Cotton, spurred anoda, and velvetleaf were grown in controlled-environment chambers at day/night temperatures of 32/23 or 26/17 C and CO2 concentrations of 350 or 700 ppm. After 5 weeks, CO2 enrichment to 700 ppm increased dry matter accumulation by 38, 26, and 29% in cotton, spurred anoda, and velvetleaf, respectively, at 26/17 C and by 61, 41, and 29% at 32/23 C. Increases in leaf weight accounted for over 80% of the increase in total plant weight in cotton and spurred anoda in both temperature regimes. Leaf area was not increased by CO2 enrichment. The observed increases in dry matter production with CO2 enrichment were caused by increased net assimilation rate. In a second experiment, plants were grown at 350 ppm CO2 and 29/23 C day/night for 17 days before exposure to 700 ppm CO2 at 26/17 C for 1 week. Short-term exposure to high CO2 significantly increased net assimilation rate, dry matter production, total dry weight, leaf dry weight, and specific leaf weight in comparison with plants maintained at 350 ppm CO2 at 26/17 C. Increases in leaf weight in response to short-term CO2 enrichment accounted for 100, 87, and 68% of the observed increase in total plant dry weight of cotton, spurred anoda, and velvetleaf, respectively. Comparisons among the species showed that CO2 enrichment decreased the weed/crop ratio for total dry weight, possibly indicating a potential competitive advantage for cotton under elevated CO2, even at suboptimum temperatures.