Agricultural pest management
Special Weed Problems
(Reviewed 10/14, updated 10/14)
Johnsongrass is one of the most troublesome of perennial grasses. It reproduces from underground stems and from seed. The mature plant grows during spring and summer in spreading patches that may be as tall as 6 to 7 feet. Johnsongrass can be a serious problem, especially in young apricot 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 selective postemergence herbicides such as fluazifop-p-butyl (Fusilade DX), glyphosate (Roundup), or others will often be required for control of johnsongrass. Johnsongrass is most effectively controlled by Fusilade DX when it is between 8 and 18 inches tall. A second application can be applied 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.
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 preemergence herbicides. If bermudagrass develops in localized areas, immediately spot treat it with postemergence herbicides such as glyphosate (Roundup). 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.
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 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 (2 lb a.i./acre) plus 2,4-D. The use of 2,4-D and other materials may require special approval from the county agricultural commissioner during certain times of the year. In organic orchards, cultivation at 2- to 3-week intervals during the growing season will eventually deplete the root system and starve the plant.
Hairy fleabane is a summer annual plant that can emerge anytime during the year. This plant can withstand several mowings and still produce seed. In addition, it can interfere with moving sprinkler and drip irrigation lines. Shallow cultivation when weeds are in the seedling stage provides effective control. Preemergence herbicides that provide the most effective control include flumioxazin (Chateau SW), isoxaben (Trellis), indaziflam (Alion), and rimsulfuron (Matrix SG).Postemergence herbicides, like paraquat and glyphosate both can control this species when it is small (less than 18-21 leaves), but once plants bolt (sending up flowering stalks), they will not control it. Glyphosate at 1 lb a.i./acre will control plants up to 13 leaves; for plants with 14 to 21 leaves 2 lb a.i./acre is required. Plants larger than 21 leaves may not be adequately controlled. Tank-mixing glyphosate plus 2, 4-D provides excellent control when these weeds are small. Be careful to follow all label and permit restrictions when using 2, 4-D to avoid crop injury. Fleabane has been identified as glyphosate resistant. 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 (MARE'S TAIL)
This summer annual weed can emerge anytime during the year. It 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. Control measures are similar to hairy fleabane.
Yellow nutsedge is a perennial weed that reproduces 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 when they have four or fewer leaves, which is about when they begin to produce new tubers. Repeat treatments are often 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 rimsulfuron (Matrix SG).
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 high label rates of glyphosate to control it. A low rate preemergence herbicide program can also effectively manage this weed and reduce the need for preharvest treatments. Applying oryzalin (Surflan) at 2 qt/acre with glyphosate in April to the area between the tree rows in the orchard can provide season-long control.
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 widespread 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 10,000 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.
UC IPM Pest Management Guidelines: Apricot
K. J. Hembree, UC Cooperative Extension, Fresno County