Integrated Weed Management

Weed control in vineyards enhances the establishment of newly planted vines and improves the growth and yield of established vines. Growers have many weed management tools available to achieve these objectives, but the method in which these tools are utilized vary from year to year and from vineyard to vineyard.

Weed management is an integral part of an overall vineyard management system. Plants on the vineyard floor influence the presence of other pests such as vertebrates, insects, mites, nematodes, and diseases. A weed management program should start before new vines are planted. The more difficult-to-control weeds (particularly perennials) are easier to manage before vines are planted. Weeds reduce vine growth and yield by competing for water, nutrients, and sunlight. Competition from weeds is most severe during the first few years after planting in areas where vine root growth is limited due to shallow or compacted soil. Weeds growing around the trunk compete directly with vine growth and provide a good habitat for field mice or voles, which can girdle and kill young vines. Gophers are most often found in nontilled vineyards and are common where broadleaf weeds, such as field bindweed and perennial clovers, predominate. These subterranean animals feed on the roots and can weaken or kill young vines. Additionally, weeds that have dried out can become a serious fire hazard

The effect of competition from weeds is most severe in the first three years of grapevine establishment, where competition can reduce cane growth and delay fruit production. As the canopy develops in older vineyards, weed growth may be suppressed because less sunlight is striking the vineyard floor. In mature vineyards, weed growth can interfere with with harvest and cultural practices. For example, seed from mature weeds can contaminate raisins as they dry on trays in the field.

Integrated weed management practices vary considerably from vineyard to vineyard. Location in the state, climatic conditions, soils, weed species present, age of vines, irrigation practices, topography, and grower preferences significantly influence vineyard floor management decisions and the techniques and tools used.

In the strip under the vine row, weeds are commonly controlled either chemically or mechanically. The area between vine rows may be sprayed, mowed, or cultivated. Alternatively, mulches and flamers can be used to limit weed growth in vineyards. Use of subsurface irrigation will reduce weed growth during the warm season.

Soil characteristics play an important role in weed management. Soil texture and organic matter influence the composition of weed species present, the number and timing of cultivations required, and the activity and residual effects of herbicides applied. Sandy loam soils usually dry more quickly than clay soils and may require more frequent cultivation to achieve effective weed control. On light-textured soils, annual species, such as puncturevine, crabgrass, horseweed, and Panicum spp., and perennial weeds, such as johnsongrass, nutsedge, and bermudagrass, are more prevalent. On heavier-textured soils, perennials such as curly dock, field bindweed, and dallisgrass are commonly found. Many herbicide labels recommend using low rates of the product on soils considered high in sand or low in organic matter, in order to reduce the potential for vine injury as herbicides can move deeper in the profile. A second, or split application, may be necessary.

Weed growth is affected by the method of irrigation, amount of water applied, and the timing of rainfall received. Irrigation or precipitation also influences the frequency and timing of cultivation and the herbicides needed, as well as their residual soil activity. Frequent wetting promotes more rapid herbicide degradation in the soil. This degradation is generally faster in moist, warm soils than in dry, cold soils and also more rapid when drip emitters or micro-sprinklers, rather than furrow irrigation, are used. The first irrigation following an herbicide application is the most critical in determining how deep into the soil the herbicide moves; subsequent irrigation is less important to the movement of the herbicide.

When properly used, herbicides will usually control the weed species listed on their label. In most situations, combinations (tank-mixes) or sequential applications of herbicides will be required to provide effective, economical control. Before using any herbicide, identify the species of weeds to be controlled, then read and follow product label directions carefully.

Herbicides are traditionally discussed as belonging to two groups: those that are active against germinating weed seeds (preemergence herbicides) and those active on growing plants (postemergence herbicides). Some herbicides have pre- and postemergence activity. Herbicides vary in their ability to control different weed species. In most vineyards, herbicides are only used on a narrow strip centered on the vineyard row that comprises 15 to 30% of the total vineyard area. Check the SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL tables and consult product labels for specific weed control activity.

