Managing Weeds in Grapes in New Mexico

Guide H-334

Revised by Leslie Beck and William “Gill” Giese

College of Agricultural, Consumer and Environmental Sciences, New Mexico State University

Authors: Respectively, Extension Weed Specialist, Department of Extension Plant Sciences; and Extension Viticulture Specialist, Department of Extension Plant Sciences/Agricultural Science Center at Los Lunas, New Mexico State University. (Print Friendly PDF)

Introduction

Weeds compete with grapevines for resources such as water, nutrients, and light. However, the intensity of this competition varies during the lifecycle of the grapevines, and is more impactful during the early stages of vineyard establishment. This significant competition from weeds in the early stages of vineyard life is due to limited root development and vegetative growth of the vines. Weeds’ competition with vines for water and nutrients is most severe when vines’ root systems are shallow and not well developed. At the same time, newly planted vines, due to their smaller size and transplant shock, can quickly be overgrown by tall, fast-growing weeds such as Palmer amaranth (Amaranthus palmeri), kochia (Kochia scoparia), Johnsongrass (Sorghum halepense), and many others, which impedes the development of young vines. Some weed species can also directly reduce the vines’ growth and development by releasing growth-inhibiting allelopathic chemicals. Additionally, weeds with vine-like growth, such as field bindweed (Convolvulus arvensis) and morningglory (Ipomoea spp.), can cause physical damage to grape plants as they climb, wrap around, and ultimately strangle the grape leaves and fruit (Figure 1). Weeds can also indirectly affect the grapevine by serving as an alternative host for insects and diseases. Furthermore, weeds that grow around the grapevine trunk can provide a habitat for rodents that can damage the trunks and/or the roots, thus further reducing the healthy growth and production of vines. Most vineyards also depend on revenue from tours, wine tastings, and events that are dependent on pleasing aesthetics, which is diminished by the presence of weeds. As a result, successful weed management in vineyards will significantly improve grapevine establishment and grape production, while likely enhancing marketing efforts.

Figure 1. Field bindweed (Convolvulus arvensis) growing in vine rows and over a young grapevine.

Because weed management techniques are often species-specific, the first step to successful weed management is to correctly identify weeds, and to keep an up-to-date record of weeds that are present in the vineyard from season to season. Weeds can be identified using resources such as weed identification books, websites, Cooperative Extension Service publications, Extension personnel, and crop consultants. While it is important to know the name of a weed, it is crucial to understand as much about the lifecycle and reproductive mode and capacity of that specific plant, in order to make sound and effective management decisions. Keep in mind that common names of plants can vary, or the same name may refer to different species across regions and communities. Cross-referencing weeds by their scientific names, which are universally the same for that plant, can help confirm the correct identity of the weeds. Accurate information on the biology and lifecycle of the weeds can help growers apply weed management techniques more effectively, based on the timing of susceptibility of the species. It is important to know that weeds are often more susceptible to certain management techniques during specific stages of their development, such as when they are young and/or actively growing. All weeds, including grasses, sedges, and broadleaves, are categorized based on their lifecycle (a process including germination, vegetative growth, flowering, seed set, and death) into annuals, biennials, and perennials. Some of the common weeds found in New Mexico agricultural fields and their lifecycles are listed in Table 1.

Integrated Weed Management

A weed management program is simply a planned schedule of activities based on the biology of the target weeds, the age of the vineyard, and other economic and environmental considerations. Weed management is a process that should start before new vines are planted and continue throughout the life and production of a vineyard. The best weed management practices in vineyards are achieved by combining preventive, cultural, mechanical, and chemical methods, which are the foundation for an integrated weed management program. Using a single weed management method continuously—whether it is cultural, mechanical, or chemical—could cause a shift in the weed populations toward weed species that can resist that method, rendering it ineffective over time. Therefore, integrating varied control methods into the management program is necessary for the sustainability of weed management and grapevine performance. The goal in integrated weed management is not only to control the existing weed infestations but also to prevent any future infestations. Integrated weed management strategies could vary depending on the age of

the vines, soil conditions, weed spectrum, climatic conditions, irrigation practices, and a grower’s management preferences. Designing effective integrated weed management strategies requires a thorough understanding of the weed management methods, the biology of the target weed, and the various viticultural practices.

Site Preparation

At sites that are infested with hard-to-control weeds (e.g., bermudagrass, field bindweed, Johnsongrass, purple and yellow nutsedge, silverleaf nightshade, and Texas blueweed), efforts must be focused on controlling these hard-to-control weeds prior to planting the vineyard.

