Agricultural pest management
Special Weed Problems
(Reviewed 11/12 , updated 11/12 )
Asparagus is a perennial plant that can be a troublesome weed in some pear orchards. If the clumps are not cut back or controlled in some way, it interferes with harvest. To control asparagus, dig it out when the clumps are small, being sure to remove the root mass. If parts of the roots are missed, additional digging will be necessary to remove new plants when they grow. Asparagus can also be sprayed with glyphosate at the flowering stage, which will damage the plant but probably will not kill it.
Bermudagrass is a vigorous spring- and summer-growing creeping perennial grass. It grows from seed, underground rhizomes, and aboveground 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 with postemergence herbicides such as glyphosate. In organic orchards, geese have been used to control grasses, including bermudagrass, but be aware of food safety concerns and check if animals in the orchard are acceptable with the buyer. If confined to an area containing bermudagrass, geese will dig up the rhizomes and completely consume the plant.
Common purslane is a prostrate summer annual that reproduces from seed that germinates in April to early May. 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. A low-rate preemergence herbicide program can also effectively manage this weed and reduce the need for preharvest treatments. Oryzalin at 1 qt/acre applied with glyphosate in April to the area between the tree rows in the orchard can provide season-long control of common purslane.
Dallisgrass is a common tufted perennial grass found in orchards. Dallisgrass has a clumpy growth habit that gives it a bunchgrass appearance. The plants are heavy seed producers. Dallisgrass seeds germinate in spring and summer and form new plants on short rhizomes that develop from the original root system.
Dallisgrass can be highly invasive in newly planted orchards. Like bermudagrass, it tends to become dominant in mowed areas because mowing stimulates seed set. It also grows in areas with standing water. Dallisgrass seedlings can be controlled with cultivation or with preemergence herbicides. Treatment with glyphosate has been successful in controlling dallisgrass infestations. For organic orchards, consider using geese, which preferentially eat grasses, but be aware of food safety concerns and check if animals in the orchard are acceptable with the buyer.
Field bindweed, also called wild morningglory, 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 seed 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 will eventually deplete the root energy reserves and starve the plant. A thick layer of mulch (6 inches) will provide some control.
Hairy fleabane is a summer annual, but sometimes biennial if emergence occurs in late summer. Each plant can produce over 40,000 wind-disseminated seeds. Hairy fleabane is often found growing in the same locations as a related species, horseweed. Hairy fleabane 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. Frequent tillage or soil disturbance can significantly reduce hairy fleabane numbers. Soil-residual herbicides such as simazine and rimsulfuron can provide good control before the plants emerge. Paraquat and glyphosate can both control susceptible populations of hairy fleabane when plants are small (before sending up flowering stalks); however, California has many populations resistant to one or both of these herbicides. Even susceptible populations of hairy fleabane are difficult to control as plants get larger. For example, glyphosate at 1 lb a.e./acre will generally control plants up to 13 leaves, but for plants with 14 to 19 leaves, 2 lb a.e./acre is required. Hairy fleabane plants with more than 19 leaves are not adequately controlled with glyphosate. 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. Because hairy fleabane has developed resistance to glyphosate and paraquat in many parts of California, it is important to avoid repeated applications of one herbicide, or low rates of glyphosate, so that further resistance to this herbicide does not develop. 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)
Horseweed is typically a summer annual weed. However, plants that germinate from seed in late summer often act like biennials. Horseweed has a woody stalk and can grow up to 10 feet tall. Under ideal conditions, more than 200,000 seeds can be produced by a single plant and the seed can be disseminated by wind more than 1/4 mile. Frequent tillage before the plant reaches the rosette stage offers good control. Flaming or mowing does not provide adequate control of horseweed. Soil-residual herbicides, including simazine, isoxaben, rimsulfuron, and flumioxazin provide good control at high label rates. Use 2,4-D for the best control of emerged plants from the seedling to rosette stage. Control in California with glyphosate is variable. Horseweed is known to develop resistance to glyphosate where repeated applications are used. Thus, it is critical to monitor control efforts and follow up with hand hoeing to prevent escape of any plants that might be resistant.
Johnsongrass is a rhizome-producing perennial grass that can be a serious problem, especially in young pear 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. In nonbearing orchards, repeated applications of selective postemergence herbicides such as fluazifop-p-butyl, glyphosate, or others will often be required for control. Johnsongrass is most effectively controlled by fluazifop-p-butyl when it is between 8 and 18 inches tall. A second application can be applied to prevent rhizome production and limit regrowth. Apply glyphosate when johnsongrass is actively growing and between 12 and 24 inches tall.
Johnsongrass populations have developed resistance to glyphosate in other states where repeated applications are used. Thus, it is critical to monitor control efforts when glyphosate is used, and follow up to prevent escape of any plants that might be resistant.
Geese are also effective at controlling johnsongrass in organic orchards, but be aware of food safety concerns and check if animals in the orchard are acceptable with the buyer.
LITTLE MALLOW (CHEESEWEED)
Little mallow is an annual or biennial plant that can occasionally be difficult to control with preemergence herbicides. Glyphosate may not control plants larger than 4 to 6 inches. Mature plants are tall and woody with a large taproot that can be removed with a shovel or with cultivation. Oxyfluorfen effectively controls seedlings and young plants.
Redroot and tumble pigweed are summer annual weeds that begin germinating in spring when the temperature warms and continue germinating through the summer, even after postemergence herbicides have been used. Preemergence herbicides used in the fall and winter may not have a long enough residual activity to control these pigweeds.
Puncturevine is a prostrate summer annual plant that begins germination in early spring and continues to germinate though the summer, especially after irrigation. Populations build up on the drier edges of orchards free of competition, where the spiny seed heads are picked up by vehicles such as tractors and trucks. Seeds have an initial dormancy of 6 months to a year. Buried seeds may remain dormant for several years before germination. Seed weevils released in 1961 have kept the overall puncturevine population down, but the weed can still be a problem in some areas.
Ryegrasses are winter annual 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 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 is often 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 suspected, avoid moving resistant weeds from one field to another by cleaning equipment before moving out of a field with herbicide-resistant weeds.
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 these plants are destroyed by cultivation or an herbicide, then a new bud on the tuber is activated. Yellow nutsedge is often one of the few surviving weeds in herbicide-treated tree rows. In established orchards, if a nutsedge infestation develops, spot-treat it with glyphosate. For best results, treat young plants before more than five leaves have formed, which is about the stage they begin to produce 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.
UC IPM Pest Management Guidelines: Pear
R. B. Elkins, UC Cooperative Extension, Lake County