Preemergence herbicides are active in the soil against germinating weed seedlings. These herbicides are applied to bare soil and then moved into the soil with rain or irrigation where they affect germinating weed seeds. However, some herbicides like trifluralin, require mechanical incorporation. If herbicides remain on the soil surface without being activated within a reasonable amount of time through rain, irrigation, or mechanical incorporation, some will degrade rapidly from exposure to sunlight, resulting in reduced weed control. Large weed seeds, such as wild oat, may germinate in the soil below the herbicide zone and will not be controlled by a treatment.

Postemergence herbicides are applied to control weeds already growing in the vineyard. They can be applied alone as strip sprays, combined with preemergence herbicides early in the season, or be applied as spot treatments during the growing season. Protect young vines from contact with postemergence herbicide sprays. Be sure to check and follow individual label instructions.

Application equipment must be accurately calibrated to apply the proper amount of carrier and herbicide to the soil and young growing weeds. For safe application and to minimize drift, spray equipment should be equipped with a short boom that has nozzles designed to minimize the amount of very small spray particles generated. Avoid spray nozzles and spray pressures that produce spray droplets less than 200 microns in size (very fine and fine-sized droplets) as these tend to drift more. Nozzle technology has advanced significantly in recent years and all major nozzle manufacturers have developed one or more nozzles that will minimize drift. Check your nozzle catalog or the internet. Off center (OC) nozzles are often used alone or on the end of the boom to direct herbicide applications toward the center of the row. Some herbicides require special use precautions as indicated in the table below. Always read and follow the entire product label before using any pesticide.

A backpack sprayer or low-volume controlled droplet applicator can be used to effectively control weeds in small areas. This method works especially well for perennial weeds. Extreme care should be exercised to avoid drift of herbicides, such as glyphosate (Roundup), oxyfluorfen (GoalTender), and paraquat (Gramoxone SL2.0), to vine leaves, green stems, or suckers. Use a shroud or shield with this equipment to help prevent spray drift and protect the vines and other desired vegetation.

Several species of summer and winter annual and perennial weeds can be found infesting California vineyards. The total number and species of weeds, even within a vineyard, vary from area to area and year to year. Conduct weed surveys at least twice each year; once in late winter and again in late spring or summer. The results from these surveys will show the spectrum of weeds present and will help to determine the most effective control strategy.

Summer Weed Survey

A survey in summer will tell you the spectrum of weeds present and determine the effectiveness of herbicides or cultivation practices. Keep records of your weed surveys (example form—PDF) to track weed population information from year to year to better understand ongoing weed control problems such as perennial weeds or herbicide resistance.

Winter Weed Survey

By surveying weeds in late winter and keeping track of your observations (example form—PDF), you can identify any species that escaped control from earlier management. Surveying will help you determine if a change in herbicides or cultural methods is needed.

How to Survey Your Fields for Summer and Winter Weeds

• Survey your vineyard in late winter to identify winter annuals and again in summer after perennials and summer annuals have germinated.
• If you use cultivation for weed control, monitor far enough ahead of cultivation so weeds do not become too large and difficult to dislodge from the soil.
• Pay particular attention to perennials. Sketch a diagram of the vineyard and mark areas where perennials are found. A hand held GPS unit also works well for recording locations of perennials. Check for re-growth of perennials a few weeks after cultivation.
• Pay attention to low-lying areas or where water tends to accumulate. These are usually problem areas for weed growth.
• Survey areas around the vineyards as potential sources for wind disseminated weed seeds such as marestail, fleabane etc.
• Keep records of your survey results and control techniques used. By knowing what species are present, you will be able to make appropriate decisions on cultural and chemical controls.

Information collected over a period of years can help you determine shifts in weed populations and the effectiveness of your management operations.

Control annual and perennial weeds before planting a vineyard to reduce competition during vineyard establishment. It is especially important to control established stands of perennial weeds before grapevines are planted. This will also reduce potential injury to young vines from herbicides that would have been used to control these perennial after the vines are planted. Field bindweed, johnsongrass, dallisgrass, bermudagrass, and nutsedge are especially troublesome perennial weeds.

Nonchemical Controls

A cycle of cultivation and irrigation is an especially effective weed control method used before planting vines. First cultivate to remove existing weeds, this is followed by irrigation to encourage germination of new weeds, and concluded with a second cultivation to kill the newly emerged weed seedlings. Frequent cultivation lowers weed seed populations in the soil, thus reducing weed density. At least two cycles of cultivation, 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.

