Growth and reproductive potential with respect to season and photosynthetic gas exchange behavior under elevated (short-term) CO2 at varying temperature, relative humidity (RH), and irradiance levels were investigated in ragweed parthenium (also known as carrot grass and congress grass), a noxious weed in India. Lower values of biomass, relative growth rate, net assimilation rate, crop growth rate, leaf area duration, leaf area index, and numbers of flowers and seeds in winter compared with summer stands showed that ragweed parthenium is greatly suppressed by low temperatures during winter. This was due to constrained vegetative growth, seedling emergence, and seed to flower ratio. The species showed maximum photosynthetic response to temperature at 25 to 35 C, and the net photosynthetic rate was reduced considerably at a low temperature (7 C). These temperatures approximately corresponded to the normal temperatures experienced by summer (25–35 C) and winter (7 C) stands of ragweed parthenium. Elevated CO2 enhanced leaf net photosynthetic efficiency, maximum photosynthetic rate, and water use efficiency (WUE) but decreased the light compensation point for net photosynthesis, stomatal conductance, and transpiration rate. The interactive effects of elevated CO2 and temperature resulted in a decrease in light-limited and light-saturated net photosynthetic rates and WUE. The interactive effects also reduced an elevated CO2-induced decrease in light compensation point relative to elevated CO2 alone. Stomatal conductance was insensitive to photosynthetic photon flux but was greatly influenced by RH. Leaves of the species may show increasing rates of net photosynthesis with a rise in CO2 and temperature. However, excessive increase in transpiration with temperature, especially at 47 C (noon temperature during summer in the plains of northern India), appears to be disadvantageous for the leaves when conservation of water is of prime importance.