Problem weeds include burning nettle , annual sowthistle , shepherd's-purse , London rocket , purslane , hairy nightshade , chickweed , and nutsedges . If problem weeds are present in significant numbers, the best strategy is to rotate to a crop in which they can be successfully controlled. Planting date can have an impact on weed problems in a given region. For instance, fields planted between October 1 and October 15 in the southern desert are often fairly weed-free. Sanitation is critical in a weed management program.
Some weeds can produce thousands of seeds in a single season.
To reduce seed production, disc or mow harvested fields before weeds flower and produce seeds. If weedy species are present, cultivate areas around the field such as field edges, fence lines, roadsides, and irrigation ditches regularly to prevent weed seed production. Cultivation equipment and irrigation water must also be kept free of weed seeds and vegetative propagules to avoid spreading weed populations. Preplant plowing, followed by irrigation and one or two discings before bed formation, will destroy many weeds. Deep plowing and inverting soil to a depth of 16 inches has significantly reduced sowthistle and groundsel infestations in the central coast region, and may also serve to reduce certain disease organisms.