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 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 two years by treating with a high dose of glyphosate or by deep plowing, 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 is a nonpesticidalmethod of controlling soil-borne pests by placing clear plastic sheets on moist soil during periods of long day length. The plastic sheets trap the sun's radiant energy in the soil, heating the upper levels to temperatures (108–131°F at a depth of 2 inches) that kill many disease-causing organisms (pathogens), nematodes, and weed seedlings. In areas where summer fog is prevalent, solarization should be done during the warm late summer or early fall when foggy days are less frequent. Solarization leaves no toxic residues and can be easily used on a small or large scale. Soil solarization also improves soil structure and increases the availability of nitrogen (N) and other essential plant nutrients. (For additional information see Soil Solarization, (PDF) UC ANR Publication 21377.)

Chemical Control

Seedling and established annual weeds can be controlled with preemergence or postemergence herbicides before planting. Before planting a vineyard use a preemergence herbicide in conjunction with a rotation crop. Make sure the residual period of the herbicide is not so long that it will interfere with planting the vines. Postemergence herbicides generally have little or no soil residual and are safer to use before planting vines. Many growers prefer to use preemergence herbicides only after the vines have been planted and soil settled around the plants to avoid possible exposure of the vines' roots to herbicides that may be in the backfill soil. Follow all label precautions and directions, including requirements for protective equipment.

Grapevines are most sensitive to weed or cover crop competition during the first few years of growth and where vine rooting depth is limited. Weedy vineyards may require several more years to become economically productive than weed-free vineyards. Regardless of the method used to control weeds, damage to the vine trunk or roots by chemicals and mechanical equipment should be avoided. As grapevines become established, direct competition from weeds is lessened.

Cultivation

Some growers prefer to manage weeds without herbicides for up to 2 years after planting in order to prevent damage to vines. This usually requires hoeing, cultivating, or the usage of weed knives (set less than 2 inches deep around vines) several times during the spring and summer.

The area between the vine rows is usually mowed or cultivated. Weeds are most effectively controlled when they are in the seedling stage. To minimize injury, particularly when vines are young, hand tools are used to cultivate close to the vine. Mechanical cultivators available for use in the vine row include weed knives, spyder cultivators, and rotary tillers. Rotary tillers such as a Weed Badger, Kimco, or Clements Hoe are most effective if used on loose soil free of large rocks. Hand-held mechanical flails (Weed Eaters) may be used, but can injure vine trunks.

Discs or mowers can be used between the rows. Mechanical control of weeds must be done repeatedly when weeds are small. The equipment should be set to cut shallowly to minimize damage to vine roots. As weeds mature they become more difficult to control, may clog equipment, and will produce seed.

Cover Crops

Planted cover crops can be used to reduce weed populations between vine rows. The cover crop and management chosen for vineyards will differ from one area of the state to another. Select a cover crop that will not compete with young vines or decrease the cover crop stip width. Cultivation prior to planting a winter annual cover crop may reduce weed growth. Mowing cover crops at their recommended height will help preserve surface cover.

Mulches

Weeds in the vine row can be controlled using mulches. Organic mulches (cereal straw, green waste, composted wood chips) or synthetic mulches (polyethylene, polypropylene, or polyester) can be used around young vines. Apply mulches when the soil surface is free of weeds. Mulches prevent the germination and growth of weed seedlings by blocking light and preventing them from reaching the soil surface. They create a more uniform moisture condition, which promotes young vine growth. However, mulches may also provide a good habitat for gophers, voles, field mice, and snakes, and can be contaminated with weed seeds. Organic and some synthetic mulches do not control perennial weed growth unless all light can be completely excluded. Some woven fabric mulches offer excellent weed control for several years, but the initial cost of purchase and installation is high.

Herbicides

To control weeds after grapevines have been planted and before bearing fruit, apply a preemergence herbicide (e.g., oryzalin, napropamide, or oxyfluorfen) as a band down the vine row or in an area three to six feet around each vine. Herbicides can also be applied to control weeds after they emerge. Selective grass herbicides (e.g., sethoxydim, fluazifop, and clethodim) are available for the control of annual grasses and the suppression of perennial grasses. To be effective, these grass herbicides require the addition of a nonphytotoxic oil or a nonionic surfactant. These materials do not control nutsedge or broadleaf weeds and clethodim is the only selective grass herbicide that will control annual bluegrass. Paraquat can be used to control most weeds and grasses near young vines if they are protected with shields or wraps. The systemic herbicide glyphosate will control broadleaf weeds after emergence, but should only be used around mature vines that have brown bark. Glyphosate should not be allowed to contact leaves or green shoots as substantial crop injury can occur. Follow all label precautions and directions, including requirements for protective equipment.