In sites that are heavily infested with annual weeds, tillage or cultivation can be used for control. Because cultivation stimulates germination of weed seeds, growers can reduce the existing populations and stimulate seed germination, controlling the young weed seedlings soon after emergence with more cultivation or herbicides. Depending on the level of infestation, this strategy could be repeated several times to reduce the weed seed bank as much as possible prior to planting the vineyard.

Cultural Control

Using ground cover vegetation or mulches can reduce the weed density in vineyards. Although weed densities can be reduced by maintaining ground cover vegetation between vine rows, the cover vegetation—if not managed properly—can also compete with the grapevines, especially during vineyard establishment. Based on observations in New Mexico, a weed-free strip of 24 inches wide along the vine row (12 inches on either side of the plant) may be sufficient to keep competition between the young vine and ground cover vegetation under control. Under optimal conditions, it usually takes three years for a vineyard to be considered an established vineyard. Recommended ground cover vegetation that could tolerate mowing, flooding, and traffic falls into two categories: grasses (either resident or established) and perennial nitrogen-fixing legumes (examples include clover and vetch species). In areas where southern root-knot nematodes (Meloidogyne incognita) are a concern, care must be taken to select ground cover vegetation that is not a host for this pest. Maintaining ground cover vegetation in vineyards can also minimize soil erosion, increase soil organic matter and beneficial microbial populations, decrease water/irrigation loss through evaporation, and improve equipment and personnel access during wet conditions (Figure 2).

Figure 2. Mature vineyard with residential ground cover vegetation. Note: Weeds along the vine rows are controlled by application of a post-emergence herbicide.

Organic or synthetic mulches can also be effective for weed management in vineyards. Mulches can prevent weed seed germination by preventing sunlight from reaching the soil surface. Therefore, the effectiveness of mulches is directly dependent on the timing of mulch application and the thickness of the mulch layer. If the mulch is applied after the weed seeds are germinated, the weed control effect will be minimal. If the mulch layer on the vineyard floor is not thick enough (which could vary depending on the type of mulch), sunlight will penetrate the layer and weed seeds will continue to germinate. It is important to understand that mulches, even when applied and maintained correctly, will not prevent the regrowth of perennial weeds in the vineyards, and additional weed control methods (e.g., herbicides) must therefore be implemented to control perennial weeds. Mulches also enhance the moisture conditions of the vineyard floors, which is especially important in improving the growth of newly planted vines.

Mechanical Control

Mechanical weed control is best described as a nonselective control option that is particularly effective against annual weeds (Figure 3). Mechanical control is physical weed removal by tools such as hoes, disks, cultivators, mowers, hands, etc. These devices are designed to damage, uproot, or cut weed seedlings. The effectiveness of mechanical tools varies depending on the weed size and weed growth type. Cultivation and hoeing are best accomplished when weeds are in the seedling stage (<3 inches) since it becomes more difficult to cultivate or hoe as weeds get bigger.

Figure 3. Mature vineyard with no ground cover vegetation. Note: Weeds under the vine rows and between the vine rows are controlled by mechanical means.

Excessive cultivation can lead to undesirable consequences such as soil erosion, reduced soil organic matter, and breakdown in soil structure that results in compaction and reduced rain/irrigation permeability. Also, if not done carefully, cultivation could injure vine roots. Cultivated soils can also restrict equipment access for vineyard operations; therefore, it is important to avoid excessive cultivation events, and to time this practice to limit the possible negative effects of cultivation as much as possible. Mechanical control of weeds in between the rows can be achieved using shallow cultivators or mowers. However, mechanical weed management in the vine rows requires specialty tillage equipment (examples include weed badger, French vineyard plough, or Clemens radius) to minimize potential damage to the vines. Additionally, mowing practices will not be effective in controlling small weed seedlings or perennial weeds with prostrate growth habits since any plant structures located below the mowing height will most likely survive and recover over time.

Prescribed burning is a method that utilizes direct flames and/or intense heat to kill young weeds. It is most effective when used in the seedling stage of young annual weeds (Figure 4). However, caution must be taken to avoid flaming the vines, irrigation tubing, and other fire hazards when using this method for weed management.

Figure 4. Using a propane flame weeder to control weeds in the vine row.

Chemical Control

Herbicides are the most commonly used weed management tool in most cropping systems, including vineyards. Various types of herbicides are registered for use in New Mexico vineyards (Table 2). Herbicides vary greatly in their target species and sites of action, so it is important to select the appropriate herbicide for the conditions and weeds present. The following are the terms describing the times at which herbicides may be applied, and they refer to the stage of development of the weeds.

