Weeds can directly compete with grapevines for water, nutrients, and light. Weeds may also impact vines indirectly, by serving as alternate hosts for insect pests and pathogens; providing habitat for rodents; altering the distribution of irrigation water; interfering with the deposition of pesticides; and impeding the movement of workers and equipment. Successful weed management can be achieved by employing a combination of strategies, which includes the use of herbicides, to eliminate unwanted vegetation from vineyards. Herbicides, however, act upon different weed species and in different ways; care must be taken to ensure that the selected products will be effective at controlling the weed species that are present in the system. Herbicides are also capable of causing serious injury or death of vines and/or may have significant unintended consequences on neighboring ecosystems. Consequently, applicators must ensure that an acceptable level of crop safety can be achieved and that off-site/off-target damage is prevented.
The most appropriate choice of herbicide (or combination of herbicides) will be affected by numerous factors including:
- mode of action of the herbicide,
- the kind of weeds to be controlled,
- the size and/or age of the weeds to be controlled,
- soil type and herbicide incorporation strategy,
- the quantity and quality of the spray water,
- the age and health of the vines.
Application Rate and Equipment
The rate or amount of herbicide to be applied is shown as the amount per acre of the actual portion treated (i.e., the area directly under the vine row, or the area between vineyard rows), and not the total herbicide to be applied to an acre of vineyard. Thus, a 10-foot row spacing with a 2-foot band of herbicide sprayed under the vines would be two-tenths (one-fifth) of an acre to be treated or 20% of one acre of vineyard.
To reduce the hazard of injury to vines as well as to ensure the maximum effectiveness of the herbicides applied, select your equipment and its use carefully. Herbicides need to be applied with a fixed boomtype applicator equipped with flat, fan-type nozzles. The height of the boom depends on the height of weeds, the nozzles, and their spacing. Take care to ensure a uniform spray pattern and, thereby, a uniform herbicide deposit. Similarly, the speed of travel must be closely regulated. It is limited not only by safety, but also by the capacity of the pump. Regulating the pressure does not sufficiently alter the output. In general, use speeds of around 1.5 to 2 miles per hour.
When using standard flat fan nozzles with droplets in the ‘fine’ category, use low pressures (20 to 35 psi) to ensure coverage and penetration while minimizing spray drift. If using anti-drift nozzles, also called air induction nozzles, the pressure will have to be higher (40 to 50 psi) to achieve the correct gallons per minute from the nozzle. This higher rate is acceptable with an air induction nozzle because the droplet size is larger (coarse or very coarse) and will not drift as much. With either nozzle choice, be sure to check the nozzle catalog to verify pressure with the desired gallons per minute.
NOTE: With glyphosate or any broadleaf systemic herbicide, reducing drift is critical. Therefore, reduce pressure to 20psi or less when using standard flat fan nozzles or use air induction nozzles that produce a very large droplet.
Constant agitation of the spray mixture is essential with wettable powders and dry flowables, and agitation must be vigorous when using oils. A defoaming agent can be used. Follow quantity and directions on label.
General Precautions
- Do not apply any herbicide to grapes unless there is a label registration for its use on grapes.
- Check all herbicide and pesticide uses with the processor before using.
- Do not use a combination of herbicides or other chemicals with herbicides unless the combination has been thoroughly tested and confirmed to not cause phytotoxic effects.
- Avoid herbicide application to any part of the vine. Do not use weed sprayers to apply other pesticides to the canopy.
- Do not repeatedly use the same mode of action (MOA) throughout the season (Table 1). The Herbicide Resistance Action Committee (HRAC) codes all chemicals based on the MOA. Using different HRAC codes or MOAs is part of a good herbicide resistant management program. For more information on controlling resistant weeds, see Herbicide Resistant Weeds and Their Management (PNW437).
Factors Affecting Control
Classification of Herbicides
Herbicides differ with respect to how they are used. When describing herbicides, we can refer to them as being either ‘pre-emergence’ or ‘post-emergence’ applied. Pre-emergence herbicides are soil-applied products that act primarily on germinating weed seeds or young weed seedlings. They persist in the environment to provide extended weed control within or across seasons.
Products classified as post-emergence herbicides are applied directly to the foliage of emerged weeds to elicit control. Post-emergence herbicides can be further characterized as being either ‘contact’ or ‘systemic’ products. Contact generally refers to herbicides that only affect the tissues that are directly treated with the herbicide-these types of herbicides do not move (translocate) to untreated parts of the plant following application. Systemic or translocated herbicides can move from treated plant parts to untreated tissues via the xylem or phloem. Although some herbicides may exhibit both pre- and post-emergence activity, a combination of both foliar and soil-applied products are typically used to successfully manage weed communities in vineyards.
Often, herbicides are referred to as being either ‘selective’ or ‘non-selective’ (which is synonymous with ‘broad-spectrum’). A selective herbicide is one that is effective at controlling some species but not others. Conversely, a non-selective herbicide is an herbicide that can control many different types of weedy pests. The most well-known examples of herbicide selectivity are: 1) the abilities of the ‘-fops’ and the ‘-dims’ (e.g. fluazifop and clethodim) to suppress grasses but not broadleaf species; and 2) the control of broadleaved weeds, but not grasses, by synthetic auxins (e.g. 2,4-D).
Kinds of Weeds to be Controlled
Weed species can be described by the length of their life cycle. Annual weeds (both winter-and summer-germinating) emerge, grow, flower, and set seed all within the course of a year. Biennial weeds complete their life cycles over the course of two years whereas perennials can persist across multiple seasons. Not all herbicides are equally effective against all three types of weed species. For example, although the seedlings of perennial weeds may be controlled by pre-emergence herbicides, mature plants are unlikely to be impacted.
