Yellow Sweetclover and White Sweetclover
Yellow sweetclover and white sweetclover are usually biennial but are occasionally summer annual or short-lived perennial weeds. They are difficult to control in onion and garlic because none of the registered herbicides will control them. Avoid growing onion or garlic crops in fields known to be heavily infested with these weeds.
Yellow and purple nutsedge are perennial weeds that reproduce from underground tubers, which can survive for several years in the soil. Each tuber contains several buds that are capable of producing plants. Although only one bud germinates at a time to form a new plant, a new bud is activated if a germinated bud or plant is destroyed by cultivation or an herbicide. To best achieve control of nutsedge, use continuous cultivation during a summer fallow period, or rotate to crops where effective herbicide and cultural control methods can be used. Deep plowing with specialized moldboard plows will bury tubers 10 to 12 inches and can significantly reduce nutsedge numbers. Dimethenamid provides partial control of nutsedge if applied before its emergence. Fumigation with metam products does not control purple nutsedge and only partially controls yellow nutsedge.
Annual bluegrass is mainly problematic in organic onion and garlic production. This weed can reach very high numbers in a field and become difficult to control, as cultivation and organic herbicides do not control it. While this weed was previously a problem in conventional production in the lower Colorado Desert, it has become less of an issue because clethodim controls it. In organic systems, management tactics include frequent handweeding or cultivation, preventing the weed from producing seed, using mulches that are several inches thick, and cleaning equipment before moving from infested fields to uninfested fields to prevent new infestations.
Dodder is a parasitic weed that can build up in onion fields in the San Joaquin Valley and coastal valley growing areas. Avoid fields with a known history of this weed. DCPA (onion) and pendimethalin (onion and garlic) suppress or control this weed.
Greater swinecress is a winter or summer annual, and sometimes biennial, broadleaf weed. It is a major problem in onion and garlic production in the low desert. Most herbicides registered for onion and garlic do not control swinecress especially in the Imperial Valley. Bromoxynil partially controls swinecress.
Littleseed canarygrass is a winter annual grass weed has become more of a problem in the low desert in recent years. It has become resistant to many herbicides that are selective to grasses, including clethodim and fluazifop. Napropamide, pendimethalin, and glyphosate control canarygrass. Canarygrass can also be managed by rotating to a crop (such as broccoli) for which other registered herbicides (such as trifluralin) are effective options.