Weeds Pages
JOHNSONGRASS
Johnsongrass can be a serious problem, especially in young prune 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 postemergent herbicides (fluazifop-p, glyphosate) will often be required for control of johnsongrass. Geese are also effective at controlling johnsongrass in organic orchards. In new plantings, trifluralin or norflurazon will control seedling johnsongrass but not established (rhizomatous) johnsongrass.
BERMUDAGRASS
Bermudagrass is a vigorous spring- and summer-growing perennial. It grows from seed, but because of its extensive system of rhizomes and stolons can also 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 (Roundup, Touchdown, etc.). 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
Dallisgrass is a common perennial weed found in orchards. It can be highly competitive 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. It 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 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 will eventually deplete the root system and starve the plant.
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. 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. Hairy fleabane and a close relative, horseweed, have developed resistance to glyphosate. Thus, it is critical to monitor control efforts and follow up with hand weeding to prevent escape of any plants that might be resistant.
HORSEWEED (MARE'S TAIL)
This annual weed can grow up to 10 feet tall with a woody stalk. It can interfere with harvesting practices if not controlled. Like hairy fleabane, this weed can withstand mowing and cause similar problems. Control measures are also similar to hairy fleabane.
YELLOW NUTSEDGE
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 nutsedge 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 often necessary to control late germinating plants. Nutsedge can be suppressed by a preemergent application of norflurazon (Solicam).
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, which 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 quart/acre applied with glyphosate in April to the area between the tree rows in the orchard can provide season long control.
RYEGRASSES
Ryegrasses are winter annuals 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 weed 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 mechanisms of strategies. 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 weed resistance. Consider scheduling known resistant fields as your last ones to be planted, harvested, etc.