Weed Management in Organic Kiwifruit Vineyards

It is especially important for organic vineyards to be as clean and weed-free as possible, since there are no organic herbicides with high efficacy available. Furthermore, weeds can serve as a hosts for pest and disease organisms and make an excellent habitat (especially grassy weeds) for destructive rodents. Perennial weeds cause the most problems, but some annual weeds, such as horseweed, which can grow among the vines to a height of six feet in one season, can be very troubling too.

Weed control in organically managed vineyards requires special attention to preventing weed problems before they start. Any method that reduces the amount of weed seed in the vineyard will diminish weed populations over time; therefore, one of the best ways to prevent weed problems is to control existing weeds before they go to seed. Cover crops or repeated mowing of weeds in row middles, coupled with mechanical control of weeds in the vine row, are key components of an organic weed management program.

Transitioning mature, full-canopied kiwifruit vines to organic production will require less intensive weed management than starting a new organic vineyard. Mature, shady vineyards often have limited weed growth when compared to newly planted vineyards, because the full vine canopy cover blocks sunlight for weed growth.

Weed management during the season before vines are planted is a critical practice for ensuring that young vines can become established with reduced competition from weeds. Two methods of managing weeds prior to planting are cultivation and soil solarization.

Cultivation

Repeated cycles of irrigation to germinate weeds, followed by cultivation to destroy young weeds, can reduce the amount of weed seed in vineyard soil. Do not work the soil when wet.

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 perennial weeds as well as annual weeds. 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 so deeply that they cannot germinate. Use a soil-inverting plow such as a Kverneland plow, because a standard moldboard plow will not sufficiently invert the soil. Avoid additional deep tillage operations, because they may bring buried weed seeds back to the surface.

Soil solarization

Soil solarization can significantly reduce weed populations in the planned vine rows. Soil solarization traps the sun's energy beneath a layer of clear plastic, increasing the temperature in the top 6 inches of soil to levels lethal to many weed seedlings as well as vegetative structures of perennial weeds. In areas like the Central Valley, soil solarization is used during summer months to heat the soil to more than 130ºF at a 3-inch depth.

The effect of solarization diminishes at greater depths and it does not control perennials as well as annuals. Seedlings of perennial weeds such as bermudagrass, johnsongrass, and field bindweed are controlled, but not their belowground vegetative structures at deeper depths in the soil. Yellow nutsedge is partially controlled while purple nutsedge is usually not significantly affected by solarization.

Effective soil solarization begins with preparing a smooth seed bed so that the plastic can be placed in direct contact with the soil surface. Disc the field to break up clods, smooth the soil, and remove any material such as rocks and weeds that will puncture or raise the plastic sheets.

Irrigate the area to be solarized either before or after applying the plastic, because wet soil conducts heat better than dry soil. If irrigating before the plastic film is laid, first allow the soil to dry somewhat to avoid compaction by heavy equipment, then immediately cover the soil with plastic. Alternatively, a drip system or microsprinkler line (with only the spaghetti tubing) may be installed on the soil surface and covered with plastic sheets to allow irrigation after the plastic was placed. 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.)

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 nutsedge or common purslane after planting the vineyard). Contact plastic suppliers well in advance so they can formulate plastic tailored to your needs.

Cover the planned vine row with plastic from 6 to 10 feet wide. The width depends on the management program planned for the row middles. 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.

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 can germinate.

Vine-Row Management

Manage weeds in the vine row with in-row cultivation, mowing, mulches, hand hoeing, or flaming. The choice of method depends in part on the type of irrigation system.

Furrow-irrigated vineyards

Furrow-irrigated vineyards are well suited for in-row cultivation. Cultivation is probably the most widely used method of weed control in organic systems. Other options include the use of organically approved herbicides, flaming, and the use of weeder geese.

Microsprinkler-irrigated vineyards

Few options are available in organic vineyards with microsprinklers. In-row cultivators may damage irrigation lines and emitters present on top of the soil surface. However, irrigation lines can be attached to the trellis with the microsprinklers facing upside down to allow for in-row mowing, cultivation, or flaming underneath. Hand hoeing, possibly flaming, organically approved herbicides, and weeder geese could also be used for weed management.

Plastic-coated cartons or wraps can help protect trunks of young vines from flame or herbicide injury; also keep flamers away from the plastic irrigation tubing. Mulches can suppress weed growth, but weeds that emerge through the mulch must be removed by hand hoeing or with organic herbicides.

Drip-irrigated vineyards

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, mowing, flaming, or mulches can all be used.

Middles Management

Similar to many conventionally managed vineyards, weeds in the row middles of organic kiwifruit vineyards are commonly managed with cover crops, tillage, and mowing.

