We examined how anticipated changes in CO2 concentration and temperature interacted to alter plant growth, harvest characteristics and photosynthesis in two cold-adapted herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and orchard grass (Dactylis glomerata L. cv. Potomac). Plants were grown at two CO2 concentrations (362 [ambient] and 717 [elevated] μmol mol−1 CO2) and four constant day/night temperatures of 15, 20, 25 and 30°C in controlled environmental chambers. Elevated CO2 significantly increased total plant biomass and protein over a wide range of temperatures in both species. Stimulation of photosynthetic rate, however, was eliminated at the highest growth temperature in M. sativa and relative stimulation of plant biomass and protein at high CO2 declined as temperature increased in both species. Lack of a synergistic effect between temperature and CO2 was unexpected since elevated CO2 reduces the amount of carbon lost via photorespiration and photorespiration increases with temperature. Differences between anticipated stimulatory effects of CO2 and temperature and whole plant single and leaf measurements are discussed. Data from this study suggest that stimulatory effects of atmospheric CO2 on growth and photosynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.