Competition and proliferation of four smartweed species in cotton

Several species of smartweed may infest U.S. cotton. The most common are Pennsylvania smartweed (Polygonum pensylvanicum), pale smartweed (P. lapathifolium), and ladysthumb (P. persicaria). In North Carolina, another species, tufted knotweed (P. caespitosum), is common in lawns and wooded areas and sometimes found in cotton. Control of these species by herbicides may differ (e.g. Command herbicide controls Pennsylvania and pale smartweed but doesn't control ladysthumb) so an identification to species is important. Little is known about the competitive effect of these species in cotton so separate studies were conducted to evaluate competition and seed rain of these four species in 'Stoneville BXN 47 cotton'. Studies were conducted at Clayton, NC in 1998. Each species was planted five inches from the cotton row at 0, 1, 2, 4, 8, 16, and 32 plants per 30 row feet. An additional treatment had no cotton and one weed per 30 row feet to simulate a no-competition environment. Undesirable weeds were removed throughout the season. Height of four cotton and weed plants and diameter of weed canopies were determined biweekly throughout the season. At seasons end just before cotton harvest, all seed remaining on plants were hand harvested then all weeds were carefully removed and fresh and dry weights of four weeds were obtained. To account for seed fallen prior to harvest, seed on the ground were counted within four four-inch diameter randomly-placed rings. Cotton was then harvested and lint yield determined.