Agricultural pest management
Special Weed Problems
(Reviewed 4/17, updated 4/17)
ANNUAL SOWTHISTLE, COMMON GROUNDSEL, and PRICKLY LETTUCE
Annual sowthistle, common groundsel, and prickly lettuce, which are in the Asteraceae (sunflower) family. are closely related to lettuce and are often pests in lettuce fields because they are not controlled by lettuce herbicides. Avoid growing lettuce in fields known to be heavily infested with these weeds. Cultural controls, such as crop rotation, preirrigation and cultivation, and cultivation during the growing season, are the best ways to manage these weeds.
Shepherd's-purse is an annual weed in the mustard family that is not controlled by most of the lettuce herbicides. It is also difficult to control with hand hoeing and cultivation because of its high seed production and resultant dense populations. Select fields that are not heavily infested with this weed. Management tools, such as crop rotation, preirrigation, cultivation during the growing season, and the use of pronamide (Kerb) are the best choices for control of this weed.
Burning nettle is an annual weed that competes with lettuce and also makes harvest difficult because it has stinging hairs on the plant surfaces. Select fields that are not heavily infested with this weed. Control is best achieved with the use of crop rotation, preirrigation and cultivation, and cultivation during the growing season, or with pronamide (Kerb).
Nutsedge is a serious weed in spring- and summer-planted lettuce. Yellow and purple nutsedge are perennial weeds that reproduce from underground tubers capable of survival for several years in the soil. Each tuber contains several buds that are capable of producing plants. Only one bud at a time sprouts to form a new plant; however, if that bud or plant is destroyed by cultivation or an herbicide, then a new bud sprouts. Control is best achieved by continuous cultivation during a summer fallow period or by rotating to crops where effective herbicide and cultural control methods can be used.
Three-week or older purslane plants that are cut by cultivation knives still have sufficient resources to set seed. Cut up or uprooted plants can also reroot and continue to grow. As a result, purslane numbers have increased in some areas. Rogueing and carrying the purslane plants to the edge of the field for subsequent disposal will reduce future problems with this weed. If purslane numbers continue to increase, then treat the entire bed top with any of the labeled preemergence herbicides effective on purslane for control.
UC IPM Pest Management Guidelines: Lettuce
R. F. Smith, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to Weeds:D. W. Cudney, Botany, UC Riverside
W. E. Bendixen, UC Cooperative Extension, Santa Barbara County
C. E. Bell, UC Cooperative Extension, San Diego County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis