of damage to trees, the type of irrigation system, and food safety requirements.

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 among others. The Wonder Weeder, commonly used in organic orchards in the Pacific Northwest, uses a Lilliston-style cultivator head, which derives its cutting action from a rolling contact with the soil.

• 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 through the trees, with the microsprinklers positioned upside down or attached to stakes.
• Drip lines may be buried to avoid damage.
• Furrow-irrigated orchards are amenable to in-row cultivation.

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 more protected. 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; young trees, before their bark is thick enough, can be permanently stunted or killed. 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 attached to stakes or suspended in the trees with the emitters positioned upside down. Drip lines may be buried.

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. They require high water volumes (70 gallons per acre) to achieve optimal spray coverage and none of these compounds have residual activity. The organic herbicides that are currently available are contact herbicides, so they usually do not provide effective control of established perennial weeds. Repeat applications are necessary to control new weed flushes. Add an organically acceptable surfactant to improve efficacy. Avoid spraying pear foliage as the materials will affect any green tissue. Check with your organic certifier to determine the current status and any use restrictions for organically acceptable herbicides.

Weeder geese

Before using animals as part of a weed management program, contact fresh-fruit buyers to make sure their presence in the orchard is acceptable. 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.

Grazing sheep

As with geese, make sure putting sheep in the orchard is acceptable with fresh-fruit buyers. Sheep may also be grazed in orchards, although there is no research on their use in California pear orchards. For further information on the integration of orchards and livestock, see the ATTRA Sustainable Agriculture Program’s website.

Management between tree rows

Consider planting a winter annual cover crop in the area between tree rows. Resident vegetation does not usually grow uniformly enough to compete well with newly invading weeds and often harbors weed species that continually colonize the tree row. Sprinkler-irrigated orchards with a perennial cover crop have been documented to use up to 30% more water than clean-cultivated orchards. Orchards with winter cover crops also use more water than clean-cultivated orchards, but not necessarily more than orchards with resident vegetation, which is the standard floor management practice in most orchards. Thus it is important to account for the additional water needs of the cover crop so that it does not compete with trees for available water.

When choosing a cover crop to seed in a pear orchard, consider both the advantages and disadvantages. Certain types of cover crops can promote pest problems, such as true bugs (e.g., lygus bugs, stink bugs, and Irbisia solani) in white Dutch clover or when bur clover dies in late spring, and gophers with the use of perennial clovers. Some cover crops, particularly grasses, can promote fruit russeting by supporting large populations of russet-causing bacteria.

A winter annual cover crop that reseeds itself, for example subterranean clovers, should compete against weeds and reduce the potential for problems in the future. To ensure success, plant cover crops in pear orchards soon after harvest from September through mid-October, 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 chopping, and spraying. To reduce this damage, cover crops may be seeded in alternate middles and these operations carried out in the nonseeded middles.

If there is a potential for frost, mow the cover crop once before pear bloom to minimize frost damage; most cover crop species will regrow and flower later in the season. Sheep have been successfully used in California vineyards for this purpose, but there is no data at this time to support this practice in pears, and food safety issues should be considered. If frost protection is not needed or is achieved in another fashion, the cover crop will be most competitive if left unmowed. An exception is subclover, which will compete with taller weeds best if mowed before mid-March. After most species in the cover crop stand 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.