If you choose not to use synthetic pesticides, then your goal should be to maintain the vineyard as clean as possible. Weeds serve as an excellent host for insect pests fungal pathogens and should be routinely controlled by all grape growers. Vineyards differ from orchards in that the foliage and fruit in vineyards are typically much closer to the ground.
While perennial weeds are definitely considered more of a problem, annual weeds, such as horseweed (marestail) (Conyza canadensis), are capable of growing within the vinerow up to the clusters to a height of six feet in a single season and serve as a host for disease. Weed control in organically managed vineyards requires special attention to prevent weed problems before they start. Cover crops planted in the row middles and mechanical control of weeds in the vine rows are key components of an organic weed management program. In some cases, particularly where the weed population is high, it may be desirable to use conventional herbicides for the first couple of years after planting and then transition into organic production. This usually helps to reduce weed pressure, making weed management easier once organic certification has been obtained.
Weed Management Before Planting
It is important to have little or no weed competition at the time of planting vines. This makes weed control before planting critical. Take measures to deplete the soil weed seed bank. A summer fallow treatment of irrigation (to encourage weed seed germination) followed by tillage (to uproot newly emerged weed seedlings) will desiccate weeds and reduce the number of weed seeds in the soil. Repeat this cycle several times to further deplete weed seeds in the soil. Alternatively, weed seeds located in the top four inches of soil can be buried with a soil-inverting plow such as a Kverneland plow to depths from which seeds cannot emerge. Here, the soil should not be deep-plowed again for at least three years to help prevent bringing up viable seed. A moldboard plow will not sufficiently invert the soil to be effective.
Soil solarization can be used in the area planned for vine rows to significantly reduce weed populations. The soil in the area designated for solarization must be moist and at least six feet wide in each row. The plastic should be buried 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 areas where summer fog is not a concern, solarization should begin when day length is as long as possible, or at least be started by the beginning of August to have sufficient time (4 to 6 weeks) to complete the process. Use clear plastic with a UV inhibiting component to prevent breakdown before the process is completed. Black plastic suppresses weed seed germination but will not heat the soil sufficiently for solarization. Solarization may not be successful in suppressing species like nutsedge. The plastic should be removed before planting the vines.
Weed Management after Planting
Mechanical cultivation uproots or buries weeds. Weed burial works best on small weeds. 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 grape 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. For tillage to be successful on perennials, the top should be removed by cultivating to a depth of 3-4 inches. This will cause the underground portion of the plant to regenerate a new top, forcing the weed to use a greater portion of the reserves available. Repeated cultivation may eventually kill these weeds by eliminating the amount of reserves available for growth. Trip mechanisms on under vine vineyard cultivators are often used to prevent damage to the vines. These mechanisms move the knife or cutting blade before it hits the vine. 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 be used for weed control in the vineyard. Mulches block 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 germinating weeds. Materials vary in the depth necessary to accomplish this. In general, the larger or looser the mulch pieces, the deeper the mulch needs to be. Organic mulches must be maintained in a layer at least four inches thick. Organic mulches breakdown with time and the original thickness is typically reduced by 60% after one year. Winter cover crops grown in vineyard middles can be the source of organic mulch. The process, known as "mow-and-throw" or ‘mow and blow', involves cutting the cover crop and moving (‘throwing' or ‘blowing') it in the vine row to the base of the vines. Weeds that emerge through the mulch can be controlled using an organic contact herbicide or with hand hoeing. Cover crops planted under the vine row may compete with weeds, but they may also increase competition with the vine, possibly reducing grape yields. Mulches may harbor voles, gophers, and field mice, which feed on vine trunks and roots.
Several contact-type herbicide products are approved for use in organic vineyards. Many of these products can damage any green vegetation contacted, including the leaves and young stems of grape vines. Apply products as directed sprays to the base of woody stems and trunks. These herbicides only kill plant tissue that they contact; so good coverage (60 or more gallons per acre spray volume) is essential. Adding an organically acceptable surfactant is recommended. These materials all lack residual activity and repeated applications will be needed to control new flushes of weeds. These products are more effective on broadleaved weeds than grasses. Follow all label precautions and directions, including requirements for protective equipment.
Flamers can be used for weed control in the vine row. Propane-fueled models are the most commonly used. Flaming works on the principle that heat causes the sap of plant cells to expand, causing the cell membrane to rupture. This process occurs in most plant tissues at about 130ºF. To work effectively, weeds must have less than two true leaves. Flaming is usually less effective on grasses because their growing point is at or below ground level. Weeds that have been killed by flaming change from a glossy to a matte or dull finish. This occurs very rapidly in most cases. Foliage that retains a thumbprint when pressed between thumb and finger has been adequately flamed; it is not necessary to burn the plants. Typically, flaming can be done at 3 to 5 mph, depending on the heat output of the unit. Avoid flaming during windy conditions, as wind can displace the flame, resulting in poor weed control or vine injury. Repeated flaming can be used to suppress perennial weeds such as field bindweed. Care must be taken to avoid igniting dry vegetation, which could not only injure the vines, but start a wildfire.
Before using any animals for weed suppression in vineyards, check federal, state, and local food safety regulations and comply with them.
Sheep are often used for weed control in organic vineyards 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 browsers, and are often used to control the brush around vineyards. If goats are used in the vineyard, they must be carefully managed to avoid damage to the vines. Unless the animals used have been otherwise trained, goats and sheep should be removed before bud break to reduce the chance of damage to young shoots.
Geese can also be used in vineyards. 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 vineyard weeds, johnsongrass and bermudagrass; geese will dig them up and eat them if confined. Generally, four geese per acre are needed. Consult the following website for further information on geese http://www.metzerfarms.com/UsingWeederGeese.cfm. In most cases, all animals used in vineyards will require some form of protection from predators (dogs, coyotes, etc.).
Vineyard Irrigation –Special Considerations:
While some vineyards are not irrigated (dry-farmed or rain-fed), most are irrigated in one form or another. Vineyards that are flood or furrow irrigated are amenable to most of the previously described forms of organic weed control. When microsprinklers or drip irrigation is used, some considerations must be made when choosing a weed control method. In-row cultivators may damage irrigation lines and emitters. However, surface lines can be suspended in the vines or on stakes to allow for in-row mowing, cultivation, or flaming underneath. If the microsprinklers are suspended upside down, hand hoeing, possibly flaming, organically approved herbicides, and weeder geese could also be used for weed control.