Weed control methods for cranberries and strawberries are treated in separate articles in “Section O. Small Fruits” in this handbook.
Orchard floor management can influence tree health, vigor, and productivity. Weeds compete for soil moisture, nutrients, and can host viruses, and vertebrate or invertebrate pests. Newly planted orchards are more sensitive to weed competition, but mature orchards are also affected, albeit the competition effects may take longer to be noticeable. Weeds affect irrigation management by depleting soil water and blocking sprinkler distribution. In certain cases, weeds can compete for pollinating bees in spring. The common dandelion, for example, blooms about the same time as pears and is a preferred nectar source in spring.
Multiple practices are available for weed management in orchards, but when a single practice is used over time it may lead to weed shifts or selection of herbicide resistance. A weed shift is a change in the species composition of a weed population in response to the repeated use of a weed management practice. Over time, a few species become dominant in the area, and these weed species are tolerant to the weed management practice. In berry fields and orchards, weeds are controlled by mowing or flailing row middles and applying herbicides within the tree or plant row. Repeated use of the same or similar weed control practices can result in a weed shift to species that tolerate these practices. Examples include prostrate weeds that tolerate flailing, deep-rooted perennials that tolerate cultivation or survive during the summer dry season, and weeds that either resist the herbicide or are selected from a natural population of susceptible biotypes.
Preventing weed shifts Weeds that survive cultivation, mowing or flailing, specific herbicide treatments, or other routine cultural practices must be eliminated before the tolerant species or biotypes become established. Combine a variety of weed control practices or treatments, rotate practices and herbicides, and spot treat with a hoe or registered herbicide when the weed first appears. Also, clean equipment when moving from an infested field.
Herbicide resistance Repeated use of a single herbicide site-of-action selects for weed biotypes that are resistant to the herbicide. The resistant biotypes are present at very low frequencies in the population, and are selected by repetitive use of the same herbicide site of action.
Managing herbicide (and glyphosate) resistance Repeated use of glyphosate in orchards in western Oregon has selected for a resistant biotype of Italian ryegrass. Overreliance on herbicides with a single mode of action for orchard floor maintenance, or within the grape and berry plant row, increases the risk of selecting for resistance in other weed species. In western Oregon, there are confirmed cases of Italian ryegrass resistant to herbicides of Groups 1, 2, 9, 10, and 22. To reduce the risk of selecting for weeds that are resistant to herbicides, alternate herbicides with different sites of action and tank-mix with herbicides that have different sites-of-action. Refer to Section C. “Agrichemicals and Their Properties” and the subsection “Managing Herbicide-Resistant Weeds” in this handbook for more information.
Steps to slow herbicide resistance
- Use multiple herbicide modes of action, including those with residual effects, and tank mixing herbicide with different site of action.
- Apply herbicides at the recommended stage of weed growth as stated on the label. Smaller weeds are typically easier to control than large weeds if they are glyphosate resistant.
- Use full rates of the herbicide to delay resistance selection.
- Include non-chemical methods, like cultivation, mowing, and flame weeding.
- Do not let weeds produce seed—or even pollen, in the case of Italian ryegrass.
More information is available from http://www.ipm.ucdavis.edu/IPMPROJECT/glyphosateresistance.html
Sod covers In many orchards and berry fields, native or planted grasses in row middles are managed by mowing or flailing. Sod, or living mulch, reduces soil erosion on sloping sites, improves traffic conditions in wet weather, and increases water infiltration and drainage. New sod varieties are being introduced into various horticultural cropping systems. They include dwarf cultivars that respond to minimal management practices such as drought, low fertility, or sublethal rates of postemergence herbicides.
Managing berry crops, vineyard, and orchard vegetation Successful vegetation management in orchards, vineyards, and berry fields requires a comprehensive, year-round approach that uses a combination of weed control practices, and alternates them over several years. Developing these strategies requires identifying each weed and gathering information about the effectiveness of each weed control practice. Consider costs and select herbicides that may be applied together, or in split applications that control the weeds in the orchard or berry field. Note the site of action of each herbicide. The site of action indicates how the herbicide works in the plant. The corresponding herbicide group number for each site of action is included for each herbicide entry in this handbook. Alternating herbicide use based on group number may reduce the chance of developing resistant species or biotypes. Often, a combination of mechanical, herbicidal, and sometimes hand removal or spot treatment with herbicide sprays or wipers, will give the most effective year-round control.
