Technical Abstract: Invasive plants are estimated to cost U.S. agricultural and forest producers 34 billion dollars each year from lost productivity and increased costs for weed control. While much effort is being spent studying these exotic plant pests, little work has been done into how invasive plants might react to the increasing carbon dioxide (CO2) concentration in the atmosphere. We grew a variety of invasive plants under ambient and elevated levels of CO2 from the seedling stage through flowering at which time they were harvested and measured. Virtually all the plants studied grew larger (9-40%) when exposed to elevated CO2; however, the response to additional CO2 varied greatly among species. In general, plants responded as expected according to their type, with grasses having less response than other species. This suggests that, while both types of weeds are likely to become worse pests as atmospheric CO2 increases, grass weeds may not be as big a problem as other weeds. Also, several weeds showed a delay or decrease in flowering when exposed to elevated CO2 which suggests that, while invasive weeds may grow larger under a future, high CO2 environment, their ability to spread might be reduced; this possibility certainly deserves further research.