Growth and Competition of Black Nightshade (Solanum nigrum) and Palmer Amaranth (Amaranthus palmeri) with Cotton (Gossypium hirsutum)

Black nightshade plants were controlled by hoeing in the same cotton plots each year (1982 to 1986) for 3 to 15 weeks after crop emergence to evaluate the influence of several black nightshade-free periods on cotton yield, reproduction of black nightshade, and longevity of weed seeds in soil. Similar information, although limited, was also collected for Palmer amaranth that escaped the initial herbicidal treatment each year. Except for 1982, black nightshade competing with cotton for the duration of the growing season in nonhoed plots severely reduced yields (60 to 100%), with greatest yield reductions (82 to 100%) occurring in 1983 and 1984 when 0.5 to 0.7 cm of rain fell within 10 days after cotton planting. When combined with cultivation, a 3-week nightshade-free period at cotton planting was of sufficient duration to protect cotton yields. Weed seed production for all hoed treatments was less than 1% of the nonhoed treatment, and after five consecutive cotton crops (1982 to 1986), the amount of both black nightshade and Palmer amaranth seeds in soil was similar for all hoed treatments. These populations were 60 to 80% and 95 to 97% less than beginning populations of black nightshade and Palmer amaranth in 1982, respectively. After 5 yr of continuous treatments, cotton was grown in 1987, with standard cultivation as the only method of weed control, to evaluate how the weed-free periods in 1982 to 1986 influenced weed seed populations in the soil and the resultant effect on weed competition and cotton yields. Reduction of cotton yields in 1987, in the absence of weed-free periods, indicated that black nightshade seed survival in soil appears to be sufficiently long for ample establishment of this weed to compete with cotton. Thus, fields will have to be kept weed free for greater than 5 yr to reduce black nightshade populations to a level that will not reduce cotton yields.