We investigated seasonal root production and root turnover of fertilized and well-watered monocultures of Phalaris for 2 years using minirhizotrons installed in six newly designed temperature gradient tunnels, combined with sequential soil coring. Elevated atmospheric CO2 treatments were combined with two cutting frequencies and three warming scenarios: no warming, +3.0/+3.0 and +2.2/+4.0°C (day/night) atmospheric warming. The elevated CO2 treatment increased both new and net root length production primarily when combined with atmospheric warming, where the constant warming treatment had a greater positive effect than the increased night-time warming treatment. Responses to elevated CO2 were greater when the swards were cut more frequently and responsiveness varied with season. For Phalaris swards, 17% of total net primary productivity went belowground. On account of root turnover, only one-third of the new roots produced in the year following establishment could be expected, on average, to be recovered from soil cores. The interaction between the effects of CO2 and warming, combined with the differential effects of the two warming treatments, has important implications for modelling belowground responses to projected climate change.