Herbicide application in the vine row should be used in conjunction with mowing or cultivation between the rows. Mowing may be required four to eight times during spring and summer, or whenever weeds are 6 to 8 inches high. Cultivation is required after each irrigation, because irrigation causes weed seeds to germinate.

Under normal growing conditions it will take at least three years for a vineyard to become established. Established vines are more tolerant of many herbicides than are newly planted vines, thus increasing the options available for weed control. Weeds are generally controlled between vine rows with discing or mowing and in the vineyard row with a strip application of herbicide or a basal treatment of herbicide around each vine.

Cultivation

Cultivation can be used in established vineyards to control annual and biennial weeds and the seedlings of perennial weeds. Control seedlings of field bindweed, bermudagrass, and johnsongrass before they are three weeks old, which is before they begin to form perennial structures such as rhizomes. Cultivation to remove established perennials in an irrigated vineyard often increases the weed problem. Frequent cultivation near vines can injure vine roots or the trunk, which reduces the vine's ability to uptake nutrients and allows soil pathogens (crown gall and collar rot) access. Irrigation tubing must be suspended high enough off the soil surface to accommodate in-row cultivation.

Flaming

Flaming can be used to control young weeds in mature vineyards. Use either a single flame directed to the base of the vine or several burners on a boom to flame the weeds between the vineyard rows. For more on flaming see the section WEED MANAGEMENT IN ORGANIC VINEYARDS.

Mulches

Mulches can also be used for weed control as discussed in the section WEED MANAGEMENT IN NEW VINEYARDS. Organic mulches degrade and they may become a perfect growth medium for weed species such as common groundsel, prickly lettuce, common sowthistle, and tall annual willowherb. Escaped weeds should be managed with herbicides, flaming or hand removal and the mulch replenished annually.

Herbicides

After grape harvest, preemergence herbicides can be applied alone or in combination with other herbicides. This treatment can be split into two applications (fall and spring) or applied as a single treatment in the winter, where it is combined with a postemergence (foliar) herbicide to target weeds present in the vineyard. It may be necessary to use postemergence herbicides repeatedly as new weeds germinate. For greatest safety, direct the herbicide spray at the soil or weed foliage and not at the vine leaves, shoots, or less than 2-year old wood. Using a visual weed-seeking sprayer can reduce herbicide use if weed density is sparse or appear spotty.

Two or more herbicides may need to be combined in one spray application (tank mix) to achieve adequate weed control. It is critical to identify the weed species present in the vineyard to determine which herbicide, or combination of herbicides, will provide the most effective control. This is described in detail in the above MONITORING section. Combinations may include one or more preemergence herbicides or a mixture of preemergence and postemergence herbicides. Read and follow label directions carefully before combining herbicides, including requirements for protective equipment.

Cover Crops

Cover crops are often used to replace the resident weed vegetation found on the vineyard floor. As with resident vegetation, keep cover crops away from young vines. Winter annual cover crops are often fall-seeded cereal crops such as oat, cereal rye, or barley. Others, such as 'Blando' bromegrass, 'Zorro' fescue, or subterranean clovers, are commonly used in no-till vineyards. Cover crops are seeded into a prepared seedbed between vine rows in late September through mid-November. Most of these cover crops will reseed themselves if mowed in January or early February and, if allowed, will re-establish by April or May.

Where late frosts are a hazard, mow cover crops just before budbreak. If reseeding is desired, mow after the cover crop matures to greatly increase the number of seeds for the next season. Periodically changing cover crop species reduces the potential for buildup of disease pathogens, weeds, rodents, and insect pests. For more information on cover crops, consult UC ANR Publication 21471, Covercrops for California Agriculture, or UC ANR Publication 3338, Cover Cropping in Vineyards: A Growers Handbook.