Agricultural pest management
Weed Management in Organic Vineyards
(Reviewed 6/06, updated 1/14, corrected 3/14)
If you choose not to use synthetic and systemic pesticides then you should go the extra distance to try to keep the vineyard as clean as possible. Weeds serve as an excellent host for fungal disease and should be routinely controlled by all grape growers. Vineyards differ from orchards because the foliage and fruit are typically much closer to the ground. Mildew spores are easily splashed back onto vines from weed hosts. Perennial weeds are definitely the worse problem, but annuals, such as marestail (Conyza canadensis), can grow among the vines and grapes to a height of 6 feet in one season and serve the same host function as the other weeds. Weed control in organically managed vineyards requires special attention to preventing weed problems before they start. Cover crops planted in middles and mechanical control of weeds in the vine row are key components of an organic weed management program.
WEED MANAGEMENT BEFORE PLANTING
It is critical to have minimum or no weed competition at the time of planting new vines so weed control before planting is important. Take measures to deplete the soil weed seed bank. A summer fallow treatment of irrigation followed by tillage and then drying can reduce weed seed numbers in the soil. Repeat this cycle several times to further deplete weed seeds in the soil. Weed seeds located in the surface 4 inches of soil can be buried to depths where they cannot emerge with a soil-inverting plow such as a Kverneland plow; a moldboard plow will not sufficiently invert the soil to be effective.
Soil solarization of the planned vine row can also significantly reduce weed populations. The soil must be moist and the width of the solarized area should be at least 6 feet. Bury all 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.
Solarization must be done during summer and should be started at least by the beginning of August to have sufficient time (4 to 6 weeks) to complete the process. Clear plastic or a plastic with a coating that suppresses weed seed germination can be used. 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.
WEED MANAGEMENT AFTER PLANTING
Mechanical cultivation uproots or buries weeds. Weed burial works best on small weeds, while larger weeds are better controlled by destroying the root-shoot connection or by slicing, cutting, or turning the soil to separate the root system from the soil. Keep cultivation shallow to minimize damage to crop roots and to avoid bringing more weed seeds near the surface to germinate.
Perennial weeds with established root systems are difficult to kill with a single tillage operation. With tillage, the top is removed and a new top is generated using the underground reserves. For perennials, tilling 3- or 4-inches deep reduces the reserves, forcing the weeds to use a greater portion of the reserves available to regenerate. Several companies make cultivation equipment. Trip mechanisms on vineyard cultivators prevent damage to the vines. Even the best cultivators will not eliminate all weeds, thus hand hoeing is often needed. Hand cultivation alone may be effective on a small scale.
Mulches can also help with weed control in the vineyard. The mulch blocks light, preventing weed germination or growth. Many materials can be used as mulches including municipal yard waste, wood chips, straw, hay, sawdust, and newspaper. To be effective, mulches need to block all light to the weeds; therefore different mulch materials vary in the depth necessary to accomplish this. Organic mulches must be maintained in a layer 4 or more inches thick. Organic mulches breakdown with time and the original thickness typically is reduced by 60% after one year. Cover crops can be grown in the middles; in the spring "mow-and-throw" the mulch in around the base of the vines. Weeds that emerge through the mulch can be controlled using an organic contact herbicide or with hand hoeing. Do not plant cover crops under the vine row because excess competition may occur, possibly reducing grape yields.
Flamers can be used for weed control in the vine row, with propane-fueled models being most common. Fire causes the cell sap of plants to expand, rupturing the cell walls; this process occurs in most plant tissues at about 130ºF. Weeds must have less than two true leaves for greatest efficiency of the burner. Grasses are harder to kill by flaming because the growing point is below the ground. After flaming, weeds that have been killed change from a glossy to a matte finish. This occurs very rapidly in most cases. Foliage that retains a thumb print when pressure is applied between your thumb and finger has been adequately flamed. Typically, flaming can be done at 3 to 5 mph through fields, although this depends on the heat output of the unit being used. Repeated flamingcan likewise be used to suppress perennial weeds such as field bindweed. Care must be taken to avoid igniting dry vegetation, which could injure the vines, or start a wildfire.
Geese can often be used to manage grass weeds in vineyards. Geese prefer grass species and will eat other weeds and crops only after the grasses are gone and they become hungry. If confined, they will even dig up and eat johnsongrass and bermudagrass rhizomes, which they have a particular preference for. Both of theseweeds can be especially troublesome in vineyards. Generally, about 4 geese per acre are needed. They require water for drinking, and some form of protection from predators (dogs, coyotes, etc.). Young geese are preferred, as they eat larger quantities of food, although having at least one older goose, helps to protect the younger birds. Consult the following Web site for further information on geese:http://www.metzerfarms.com/weeder.htm. Other animals sometimes used in organic vineyards include sheep and goats. Sheep will effectively remove all weeds down to ground level. Goats are browsers and must be carefully managed to avoid damage to the vines. Both sheep and goats are generally used during the time when grapes are dormant and the chance of grazing damage is minimal.
Manage weeds in the vine row with in-row cultivation, mulches, hand hoeing, or flaming. The choice of method depends in part on the type of irrigation system.
Furrow-irrigated vineyards are amenable to in-row cultivation. Cultivation is probably the most widely used method of weed control in organic systems. Other options include the use of cultivation, organically approved herbicides, flaming, and the use of weeder geese.
Few options are available in organic vineyards with microsprinklers. In-row cultivators may damage irrigation lines and emitters. Surface lines can be suspended in the vines or on stakes to allow for in-row mowing, cultivation, or flaming underneath. The microsprinklers are suspended upside down. Hand hoeing, possibly flaming, organically approved herbicides, and weeder geese also could be used for weed control. Protect trunks of young vines from flamers or herbicides to avoid injury; also keep flamers away from the plastic irrigation tubing. Mulches can suppress weed growth. Weeds that emerge through the mulches must be removed by hand hoeing or with the use of organic herbicides.
Weed control options for surface drip-irrigated vineyards are similar to those available for micro-sprinklers. However, if drip lines are suspended or subsurface drip irrigation is used, cultivation, flaming, or mulches can all be used.
Similar to many conventionally managed vineyards, weeds in the middles of organic vineyards are commonly managed with cover crops and/or mowing.
Consider planting a cover crop in the area between vine rows. Resident vegetation does not usually grow uniformly enough to compete well with newly invading weeds. In addition, resident vegetation includes weed species that continually colonize the vine row. An annual cover crop that reseeds itself will compete against weeds and reduce the potential for problems in the future.
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, the cover crop will be most competitive if mowing can be avoided. 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 prevent emergence of weed seeds.
UC IPM Pest Management Guidelines: Grape
A. Shrestha, UC IPM Program, Kearney Agricultural Center, Parlier