Common reed [Phragmites australis (Cav.) Trin. ex Steud.], an aggressive invader in North American wetlands, is likely to undergo a range expansion as the climate changes. Increased atmospheric [CO₂] and temperature have been shown to cause morphological and physiological changes in many species, sometimes altering the way they respond to herbicides. To understand how climate-related environmental parameters may impact P. australis management, we grew two P. australis haplotypes (the Gulf Coast type and the Eurasian type) under ambient (400 ppm CO₂, 32/21 C) or elevated (650 ppm CO₂, 35/24 C) climate conditions. After 6 wk, the Gulf Coast type had reduced leaf area, increased stomatal conductance, and increased transpiration under the elevated conditions. The Eurasian type had lower V_cmax (the maximum carboxylation rate of Rubisco) and lower J_max (the maximum electron transport rate of RuBP regeneration) under elevated climate conditions. Results likely reflected a greater impact of higher temperatures rather than increased [CO₂], After the 6-wk period, plants were either treated with glyphosate (0.57 kg ae ha^-1) or remained an untreated control. Data were collected 30 d after treatment (DAT) and 60 DAT to evaluate herbicide efficacy. Overall, the Gulf Coast type was less responsive to glyphosate applications under the elevated climate conditions than under current climate conditions. The lower leaf area of the Gulf Coast type in these climate conditions may have resulted in less herbicide interception and uptake. Glyphosate efficacy was less impacted by climate treatment for the Eurasian type than for the Gulf Coast type.