Soil-active herbicides Persistent, soil-active herbicides can be applied during the winter dormant season, and then activated with rain or sprinkler irrigation if dry conditions persist. Apply lower rates on sandy or gravelly soils, or soils containing lower clay or organic matter contents or cation exchange capacities. Control existing vegetation by mixing with a postemergence contact or translocated herbicide. After establishing an effective weed control program, rotate soil-active herbicides applied in fall and/or early spring to improve year-round weed control and reduce possible crop injury.
Postemergence herbicides Contact herbicides such as paraquat (Gramoxone), glufosinate (Rely 280), and carfentrazone (Aim) can be used to control existing vegetation, but they lack residual control and are nonselective in broadleaf crops. Paraquat is a restricted-use herbicide and requires careful handling and secure storage. Glyphosate (Roundup) controls many weeds but must be applied at the correct stage of weed growth to obtain maximum movement into the roots (see label for details). Avoid applications to green bark, low limbs, tree trunks that are wounded, or suckers with buds that are beginning to open. This is important for translocated herbicides, such as glyphosate, on crops that are very sensitive, such as grapes and raspberries. Green bark is vulnerable to repeated herbicide applications and may need protection in the first 2 or 3 years after planting. Herbicides are prone to enter through green bark and wounds on stems than through mature bark. White latex paint that is often applied to young orchard trees does not provide adequate protection.
There are at least three nonselective OMRI-listed and NOP-approved organic herbicides now available for use in orchard, vineyard, and berry crops: AXXE (contains ammonium nonanoate), Suppress (a mixture of caprylic and capric acids) and Finalsan Total Vegetation Killer (contains ammoniated soap of fatty acids). Like the other contact herbicides listed above, these products do not provide residual control of emerging weeds. These herbicides are most effective if weeds are less than 6 inches tall, there is bright sunlight, or air temperatures are 70°F or higher. Shielded or hooded sprayers are needed to prevent contact with leaves and stems of low-growing crops.
Several selective postemergence herbicides are registered in horticultural crops. They usually work best if applied to seedlings less than 4 inches tall. Time the application so the maximum number of seedlings have emerged but the largest seedlings are not too large to control. Environmental conditions also may influence the crop’s tolerance of the herbicide. Hot weather can increase risk of injury from many postemergence herbicides unless conditions are so dry that the plant is not growing vigorously. Conversely, poor growing conditions for weeds often diminish the effectiveness of postemergence herbicides. The grass herbicides sethoxydim (Poast) and clethodim (Select) are more effective when weeds are actively growing before and after the herbicide treatment.
Surfactants can make the difference between good and poor weed control. Crop oils or other nonphytotoxic adjuvants are required on many postemergence herbicides; in specific cases, nitrogen solutions (e.g., 2.5% UAN or AMS) may be required and may improve grass control. Read the label carefully for this crucial information.
Warning Using 2,4-D or similar materials on horticultural farms involves risk to the crop to which it is applied and to crops in nearby fields. However, there may be instances in which guidance in 2,4-D use will enhance weed control with minimal chance for crop injury. Be careful to clean all 2,4-D from your equipment, or use separate sprayers to avoid possible crop injury. Never use a volatile formulation of 2,4-D or similar material. Buy only a product that lists the intended crop on the label.
The information provided in this handbook is not intended to be a complete guide to herbicide use. Before using any chemical, read the label recommendations on the container. Before a chemical can be recommended for a specific use, it must be thoroughly tested. Following the recommendation on the manufacturer’s label can prevent many problems arising from the improper use of a chemical. Any use of a pesticide contrary to instructions on the printed label is illegal and is not recommended.
Note To selectively control weeds, and to minimize the chance of injuring trees or berry plants, herbicides must be applied at the correct rate and time. Get more consistent results by reading the label and other information about the proper application and timing of each herbicide. Suggested rates in this guide are stated as pounds of active ingredient per acre (lb ai/a) or pounds of acid equivalent per acre (lb ae/a). See the product label for specific amounts of a particular formulation to apply per treated acre. For band applications under tree or in berry rows, reduce the quantity of herbicide applied proportionally to the area within the row actually sprayed. Numerous tank-mixes are labeled for orchard use. Growers can also assume responsibility for mixing products themselves unless mixing is prohibited by the label. Livestock grazing in orchards and vineyards often is prohibited if herbicides have been applied for weed control.