Consider planting a cover crop in the area between vine rows, because resident vegetation does not usually grow uniformly enough to compete well with newly invading weeds. In addition, resident vegetation includes weed species that can colonize the vine row. An annual cover crop that reseeds itself will compete against weeds and reduce the potential for problems in the future. Do not plant cover crops under vines because excess competition may occur, possibly reducing kiwifruit yields.

If there is a potential for frost and the cover crop is tall, mow once before the cover crop blooms 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 with a ringroller. The ringroller will allow more seed production and also create a surface mulch that will help to prevent emergence of weed seedlings.

Cultivation

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 where they can germinate.

Perennial weeds with established root systems are difficult to kill with a single tillage operation, because only the top of the plant is removed and established plants can often generate a new top using underground energy reserves. However, repeated cycles of tilling 3 or 4 inches deep and subsequent regrowth can reduce the plant's stored reserves, and eventually suppression of these weeds can be achieved.

In-row cultivation

In-row cultivators are equipped with a sensor or trigger mechanism that pivots the cutting arm around the vine to minimize injury. Several companies make cultivation equipment; those that have performed well include equipment from Bezzerides, Kimco, and L&H Manufacturing.

In-row cultivation is amenable in furrow-irrigated vineyards. Some precautions to take with other irrigations systems include:

• In sprinkler-irrigated vineyards, take 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 from the vine.
• Protect microsprinkler irrigation lines and emitters from damage by suspending the surface lines in the vines or on stakes with the microsprinklers positioned upside down.
• Bury drip lines to avoid damage. This may also help to inhibit weed seed germination, as the top few inches of the soil will stay dry. This effect, however, will depend upon the depth of burial. Shallow drip-line burial that provides water to the top 3 inches of soil may not reduce weed emergence.

Mulches

Mulches can help with weed management in the organic vineyard by blocking light and preventing weed germination or growth. Many materials can be used as mulches including municipal yard waste, wood chips, straw, hay, sawdust, newspaper, and others approved for use in organic agriculture.

To be effective, mulches need to block all light to the weeds; therefore different mulch materials vary in the depth necessary to accomplish this. Mulches, such as yard waste, wood chips, straw, or hay must be maintained in a layer four or more inches thick. Organic mulches break down with time and the original thickness typically is reduced by 60% after one year. Thus, regular additions are needed to maintain weed control.

Another option is to combine weed control in the row middles with management efforts in the vine-row. This is known as "mow-and-throw": cover crops grown in the row middles are mowed during the spring and placed into the vine row to serve as a mulch around the base of the vines. Weeds that emerge through the mulch can be controlled using an organic contact herbicide or with hand hoeing.

Organic Herbicides

Several approved contact herbicides are available to use in organically certified vineyards, but these materials are costly and tend to be less effective than synthetic herbicides. Check with the organic licensing organization to determine current status and any use restrictions for organically acceptable herbicides.

These oil-based products (clove oil = Matratec; clove oil + cinnamon oil = WeedZap; lemongrass oil = GreenMatch EX) damage green vegetation, including the leaves and young stems of kiwifruit vines. Apply these products directly to weed foliage.

Because these herbicides only kill contacted tissue, good coverage is essential and most effective control will be obtained when weeds are small. Best results have occurred when spray volumes are 60 gallons per acre or more and applications are made during warm, sunny conditions. Adding an organically acceptable adjuvant is also recommended. Because these materials lack residual activity, repeated applications will be needed to control each new flush of weeds.

Flaming

Flamers can be used for weed control in the vine row, with propane-fueled models being most common. Heat causes the cell sap of plants to rapidly expand, rupturing the cell walls. This process occurs in most plant tissues at about 130ºF.

It is not necessary to "burn" the leaf tissue. 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 thumbprint 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. Weeds should have less than four true leaves for greatest efficiency with flaming. Grasses are harder to kill than broadleaf weeds, because the growing point of grasses is at or below the ground. When flaming is used repeatedly, grasses will eventually dominate the vineyard, while perennial weeds such as field bindweed are suppressed.

Protect the trunks of young vines from flamers to avoid injury to the cambium layer, and keep flamers away from plastic irrigation tubing. Do not flame in vineyards with a lot of dried vegetation, in order to avoid fires that can injure vines and irrigations systems and spread beyond the boundaries of the vineyard.

Weeding animals

Before using any animals, check federal, state, and local food safety regulations and comply with them. For further information on grazing animals, please consult the following websites: Targeted Grazing (University of Idaho) and Weeding With Geese (University of Missouri).

Weeder geese

Geese can often be used to manage grass weeds in vineyards. Geese prefer grass species and will only eat other weeds and crops 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 these weeds can be especially troublesome in vineyards.

Generally, about four 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.

Other animals

Sheep and goats are sometimes used in organic vineyards as well. 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 vines are dormant and the chance of grazing damage is minimal.