The growth habits of the crop and competing weed species are important determinants of crop–weed competition. Three cowpea genotypes with similar vegetative vigor but different growth habit were assessed for their relative competitiveness with two weed species. ‘Iron-Clay’ (IC) grows erect, ‘IT89KD-288’ (288) is semierect, and ‘UCR 779’ (779) is prostrate. Common purslane, a short-statured weed, and common sunflower, a tall species, were planted within the cowpea rows. Cowpea canopy height and width, leaf area, dry weight, and light intensity above and below sunflower and cowpea canopies were measured weekly from 21 d after planting. Sunflower reduced the leaf area, amount of light received, and biomass of all cowpea genotypes. Purslane reduced the leaf area of 779 and the biomass of 288 and 779, but the biomass and leaf area of IC were not affected. The presence of sunflower increased the height of IC and 288, but the presence of purslane decreased the canopy height of 779. IC reduced sunflower biomass, whereas IC and 779 reduced purslane biomass. IC and 288 reduced sunflower leaf area, whereas IC and 779 reduced purslane leaf area. The growth analysis of biomass, leaf area, and canopy height of cowpeas and weeds showed similar results. The experiments indicate that cowpea genotypes differ in their ability to compete with purslane and sunflower. IC was the most competitive genotype, suggesting that an erect growth habit may be more effective in suppressing weeds than semierect or prostrate growth habits.

Nomenclature: Common purslane, Portulaca oleracea L. POROL; common sunflower, Helianthus annuus L. HELAN; cowpea, Vigna unguiculata (L.) Walp. VIGSI. ‘Iron-Clay’, ‘IT89KD-288’, ‘UCR 779’.