Invasive plants are a major threat to the earth s biodiversity and cost U.S. producers $34 billion annually. Understanding how increased atmospheric CO2 may alter establishment, spread and control of invasive weeds is crucial to future management strategies. Here we report on the effects of elevated CO2 on growth of 2 invasive weeds important to Southeastern U.S. agriculture. Sicklepod (Cassia obtusifolia L.; C3 legume) and Johnsongrass (Sorghum halepense (L.) Pers.; C4 grass) were grown at either 375 mol mol-1 (ambient) or 575 mol mol-1 (elevated) CO2 in open top field chambers. Photosynthesis, morphology and biomass were assessed. Growth in elevated CO2 increased photosynthetic rate and water use efficiency for both species. While both species increased leaf and stem dry weights when grown under elevated CO2, the sicklepod tended to be more responsive than was Johnsongrass. Both plants tended to partition less total dry weight to reproductive structures when grown under high CO2. This study suggests that while both weeds are likely to increase in importance, sicklepod may be more of a problem than Johnsongrass in a future CO2-enriched world; this prediction may change if reproductive success is negatively impacted by elevated CO2 and this potential deserves further investigation.