Pest Management Guidelines
Special Weed Problems
(Reviewed 12/07, updated 12/07)
Currently registered preemergent herbicides will control only seedlings of annual and some perennial weeds. Repeated postemergent treatments are required to control perennials. While they are best controlled (preferably eradicated) before planting, if these weeds are still present after planting, a program is needed for their control. With these treatments, there is always some concern of injury to trees from careless application.
The potential risk for the development of herbicide resistance is greatest when the same herbicide is used repeatedly, as often is done in orchards. To prevent the development of resistance use a variety of weed control strategies, including cultural practices and alternating herbicides with different modes of action. Failure to do so can result in the rapid loss of herbicides as a pest management tool, although cultivation remains an option. If resistant populations are observed,avoid moving resistant weeds from one field to another by cleaning equipment before moving out of a field with known herbicide resistant weeds. Consider scheduling known resistant fields as the last ones to be planted, harvested, etc.
Some populations of horseweed (Conyza canadensis), annual or Italian ryegrass (Lolium multiflorum) and hairy fleabane (Conyza bonariensis) have developed resistance to glyphosate, in California.
One of the most important control strategies in managing resistant populations of these weeds is to not let the plants produce seed.
Primary species that are difficult to control are the perennial grasses (dallisgrass, bermudagrass, and johnsongrass) and broadleaf weeds (field bindweed, dandelion, and curly dock). All these species are controlled with glyphosate (Roundup, Touchdown), but this material should not be used around newly planted trees. To achieve the best control, cultivate the weeds to chop the stems, crowns, and rhizomes into small pieces, then irrigate to encourage regrowth and a lot of leaf area on the weeds. Before the weeds flower or seed, treat with one of the postemergence materials. Good coverage is important. Spot treat regrowth if it occurs. In young trees fluazifop or sethoxydim can be used to control grasses without damage to trees. Field bindweed and curly dock can also be suppressed with 2,4-D applications. Dandelion can be controlled with 2,4-D. A permit is required for this material. Use 2,4-D with caution in walnut orchards to prevent tree damage.
In mature orchards with heavy shade, perennial weeds will be a minimal problem. It will be necessary to control seedlings of these weeds with preemergent materials or spot treatments of glyphosate (Roundup, Touchdown) or paraquat before they become established. Seeds of the grass species last at least 2 years while seeds of the broadleaf weeds may last 10 to 60 years in the soil, depending upon species, so frequent monitoring is necessary for continued control. Do not allow perennial plants to reestablish or to produce seed.
Another species difficult to control is yellow nutsedge. Yellow nutsedge can be reduced by treating with glyphosate (Roundup, Touchdown) and then retreating before the nutsedge reaches the five-leaf stage. New nutlets do not have the opportunity to form if retreated at or before this stage. It can also be reduced, but not eradicated, by treating for several years with a preemergent treatment of norflurazon (Solicam), which isn't recommended for sandy soils, especially when this is followed by a postemergent application of glyphosate. Both seedling bindweed and young nutsedge can be controlled by cultivating when the soil is dry.
UC IPM Pest Management Guidelines: Walnut
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