Chapter 23

Vineyard Weed Management

Chemical Weed Control

Herbicides

When properly used, herbicides registered for use in vineyards can control most weed species. In many vineyards, combinations and/or sequential applications of herbicides are required to provide effective, economical control. Before using any herbicide, identify the weed species to be controlled, then read and follow product label directions carefully. More commonly growers apply herbicides as vine-strip (the under-vine area) treatments rather than in alleys (the inter-row area). It is not necessary to maintain a totally clean strip under the trellis all summer long.

Pre-emergence Herbicides

Pre-emergence herbicides are applied to bare soil and are leached into the soil with rain or irrigation where they are active against germinating weed seeds. They must be moved by water (rainfall or irrigation) into the top 1 to 3 inches (2.5 to 7.6 cm) of soil where weed seeds germinate. If herbicides remain on the soil surface without incorporation, some will degrade rapidly from sunlight.

Post-emergence Herbicides

Post-emergence herbicides are applied to control weeds already growing in the vineyard. They may be contact herbicides or translocated (systemic) herbicides. Post-emergent materials are intended to control small weeds. Weeds that are large or stressed for moisture during application often escape control.

Herbicide Treatment Options

Treatment Options in Young Vineyards

Weed management is critical around young vines where weeds compete for nutrients, water, and light. Weedy vineyards may take 1 to 2 years longer than those that are weed-free to become economically productive. (From an economic standpoint, however, it is important to compare the costs of weed management with the benefits of earlier production.)

Treatment Options in Established Vineyards

It takes 3 to 4 years for a vineyard to become established under normal growing conditions. Established vines are more tolerant of many herbicides than newly planted vines, thus increasing the options available for weed control. There are three programs growers can consider. They include a spring preemergence application, which is a traditional approach; the delayed pre-emergence option; and the fall/ spring split option (Mitchem et al. 2005).

Spring Pre-emergence Option: Traditionally, a vineyard herbicide program has consisted . . .

Delayed Spring Pre-emergence Option: The delayed pre-emergence herbicide program requires a spring glyphosate application. The spring application should be made prior to bud break.

Fall/Spring Split Option: The fall/spring split is the third option that growers should consider. This program begins with a fall pre-emergence application in combination with non-selective burn down herbicide like Gramoxone Max or Rely applied after harvest.

Application Rate, Timing, and Equipment

Rate

The appropriate rate of herbicide to apply in the vineyard depends on several factors. Pre-emergence herbicides are soil active, and the effective concentration to use depends upon soil type. Generally, more preemergence herbicide is needed to control weeds on finer texture soils (clays and silts) than on coarser soils (sands and gravels). For some herbicides, the rate should be increased when the soil has a high organic matter content. The proper rate of both pre- and post-emergence herbicides to be used is also influenced by the species of weeds to be controlled.

Timing

The correct timing of herbicide application in the vineyard depends on several factors. Some herbicides are more soluble than others, and move more quickly through the soil.

Equipment

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

Other Herbicide Considerations

Pre-emergence herbicides control germinating weed seeds but do not usually give acceptable control of emerged weeds. However, some herbicides (e.g., diuron and oxyfluorfen) provide pre-emergence and post-emergence weed control. Rainfall is needed to activate pre-emergence herbicides. Best control results when activation, from rainfall or overhead irrigation, occurs within a few days of application. The first irrigation following an herbicide application is the most critical in terms of how far the preemergence herbicide is moved into the soil; subsequent irrigation is less important to the movement of the herbicide.

Herbicide Resistance

When using chemical control, growers must be aware that repetitive use of a single herbicide or of a particular herbicide group with the same site of action could select for herbicide-resistant weed plants in the vineyard. Methods for reducing the chances of herbicide resistance include the following:

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