Pre-emergence herbicides are generally applied to the soil prior to weed emergence for residual weed control, and may require incorporation by either mechanical tillage or water (rainfall or irrigation). However, some herbicides, such as diuron (Karmex DF) and flumioxazin (Chateau), have both pre-emergence and post-emergence activities that can control small seedlings of certain weed species as well as provide residual weed control.

Post-emergence herbicides, such as clethodim (Select 2EC), glufosinate-ammonium (Rely 280), glyphosate (Roundup PowerMAX), carfentrazone-ethyl (Aim EC), and paraquat dichloride (Gramoxone), are all applied after weed emergence. However, the method of application could vary depending on the herbicide (see Table 2 for more details).

Particular herbicides may be labeled for non-bearing vines only, bearing vines only, or for both bearing and non-bearing vines. Non-bearing vines are classified as plants that will not bear fruit for at least one year following application of a particular herbicide.

When using chemical control, growers must be aware that repeated use of a single herbicide active ingredient, or active ingredients within a particular herbicide group with the same site of action, could select for herbicide-resistant weeds. To avoid selecting for resistant weeds, make sure to rotate herbicides with different groups or sites of action, and do not make more than two consecutive applications of herbicides from the same group against the same weed.

Herbicides kill plants by binding to a specific protein and inhibiting that protein’s function. The binding site of an herbicide is referred to as the herbicide’s site of action. Herbicide grouping is based on the site of action of herbicide; therefore, herbicides with the same site of action are assigned the same group number. Since no single herbicide controls all weeds commonly found in vineyards, tank-mixing two or more herbicides from different herbicide groups can delay or prevent the development of herbicide resistance in weeds and dramatically increase the spectrum of weed control. However, it is important to follow the label directions on tank-mixing different herbicides since some herbicides may not be compatible with each other. If such incompatible herbicides are mixed, the efficacy of one or both partners in the mixture can be reduced. When using herbicides, it is important to use proper equipment and calibrate the equipment regularly. A wide variety of equipment, ranging from a simple backpack sprayer to units with optical controllers for precise application, is available for herbicide application in vineyards.

A list of currently registered herbicides for vineyards in New Mexico, their Weed Science Society of America (WSSA) grouping, and some information regarding their usage is provided in Table 2. Be sure to read, understand, and follow the label instructions when using any pesticide. Pay particular attention to information such as required personal protective equipment (PPE), timing of application, rates of application, harvest restrictions, vineyard characteristics (e.g., soil type), restrictions, and types of weeds controlled. Many times the poor performance or non-performance of an herbicide can be traced to improper use and failure to follow label directions. Because some herbicides have long residual activities, it is important to obtain the history of herbicides used in the field before planting vines; this will help to reduce the risk of herbicide injury to vines due to carryover of previously used herbicides.

For Further Reading

H-303: Pruning Grapes to the Four-arm Kniffin System
pubs.nmsu.edu/_h/H303/

H-309: Grape Varieties for North-central New Mexico
pubs.nmsu.edu/_h/H309/

H-331: Trellis End Post Assembly Designs for Vineyards
pubs.nmsu.edu/_h/H331/

H-338: Grapevine Phenology: Annual Growth and Development
pubs.nmsu.edu/_h/H338/

Original authors: Jamshid Ashigh, Extension Weed Spcialst/Assistant Professor; Bernd Maier, Extension Viticulture Specialist; and Carry Hamilton, Program Manager.

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Leslie Beck is the Extension Weed Specialist at New Mexico State University. Her research program focuses on integrated weed management systems in landscape, horticultural, and agronomic crops. Her Extension work provides the citizens of New Mexico with accurate, applicable, and effective weed identification and management strategies in multiple cropping systems.

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Gill Giese is the Extension Viticulture Specialist and Assistant Professor at NMSU. He earned his Ph.D. at Virginia Tech, and has worked as a commercial winemaker and viticulture/enology instructor. His applied research and Extension work focuses on variety/rootstock evaluation, mitigation of frost/cold damage, soil issues, insects and nematodes, and trellising options to optimize grape yield and berry composition in New Mexico and the arid Southwest.

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Brand names appearing in publications are for product identification purposes only. No endorsement is intended, nor is criticism implied of similar products not mentioned. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

The pesticide recommendations in this publication are provided only as a guide. The authors and New Mexico State University assume no liability resulting from their use. Please be aware that pesticide labels and registration can change at any time; by law, it is the applicator’s responsibility to use pesticides ONLY according to the directions on the current label. Use pesticides selectively and carefully and follow recommended procedures for the safe storage and disposal of surplus pesticides and containers.

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Revised July 2021 Las Cruces, NM