Pest Management Guidelines
Special Weed Problems
(Reviewed 12/09, updated 12/09)
BERMUDAGRASS AND JOHNSONGRASS
Preplant herbicides such as bensulide (Prefar), napropamide (Devrinol), or trifluralin (Treflan) effectively control these perennials growing from seed but they will not control growth from established rhizomes.
Disk fields infested with bermudagrass and johnsongrass plants several times to cut the rhizomes into short segments. This will significantly enhance the effectiveness of sethoxydim (Poast), which can be used to selectively control these two weeds in all varieties of peppers. The grasses should be growing vigorously at the time of treatment and an oil or paraffin-based adjuvant should be added to sethoxydim. If regrowth occurs, a second application will be required.
Organic growers may want to try weeder geese, which prefer to eat grasses over broadleaf plants. Geese have been reported to root out rhizomes and consume them, helping to eliminate johnsongrass.
Bindweed is a deep-rooted perennial that is difficult to control once it becomes established. Herbicides are not effective against established plants. Seedlings can be easily told from established plants by the presence of cotyledons. Established plants require cultivation for control. Field bindweed is best controlled after a cereal crop when actively growing bindweed can be treated in fall with glyphosate (Roundup) or a similar material. Rotation with cotton, corn, or lettuce has helped reduce field bindweed in some areas. The frequent cultivations used in lettuce help control this weed, and in cotton, glyphosate can be applied with hooded sprayers or over the top if Roundup Ready cotton is planted. In corn, both cultivation and selective herbicides can be used to control field bindweed.
Dodder is a parasitic plant; its seedlings must attach to a suitable host to survive. In addition to peppers, other known hosts of dodder include tomatoes, safflower, sugarbeets, alfalfa, asparagus, honeydew melon, onions, carrots, nightshades, and numerous broadleaf weeds. Rotations are generally not effective in eliminating dodder because it has a wide host range and its seeds can remain viable for years. However, rotation to nonhost crops such as cotton, corn, cereals, and garlic can help reduce the seed population. The standard way to control dodder has been to destroy the host pepper plants as soon as dodder is observed. If dodder is flowering, remove the host plants from the field and burn them to kill the seed. An application of pendimethalin (Prowl H20) preemergence, which is registered in transplanted peppers only, has been shown to reduce dodder germination and emergence by 80%.
Most dodder germinates between March 1 and May 20, so late planting or transplanting can help reduce problems with dodder.
LITTLE MALLOW (CHEESEWEED)
Little mallow is an aggressive biennial plant that produces seeds that remain viable in the soil for many years. It is only marginally controlled by napropamide and, as a result, is frequently present in pepper fields. Oxyfluorfen (GoalTender) can be used preplant to effectively control this weed. It can be removed in hand weeding and thinning operations although the deep taproot makes this job difficult. Cheeseweed that escapes control can make harvest operations difficult.
The nightshade family includes black nightshade, hairy nightshade, cutleaf nightshade, groundcherry, and several others. These annual weeds are related to peppers and are resistant to many of the herbicides commonly used in pepper production; S-metolachlor is effective on the nightshades. Soil fumigation with metam sodium can be very effective if it is done properly. Cultivation is also effective. Rotating to crops where available herbicides control nightshade helps to avoid seed buildup. Roguing nightshades from the field to reduce seed deposition is effective if the weeds can be distinguished from the crop plants.
Nutsedge is a perennial that reproduces primarily through abundantly produced tubers, but can also produce viable seed. Tubers can remain viable in the soil for several years until conditions are favorable for growth. The tubers contain four to seven buds, each capable of producing a plant. Generally only one bud will germinate on any tuber; however, if the top is removed by cultivation or herbicide treatment, another bud will form a new plant. Fields infested with nutsedge should not be planted to peppers. If this is not possible, deep plowing to a depth of 10 to 12 inches with a moldboard plow has provided 95% control. The soil has to be thoroughly inverted to obtain good control. Yellow nutsedge can be particularly troublesome in Southern California because it can grow through the black plastic mulch used in this area on fresh market peppers.
Metam sodium applied before planting only delays emergence of nutsedge, especially on sandy soil, but does not provide effective control. Halosulfuron (Sandea) provides effective control of both yellow and purple nutsedge after it emerges while S-metolachlor is effective as a preemergence application on yellow nutsedge only.
UC IPM Pest Management Guidelines: Peppers
WeedsR. F. Smith, UC Cooperative Extension, Monterey County
O. Daugovish, UC Cooperative Extension, Ventura County
Acknowledgment for contributions to Weeds:
C. E. Bell, UC Cooperative Extension, Imperial County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis