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How to Manage Pests

UC Pest Management Guidelines


Special Weed Problems

(Reviewed 4/10, updated 4/10)

In this Guideline: More about weeds in alfalfa:

BERMUDAGRASS. Bermudagrass is a vigorous spring- and summer-growing perennial grass. It grows both from seed and underground rhizomes and stolons, which can be spread during cultivation. It frequently becomes a problem in mowed orchards because mowing increases the amount of light that the stolons receive, thus stimulating their growth. This grass is very competitive with the trees for moisture and nutrients. Seedlings can be controlled with preemergent herbicides. If bermudagrass develops in localized areas, immediately spot treat it with postemergent herbicides such as glyphosate (i.e., Roundup or Touchdown). 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.

BURCLOVER. Burclover is a winter annual that is relatively low growing. Seedlings may emerge in spring in milder coastal locations. Because it is a favored host of lygus bugs and other plant bugs (Calocoris sp.), its presence in the orchard can attract and host populations of these pests within the orchard. Most preemergent herbicides, other than those in the dinitroaniline family (e.g., oryzalin, trifluralin), will control burclover.

COMMON PURSLANE. Common purslane is a prostrate summer annual that reproduces from seed, which germinates in April to early May. Common purslane grows into a plant with 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. A low rate preemergent herbicide program can also effectively manage this weed and reduce the need for preharvest treatments. Oryzalin (Surflan) at 1 qt/acre applied with glyphosate in April to the area between the tree rows in the orchard 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 preemergent 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 broadleaf weed that either grows from seed, which can survive for up to 30 years in the soil, or from stolons, rhizomes, or extensive roots. Because of the seed's longevity in the soil, it is critical to destroy plants before they can produce seed. The plants may spread from stem or root sections that are cut during cultivations, however cultivation controls seedlings. If field bindweed appears in or around the orchard, spot treat with high label rates of glyphosate. Another alternative is a modest rate of glyphosate plus 2,4-D. 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 in December if winter 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. In some areas as a result of several years of weed management programs centered aroundthe use of glyphosate, shifts have occurred to the predominance of glyphosate-resistant plants.

Glyphosate at 1 lb a.i./acre will control plants up to 13 leaves; for plants with 14 to 19 leaves 2 lb a.i./acre is required. Plants larger than 19 leaves are not adequately controlled. Tank-mixing glyphosate plus 2,4-D provides excellent control when these weeds are small. Be careful to follow all labels and permit restrictions when using 2,4-D to avoid crop injury. Plants of a close relative, horseweed, have developed resistance to glyphosate in many areas. 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 interfere with 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 many 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 peach orchards. It can be controlled by repeated tillage during the dry summer months, but the soil must be fairly dry or the rhizome buds may sprout. Repeated applications of postemergent 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, fluasifop-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.

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. In established orchards, if a nutsedge infestation develops, spot treat it with glyphosate. For best results, treat young plants before more than 5 leaves have formed, which is about when 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 preemergent application of norflurazon (Solicam) or thiazopyr (Visor); thiazopyr is registered for use in nonbearing orchards only.

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 annual 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 applied to very small seedlings is effective, and two applications of selected grass herbicides (e.g., Fusilade, Poast, and Select Max) are effective in ryegrass control.

SPRANGLETOP. Mexican sprangletop is increasing at a fast pace in San Joaquin Valley orchards, field edges, and ditchbanks. It is especially increasing where the dominant part of the weed control program depends on glyphosate and where preemergent herbicide applications have stopped. Sprangletop can be controlled by postemergent applications of glyphosate when weeds are small, however, repeat applications are necessary within 14 to 21 days for complete control. Preemergent control can be achieved with several herbicides including oryzalin (Surflan), pendemethalin (Prowl), and pronamide (Kerb). Depending on the rate applied, these herbicides may lose effectiveness in late summer and not control sprangletop that emerges in fall.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Peach
UC ANR Publication 3454
J. A. Roncoroni, UC Cooperative Extension Napa County
S. D. Wright, UC Cooperative Extension Tulare County

Acknowledgment for contributions to Weeds:
K. J. Hembree, UC Cooperative Extension Fresno County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
A. Shrestha, UC IPM Program, Kearney Agricultural Center, Parlier

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