A method for investigating the thermoperiodicity of crop plants is described. On the basis of our experimental results and published data, crop plants were assessed to establish whether they were thermoperiodic or not. Tomato and maize were classified as thermoperiodic species. Cucumber, beans, peas, oats and wheat were not thermoperiodic. Strategies for climate control based on the estimation of optimum temperature are proposed. These can be used to minimize energy consumption and, together with other environmental treatments, could optimize the yield of glasshouse crops.