Integrated Weed Management

Weeds may reduce kiwifruit vine growth and yields by competing for water, nutrients, and space. They interfere with low-volume irrigation emitters, reducing the uniformity of water output and efficiency of the system.

For optimum kiwifruit yield and vine health, weeds must especially be controlled in the area around the base of the vine. Generally the larger the area that remains weed free, the greater the vine growth.

The first four years of a vineyard's life are most critical. At or around the fourth year of growth the effect of weed competition is somewhat lessened as vines become established and shade from the canopy reduces weed growth.

Weeds in the 4- to 6-foot-wide vine rows and in the area between the vine rows (middles) are controlled mechanically, through tilling and mowing, or chemically. However, row middles are typically mowed and only rarely chemically treated. During the first four years, weeds in the row middles may need to be mowed multiple times. As vines become established and shade weeds, the number of mowings required will decrease.

A well-managed annual winter cover crop between the vine rows will reduce weed competition. However, if there is danger of frost in spring, the cover crop should be mowed to help moderate vineyard temperatures and reduce the potential for chill damage.

Very few herbicides are registered for use in kiwifruit vineyards. However, combinations and sequential applications of the available herbicides can provide effective and economical control. Before using any herbicide, always read the label, determine what weed species need to be controlled, and follow the directions carefully.

Most commonly, herbicides are used only in the vine row to control weeds. This will reduce the amount of herbicide used, and reduce the need for tillage operations close to the vine row, which may result in cut surface roots and injury to trunks. Frequently two or more herbicides must be combined to achieve adequate weed control. The weed species present will determine which herbicide combinations are needed. This may include more than one preemergence herbicide or, more frequently, a combination of preemergence and postemergence herbicides.

Preemergence herbicides are used to control weeds before they emerge, usually by affecting roots or shoots of very small seedlings. They can be applied in the fall after harvest, during the winter dormant period, or split into two applications: fall and either late-winter or early spring. Most preemergence herbicides must be incorporated into the soil and activated with water. The application should be made when rain is expected within 2-3 weeks of the application. Refer to the herbicide label for guidance on the length of time specific preemergence herbicides can remain on the soil surface before efficacy is reduced due to a lack of incorporation.

Postemergence herbicides control weeds already growing in the vineyard and should be applied whenever monitoring indicates a need. They may be combined with a fall or spring preemergence treatment or applied as spot treatments during the growing season. Control will be optimized when weeds are treated at a young age.

Several species of summer and winter annual, and perennial weeds can be found in California kiwifruit vineyards. The number and species of weeds vary from area to area, within a vineyard, and from year to year. Conduct weed surveys at least twice each year; once in late winter, again in late spring or summer, and/or following each weed management effort.

Use the Weed Gallery photos to help identifying weeds. Check the susceptibility charts to determine the herbicide(s) that give the best control.

Summer weed survey

A survey in summer will tell the spectrum of weeds present and help to determine the most effective control strategy by surveying the effectiveness of previous herbicide or cultivation practices.

Winter weed survey

By surveying weeds in late winter, it will be possible to identify any species that escaped control from earlier management. Surveying will help to determine if a change in herbicides or cultural methods is needed.

How to survey fields for summer and winter weeds:

• Survey the vineyard in late winter to identify winter annual weeds, and again in summer after perennial and summer annual weeds have germinated.
• If cultivation for weed control is used, monitor at least 2 weeks before planning to cultivate.
• Pay particular attention to perennial weeds. Sketch a diagram of the vineyard and mark areas where perennial weeds are found. A hand held GPS unit also works well for recording locations of perennial weeds. Check for re-growth of perennial weeds a few weeks after cultivation.
• Pay attention to low-lying areas or where water tends to accumulate. These are often problem areas for weed growth.
• Survey areas around the vineyards as potential sources for wind disseminated weed seeds such as horseweed, hairy fleabane etc.
• Keep records of survey results and control techniques (example form—PDF). By knowing what species are present, it will be possible to make appropriate decisions on cultural and chemical controls.

Information collected over a period of years can help to better understand ongoing weed control problems such as perennial weeds, herbicide resistance, shifts in the weed population, and the overall effectiveness of the selected weed management program.

An especially effective method of weed management before planting vines is to cultivate, irrigate to germinate new weeds, then cultivate again. This reduces the weed seed population in the soil, thus reducing weed emergence. At least two cycles of cultivating, irrigating, and cultivating again should be used to make marked reductions in weed seedlings. This method will effectively control annual weeds, but is not effective on perennial weeds if irrigation is used to keep soil moist.

To control perennial weeds such as dallisgrass, bermudagrass, and johnsongrass, cultivate dry soil to cut the rhizomes into small pieces and bring them to the surface to dry. Once desiccated, rework the soil to pull new rhizomes to the surface and dry them as well. If the soil is irrigated or rain occurs before complete control of the perennial plant is achieved, this practice will be only partially effective. Working wet soil can increase the population of these weeds, because plant fragments may re-root. After rhizomes, stolons, and other perennial weed parts have been destroyed, weed seedlings, emerging after irrigation or rainfall, should be destroyed immediately; otherwise these perennial weeds can quickly reestablish.

Herbicides

Currently, there are no preemergence herbicides registered for use in kiwifruit vineyards prior to planting. It is also safer for young vines to control perennial weeds with postemergence herbicides before planting, because preemergence herbicides can persist in the soil and negatively affect newly planted vines. Established perennial grass weeds can be controlled with glyphosate followed 2 to 3 weeks later with cultivation.

If the soil and plant material can be dried after treatment by withholding irrigation in summer, increased control is achieved. However, usually this is not feasible in winter due to rainfall. Field bindweed will be partially controlled with this method. Yellow nutsedge plants will be controlled, but new plants will emerge from underground tubers. In most cases, repeated treatments of perennial weeds may be required.

Alternatively, a fumigant, such as metam sodium (Vapam), can be used to control most annual and perennial weeds and many soilborne pests. Follow directions carefully in relation to soil moisture, timing, and method of application to achieve the best results.

There are a number of ways that weeds growing at the base of the vines can be controlled, including mulching, various forms of cultivation, mowing, and herbicides.

Hoeing or using weed knives in the row and cultivating between the rows are cultural methods that must be performed frequently to reduce competition and to keep weeds from setting seed. If weeds are allowed to mature, not only do the plants often become a fire hazard, but more importantly, they produce enough seeds for many years of weeds.

When mulching, make sure the mulch does not harbor insects or disease organisms. Black polypropylene mulches are also available. These prevent germination and establishment of weed seedlings while allowing water to percolate through into the soil. Mulches work well in vineyards irrigated with low-volume emitters (drip and microsprinklers), but they are quite costly. To reduce costs, some growers install mulches only in a 2-x-2-foot or 3-x-3-foot area under the emitters.

Herbicides

Herbicides are commonly used to control weeds in a 4-6-foot-wide strip under the vine row. Removing weedy plants from around the base of the vine reduces weed competition and the potential for rodent damage.

Preemergence herbicides are the safest for use around established vines. Currently registered preemergence herbicides will control only seedlings of annual and some perennial weeds. Postemergence herbicides may be used either alone or combined with a preemergence herbicide to kill existing weeds and to provide residual control of new seedlings. Repeated spot applications of postemergence herbicides are often used to treat perennial weeds during the growing season. Be careful to prevent spraying the vines with postemergence herbicides, because injury can occur.