Greenhouse replacement-series experiments were conducted to evaluate the competitiveness of cowpea, sunn hemp, and velvetbean when grown in combination with yellow nutsedge and smooth pigweed. The effect of the cover crop species on yellow nutsedge tuber production was also evaluated. Cowpea and velvetbean were equally competitive with yellow nutsedge, but sunn hemp was less competitive. Although sunn hemp height was double that of cowpea or velvetbean, photosynthetically active radiation penetrating to the soil surface was twofold to eightfold greater than with the other two species. Leaf area per plant with sunn hemp monocultures were only 63 to 70% of cowpea and 37 to 41% of velvetbean. Increasing the proportion of cover crops in crop : weed mixtures did not significantly affect nutsedge tuber number per plant or tuber weight per plant. Cowpea was more competitive than smooth pigweed, whereas both sunn hemp and velvetbean were less competitive than smooth pigweed. The utility and efficacy of leguminous cover crop species for nutsedge and smooth pigweed suppression do not appear to be due to inherent competitiveness. Until cultivars that are more competitive become available, cultural measures should be employed that enhance cover crop modification of soil environmental conditions to minimize weed seed germination and vegetative propagule sprouting.

Nomenclature: Smooth pigweed, Amaranthus hybridus L. AMACH; yellow nutsedge, Cyperus esculentus L. CYPES; cowpea, Vigna unguiculata (L.) Walp; sunn hemp, Crotalaria juncea L; velvetbean, Mucuna pruriens (L.) DC. var. pruriens