Summary • This study examined the effects of warming, elevated atmospheric CO2 and cutting regimen on the growth of Phalaris aquatica cv. Holdfast swards. • Six temperature gradient tunnels (TGT) were used to manipulate both air temperature and atmospheric CO2 concentrations (ambient and 750 ppm). Within each tunnel, there were three temperature treatments: no warming, constant warming of +3.0°C and a daytime warming of 2.2°C combined with a night-time warming of 4.0°C and two defoliation frequencies. • Averaged across 20 months of growth, there was a positive effect of elevated atmospheric CO2 (+11%), no effect of either warming treatment and a negative effect of frequent cutting (−19%) on total above ground biomass production. The responses to all treatments, however, were strongly seasonal. • Positive responses to CO2 were statistically significant only in the spring, when plant growth was strongest. No evidence was found that high night-time warming had different effects on plant growth and plant responses to elevated atmospheric CO2, than constant warming. ©New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01025.x