Agricultural pest management
Special Weed Problems
(Reviewed 6/10, updated 6/10)
BERMUDAGRASS. Bermudagrass is a vigorous perennial weed that grows in the spring and summer. It is propagated by seed and an extensive rhizome and stolon system that is often spread during cultivation. Bermudagrass competes aggressively with trees for moisture and nutrients. Seedlings can be controlled with preemergence herbicides. Immediately spot treat any areas of bermudagrass that develop in an orchard or localized area with a postemergence herbicide such as glyphosate (Roundup). Take care not to let any herbicide drift onto the trees.
Bermudagrass frequently becomes a problem in mowed orchards because mowing increases the amount of light that the stolons receive, thus stimulating their growth. In organic orchards, geese have been used to control grasses, including bermudagrass. If confined to an area containing bermudagrass, geese will dig up the rhizomes and completely consume the plant.
COMMON PURSLANE. Common purslane is a prostrate summer annual that reproduces from seed that germinate in April to early May. Mature common purslane has fleshy stems that can root and continue to grow after cultivation or mowing if moisture is present. This weed predominates in sunny areas of the orchard, especially if low rates of translocated herbicides (e.g., glyphosate) are used as preharvest sprays. If problems develop with this weed, use higher rates of glyphosate to control it. Oryzalin (Surflan) at 1 qt/acre applied with glyphosate in April between the tree rows (middles) can provide season long control.
DALLISGRASS. Dallisgrass is a common perennial grass found in orchards. It can be highly invasive in newly planted orchards. Dallisgrass seedlings germinate in spring and summer and form new plants on short rhizomes that develop from the original root system. Dallisgrass seedlings can be controlled with cultivation or with preemergence herbicides. Dallisgrass has a clumpy growth habit that gives it a bunchgrass appearance. Like bermudagrass, it tends to become dominant in mowed areas because mowing stimulates seed set. It also grows in areas with standing water. The plants are heavy seed producers. Treatment with glyphosate has been successful in controlling dallisgrass infestations. For organic orchards, consider using geese, which eat grasses preferentially.
FIELD BINDWEED. Field bindweed is a vigorous perennial weed that grows from rhizomes, an extensive sprouting root system, or seed that can survive for up to 30 years in the soil. Because of the seed's longevity in the soil, it is critical to control these plants before they can produce seed. Seedling field bindweed is controlled with cultivation, but mature plants may spread from stem or root sections that are cut during this operation. If field bindweed appears in or around the orchard, spot treat with high label rates of glyphosate.
In organic orchards, cultivation at 2- to 3-week intervals during the growing season may eventually deplete the root system and starve plants growing on lighter soils.
HAIRY FLEABANE. Hairy fleabane is an annual plant that normally emerges in February, but it can emerge as early as October and continue germinating throughout the winter, if temperatures are warmer than average. This plant can withstand several mowings and still produce seed. In addition, it can interfere with moving sprinkler and drip irrigation lines. Paraquat and glyphosate both can control this species when it is small, but once plants bolt (sending up flowering stalks), they will not control it. Tank-mixing glyphosate plus 2, 4-D provides excellent control when these weeds are small. Be sure to follow all label and permit restrictions when using 2,4-D to avoid crop injury. A close relative, horseweed, has developed resistance to glyphosate in many parts of the Unites States. Thus, it is critical to monitor control efforts and follow up with hand hoeing to prevent escape of any plants that might be resistant.
HORSEWEED. Horseweed is a common weed in California orchards, cultivated fields, and disturbed areas. This annual weed has a woody stalk and can grow up to 10 feet tall. If not controlled, it can obstruct harvesting practices. Like hairy fleabane, this weed can withstand mowing and interfere with moving sprinkler and drip irrigation lines. Paraquat and glyphosate both can control this species when it is small, but once plants bolt (sending up flowering stalks), they will not control it. In some areas, horseweed has developed resistance to glyphosate. Thus, it is critical to monitor control efforts and follow up with hand hoeing to prevent escape of any plants that might be resistant. Tank-mixing glyphosate plus 2,4-D provides excellent control when these weeds are small as does adding ammonium sulfate at 10 to 15 lb/acre or citric acid to glyphosate. Be careful to follow all labels and permit restrictions when using 2,4-D to avoid crop injury.
JOHNSONGRASS. Johnsongrass can be a serious problem, especially in young nectarine orchards. It can be controlled by repeated tillage during the dry summer months, but the soil must be dry or the rhizome buds may sprout. Repeated applications of postemergence herbicides such as clethodim (Select Max), fluazifop-p-butyl (Fusilade), glyphosate (Roundup), sethoxydim (Poast), or others will often be required for control of johnsongrass. Johnsongrass is most effectively controlled by selective grass herbicides (clethodim, fluazifop-p-butyl, sethoxydim) when it is between 8 and 18 inches tall. A second application is usually required to prevent rhizome production and limit the chance of regrowth. Apply glyphosate when johnsongrass is actively growing and between 12 and 24 inches tall. Geese are also effective at controlling johnsongrass in organic orchards. In new plantings, trifluralin (Treflan) or norflurazon (Solicam) will control seedling johnsongrass but not established johnsongrass plants.
RYEGRASSES. Ryegrasses are annual winter grasses that are common throughout California. In 1998, two orchard sites were identified as having glyphosate-resistant ryegrass populations. More recent surveys have observed that glyphosate-resistant ryegrass is now in numerous orchards in Northern California and at least some orchards in the San Joaquin Valley. It is estimated that glyphosate-resistant ryegrass now occupies over 5,000 acres in California. 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.Paraquat is effective when applied to very small seedlings, and two applications of selected grass herbicides (e.g., Fusilade, Poast, and Select Max) are effective in ryegrass control.
SPRANGLETOP. Mexican sprangletop is spreading at an increasing pace in San Joaquin Valley orchards, field edges, and ditchbanks. This is especially true where preemergence herbicide applications have been stopped, and glyphosate has become the dominant part of the weed control program. Sprangletop can be controlled by postemergence applications of glyphosate when weeds are small. For complete control, repeat applications are necessary within 14 to 21 days. Preemergence control can be achieved with several herbicides including oryzalin (Surflan), pendemethalin (Prowl), and pronamide (Kerb). These herbicides may lose effectiveness in late summer, depending on the rate applied, and not control sprangletop that emerges in fall.
NUTSEDGE. Yellow and purple nutsedge are perennial weeds that reproduce from underground tubers that survive for 2 to 5 years in the soil. The tubers are easily spread by cultivation equipment. Each tuber contains several buds that are capable of producing plants. One or two buds germinate to form new plants; however, if destroyed by cultivation or an herbicide, then a new bud is activated. If a nutsedge infestation develops in established orchards, spot treat it with glyphosate. For best results, treat young plants before more than 5 leaves have formed. This is the approximate stage in which they begin to produce tubers. Repeat treatments are usually necessary to control late germinating plants. Where nutsedge is already well established, treat with glyphosate every 21 to 28 days during the season as new growth flushes emerge. Nutsedge can be suppressed by a preemergence application of norflurazon (Solicam) or thiazopyr (Visor); thiazopyr is registered for use in nonbearing orchards only.
UC IPM Pest Management Guidelines: Nectarine
J. A. Roncoroni, UC
Cooperative Extension, Napa County