HORTICULTURAL WEED MANAGEMENT
Ed Peachey
Revised November 12, 2008

Horticultural weed management information is available both online at http://pnwpest.org/pnw/weeds and in print in this handbook. Both vegetation and weed management files are linked to weed control suggestions for orchard, berry, and vegetable commodities in the online version, while the print version contains brief management summaries. Management information for nursery production will be linked online.
Vegetation Management in Orchards, Vineyards, and Berries*
* Weed control in cranberries and strawberries is treated in separate sections of this handbook.  DTP:  Please put footnote text at the bottom of the page on which this heading appears; delete this text
Weeds such as deep-rooted perennials compete for soil moisture and nutrients in newly planted and mature orchard and berry crops, or weeds may intercept light in newly planted or shorter crops. Other weeds may host pests including plant viruses and can compete for pollinating bees in spring. Common dandelions, for example, bloom about the same time as pears and are a preferred nectar source in spring.
Weed shifts—Excessive “weedy” vegetation in most orchards and perennial berry fields is controlled by mowing or flailing row middles and applying herbicides within the tree row. Repeatedly using the same or similar weed control practices results 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. 
Managing herbicide resistance—Repeated use of glyphosate in at least one orchard in western Oregon has selected for a resistant biotype of annual ryegrass. Overreliance on herbicides with a single site of action for orchard floor maintenance increases the risk of selecting for resistance in other weed species, and it threatens the long-term usefulness of glyphosate for weed control in orchards and other crops. Alternate herbicides with different sites of action to reduce the risk of selecting for weeds that are resistant to glyphosate. Refer to “Managing Herbicide-resistant Weeds” and “Agrichemicals and their Properties” in this handbook for more information.
Sod covers—Native or planted grasses in many orchards and berry fields often are managed in row middles by mowing or flailing. Sods reduce soil erosion on sloping sites, improve traffic conditions in wet weather, and increase 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. Consult your local Extension agent for recent information about living mulches and their management.
Managing orchard vegetation—Successful vegetation management in orchards 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; that is, how the chemical works in the plant. The site of action and corresponding herbicide group number are 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 activated with rain or sprinkler irrigation if dry conditions persist. Apply lower rates on sandy or gravelly soils, or soils containing lower clay, organic matter contents, or cation exchange capacities. Control existing vegetation by mixing a postemergence contact or translocated herbicide. After establishing an effective weed control program, use lower rates and split applications of some herbicides such as simazine, diuron, or terbacil (Sinbar) in fall and early spring to improve year-round weed control and reduce possible injury.
Postemergence herbicides—Contact herbicides such as paraquat and carfentrazone (Aim) can be used to control existing vegetation, but they lack residual control and are nonselective in broadleaved crops. Paraquat is a restricted-use pesticide 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, or suckers with buds that are beginning to open (important for translocated herbicides such as glyphosate on crops that are very sensitive, such as grapes). 
Two nonselective OMRI-listed and NOP-approved organic herbicides are now available for use within orchard crops: Matran (contains clove and wintergreen oils) and Greenmatch EX (lemongrass oil). Like the contact herbicides listed above, they do not provide residual control of emerging weeds. These herbicides are most effective if weeds are less than 6 inches tall, and there is bright sunlight or air temperatures of 50°F or more. Shielded or hooded sprayers are needed to prevent contact with 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 big to kill. Environmental conditions also may influence the crop’s tolerance of the herbicide. Hot weather can increase risk of injury from many postemergence herbicides unless it is 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 Poast (sethoxydim) and Select (clethodim) are much less effective if the weeds are not actively growing before or after the herbicide is applied. 
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 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, not only to the crop to which it is applied but also 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 wrong use of a chemical.
Note  Herbicides must be applied at the correct rate and time to selectively control weeds and to minimize the chance of injuring trees or berries. 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, or calculate the product rate using the samples in the boxes. For band applications under tree or 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, or growers can assume responsibility and mix products unless the label prohibits. Livestock grazing in orchards and vineyards often is prohibited if herbicides have been applied for weed control.
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