Weeds Pages
BERMUDAGRASS
Bermudagrass is a vigorous spring- and summer-growing perennial. It grows from seed as is often introduced that way on mowers and other equipment; but it also has an extensive system of rhizomes and stolons which can also be spread during cultivation. It frequently becomes a problem in mowed orchards because mowing increases the amount of light reaching the stolons, stimulating their growth. This grass competes strongly with trees for moisture and nutrients.
Control seedlings with preemergence herbicides. If bermudagrass gets established in localized areas, immediately spot treat it with a postemergence herbicide such as glyphosate. In organic orchards, geese can be used to control grasses, including bermudagrass. When confined to an area containing bermudagrass, geese will dig up the rhizomes and completely consume the plant.
BARNYARDGRASS AND JUNGLERICE
Barnyardgrass and junglerice are related summer annual grass weeds that grow in dense, tall or spreading clumps. Several varieties that differ in growth habit and floral appearance occur in California. These summer grasses germinate in the spring and early summer and grow through late summer, producing large quantities of seed. Control can be difficult due to the late germination timing relative to preemergence herbicide programs commonly used in orchards as well as widespread glyphosate resistance in junglerice. Both species tend to prosper in wetter areas of the orchard such as low spots or near drip emitters.
Seedlings can be controlled, in some situations, with spring applications of preemergence herbicides (water incorporation needed), postemergence grass-specific herbicides (graminicides), as well as nonchemical practices such as cultivating, flaming, hand-weeding, mowing, mulching, and employing cultural practices that maintain and promote a dense tree canopy.
COMMON PURSLANE
Common purslane is a prostrate summer annual that reproduces from seed, which germinates from April to early May. Common purslane grows into a plant with fleshy stems; fragments of these stems can root and continue to grow after cultivation or mowing, if moisture is present.
If problems develop with this weed, use higher rates of glyphosate or contact herbicides to control it. A low rate preemergence herbicide program can also effectively manage this weed and reduce the need for preharvest applications. Pendimethalin at 1 pound (a.i.) per acre applied with glyphosate and incorporated with water in April to the area between the tree rows in the orchard can provide season-long control of common purslane.
FIELD BINDWEED
Field bindweed is a vigorous perennial weed that either grows from seed (which survives for up to 30 years in the soil), rhizomes, or extensive deep roots. Because of the seed's longevity in the soil, it is critical to destroy plants before they produce seed. The plants may spread from rhizome or root sections that are cut during cultivation, but cultivation controls seedlings.
If field bindweed appears in or around the orchard, spot treat with high label rates of glyphosate. Be extremely careful to avoid glyphosate drift onto the canopy or sucker leaves. Another alternative is to use a lower rate of glyphosate (2 pounds a.e. per acre) 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 classified as a winter annual, but in California, while the greatest emergence is in the winter, it can emerge throughout the year and occasionally persists longer than a year. It can withstand several mowings and still produce seed. In addition, it can interfere with moving sprinkler and drip irrigation lines as well as harvest operations.
Most populations of hairy fleabane, and a close relative, horseweed (mare’s tail), have developed resistance to glyphosate in California; some of these populations are also resistant to paraquat. Thus, it is critical to monitor control efforts and follow up with hand hoeing to prevent escape of any plants that might be resistant.
Preemergence herbicides, like indaziflam, flumioxazin, or rimsulfuron in bearing orchards, or isoxaben in non-bearing orchards, provide the best control. Paraquat or glyphosate can control many populations of hairy fleabane when it is small; however, resistance to both herbicides has been documented. Glufosinate effectively controls fleabane. Once plants bolt (sending up flowering stalks), paraquat or glyphosate will not control it anymore. Glyphosate at 1 pound (a.e.) per acre will control plants up to 13 leaves; for plants with 14 to 19 leaves 2 pounds (a.e.) per acre are required. Plants larger than 19 leaves are not adequately controlled.
HORSEWEED (MARE'S TAIL)
Horeseweed is an annual weed with a woody stalk and can grow up to 10 feet tall. Like hairy fleabane, this weed can withstand mowing and can interfere with irrigation and harvesting (especially prune harvesting). Horseweed is controlled using the same methods as those used to control hairy fleabane. Some horseweed populations are glyphosate-resistant and paraquat resistance has also been reported.
JOHNSONGRASS
Johnsongrass is a perennial grass that can be a serious problem, especially in young orchards. It can be controlled by repeated tillage during the summer months. However, the soil must be fairly dry, or the rhizome buds may sprout.
Repeated applications of grass-specific, postemergence herbicides (clethodim, fluazifop-P-butyl, or nonselective herbicides such as glyphosate) will often be required to control 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.
RYEGRASSES
Ryegrasses are winter annuals that are common throughout California. Glyphosate-resistant annual ryegrass is widespread throughout the cropping systems of the Central Valley and other production regions. There are also documented cases of ryegrass with resistance to other modes of action in orchards, vineyards, and annual cropping systems. The potential risk for the development of herbicide resistance is greatest when the same herbicide is used repeatedly, as often is done in orchards.
THREESPIKE GOOSEGRASS
Threespike goose grass is a short-lived perennial species with a spreading, tufted or bunched growth habit. It is a low-growing plant that tolerates mowing well and is a prolific seed producer on 2 to 4 spikes in a compact inflorescence. Seeds typically germinate and established plants resume growth in the early spring and produce seed from early to late summer in orchards. Plants usually die back to the crown in the winter.
Threespike goose grass seedlings can be controlled with many preemergence herbicides, but established plants are not well controlled by contact herbicides or glyphosate. Postemergence grass-specific herbicides (graminicides) can be effective on established plants.
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 can produce plants. One or two buds germinate to form new plants; however, if the plant is destroyed by cultivation or an herbicide, then a new bud in the tuber can be activated.
If nutsedge occurs in established orchards, spot treat with glyphosate. For best results, spray young plants before 6 leaves have formed, which is when they begin to produce tubers. Repeated applications are often necessary to control late germinating plants. Nutsedge can be suppressed by a preemergence application of norflurazon.
Nutsedges thrives in sandy soils and particularly in moist areas; presence of these weeds may indicate excessive irrigation relative to soil infiltration, crop water use, or both. Irrigation management can be a part of integrated nutsedge management, if only to contain the spread of existing weeds. Stands of yellow nutsedge may also indicate irrigation leaks which should be identified and repaired.