It is possible for weed communities in vineyards to be comprised of species that are naturally sensitive to different herbicides or herbicide modes of action. They may also differ with respect to life history traits or emergence patterns. Herbicide labels list the weed species that can be suppressed or controlled by the active ingredient. The labels will also provide instructions on when to time applications to maximize herbicide efficacy and will also list appropriate tank-mix partners to help growers expand the spectrum of weed control. Always read labels to ensure that herbicide applications will be both effective and safe.
For help on weed identification, see Invasive Weeds of Eastern WA (EM005) and visit: https://wine.wsu.edu/extension/pest-management/. In addition, Washington State University Extension also provides a free Weed Identification Service. Information on this service can be found at: https://css.wsu.edu/extension/weed-identification/.
The Size and/or Age of the Weed to be Controlled
Weed control strategies may not always be 100% effective and escapes can occur for numerous reasons. One of those reasons is the size of plants at the time of application. The efficacy of post-emergence herbicides is often diminished when products are applied to large/mature plants. This can result from poor spray coverage and the ability of dense foliage to shield sensitive tissue from herbicide deposition. While plant size is mainly a concern with contact herbicides, the efficacy of systemic products can also be influenced. For example, may perennial species are tolerant of many herbicides, including translocated products like glyphosate, because their root systems and nutrient reserves support regeneration/regrowth. There are several strategies that growers can take to maximize weed control with post-emergence herbicides, including: timing applications to treat weeds while they are small/tender, applying herbicides at appropriate rates and volumes, and using label recommended adjuvants to improve herbicide contact and penetration.
The development of herbicide resistance is a significant concern for growers of perennial crops, including grapes. Weed species with resistance to glyphosate, glufosinate, and paraquat have been confirmed in California and Oregon as well as other Western states. Incomplete weed control can increase the chance of a herbicide resistant biotype reaching reproductive maturity, setting seed, and becoming established in a production system. Herbicide labels will provide instructions to applicators regarding strategies for resistance management. Additional information can be found at the Weed Science Society of America’s (WSSA) Resistance webpage.
Soil Type and Herbicide Incorporation Strategy
The length of time that pre-emergence herbicides may reside in a treated area will be influenced by multiple factors, such as soil texture and organic matter content. Soils that are high in clay or organic matter can bind herbicides tightly to the soil matrix. Conversely, coarse soils can enhance leaching potential. Herbicide persistence is also a function of herbicide chemistry. Some herbicides naturally bind very tightly to soil particles whereas others are significantly more mobile. The interactions between soil and herbicide chemistry can affect a product’s use rate within a given system. This, in turn, may influence how well a herbicide performs, as well as how long it remains in the treated zone.
To be effective, pre-emergence herbicides must be incorporated (usually 1-2″ deep) into the soil profile. Incorporation is required because these products are mostly active against newly germinated weed seedlings. Additionally, incorporation is needed to reduce or prevent volatilization and photodegradation, which can result in reduced herbicide performance. The length of time an herbicide can remain on the soil surface varies dramatically among products. Even if an herbicide does not require immediate activation to prevent product loss, an unincorporated herbicide is unable to control emerging weeds. While many growers will time herbicide applications to take advantage of naturally occurring rainfall, uncooperative weather may require the use of irrigation for activation. Some products, due to their mobility and potential for crop injury, may be incompatible with sprinkler systems; this is of particular concern when grapes are grown on shallow, coarse, sandy, or gravelly soils.
Check pre-emergence herbicide labels carefully to ensure that the product is being applied at the proper time of year to effectively target the weed species of concern in a vineyard. Apply and incorporate products as described to prevent crop injury; this includes following timing recommendations to avoid treating vineyards when and if significant crop damage could occur.
The Quantity and Quality of the Spray Water
Water is the main carrier for crop protection products, including herbicides. Consequently, the quantity and quality of spray water can impact herbicide distribution and performance. In general, higher carrier volumes have been shown to improve the efficacy of foliar-applied herbicides (glyphosate is a notable exception). Post-emergence herbicide labels provide instructions regarding the recommended application parameters (i.e. GPA, pressure, droplet size) to maximize weed control potential. Carrier volume can also affect the delivery of soil-applied products; always read the herbicide label to ensure that all herbicides are used both effectively and safely.
Water quality factors can also affect herbicide performance and safety. For example, herbicides that are weak acids (such as glyphosate) can have their efficacy reduced under alkaline conditions (pH >7). Salts and soil particles in spray water can bind to some herbicides, thereby affecting their dispersal, deposition, and uptake. Water quality recommendations exist for both pre- and post-emergence herbicides; read labels carefully.
The Age and Health of the Vines
Vine age can affect what herbicides are available for use in a production system. While competition from weeds is most severe in newly planted grapes, not all herbicides are labeled for use around young canes. Growers should be aware of re-plants in mature vineyards before making soil-applied treatments. Regardless of vine age, residual herbicides should be applied to soil that is settled and free of cracks to minimize the potential for crop injury. Avoid using herbicide-treated soil to backfill planting holes. Unless specifically noted on the label, avoid herbicide spray or mist contact with leaves, green bark, roots, or fruit to reduce the potential for crop injury.
Crop safety is paramount; however, the safety of species and habitats outside of the treated area are also a significant concern. Follow label instructions to reduce the potential of spray or volatility drift and subsequent damage to off-target organisms. Surface and groundwater advisories and buffer zone recommendations are included on several herbicide labels; read and follow all labeled guidelines to prevent contamination of aquatic systems.