The best strategy for controlling nightshade species (hairy, black, and cutleaf nightshade) is to plan a crop rotation sequence that prevents population build-up. Choose rotation crops, such as corn, sorghum, cereals, or sugar beets, which can be managed with herbicides that kill nightshades. Nightshades are prolific seed producers; once a nightshade numbers increase, it may take several rotations to reduce infestations significantly. Tillage, a combination of two or three different herbicides effective on nighthades and multiple herbicide applications may be necessary to control nightshade infestations in potatoes. Dimethenamid-p, flumioxazin, and rimsulfuron are herbicides that have been shown to be effective at controlling hairy and black nightshade.
Yellow nutsedge is very serious problem in the Kern County area and must be carefully managed at harvest since nutsedge rhizomes can reduce potato quality by penetrating tubers. This weed does not tolerate shade; once potato vines have closed over, further nutsedge emergence and growth is usually suppressed. In early-season potato areas, herbicides may not be necessary if the potato canopy closes over the row before nutsedges begin to emerge. In fields where nutsedges will most likely emerge before row closure, plant to use a preemergence application of EPTC before March 1; if necessary and allowed by label, apply EPTC again after approximately 1 month. In regions with high organic matter content soils, apply s-metolachlor to the soil surface and incorporate before potato emergence.
Before harvest, thoroughly apply vine-killing agents to suppress nutsedge growth after vine death and to help prevent tuber damage. Several herbicides can be used to control or suppress nutsedge in rotation crops; some very effective herbicides are available for grains or alfalfa. It is most likely not possible to eradicate nutsedge, however, even when weed control is used during fallow periods.
Quackgrass is a problem in the northern California potato-growing areas. Its rhizomes may penetrate potato tubers, reducing their quality. Quackgrass growth can be suppressed for one growing season and rhizomes reduced, but not eradicated, with a preplant EPTC application. The area to be treated must be tilled thoroughly to cut quackgrass rhizomes into small pieces before applying the herbicide. Incorporate the EPTC into the soil by discing 6 inches (15 cm) deep in two directions. Fall treatments with EPTC may provide additional control.
Before potatoes are planted, glyphosate can be used to control actively growing quackgrass that is at least 8 to 10 inches tall. For complete control, repeated applications may be necessary. Wait at least 5 to 7 days after the last application before preparing seedbeds. After planting, if quackgrass is a problem, clethodim or sethoxydim may be used. Sequential treatments may be necessary.