The effect of temperature on germination rate was assessed for seeds of 31 wild plant species and four cultivated species growing in the UK. The temperature at which seed first germinated ranged from 2 °C (Brassica rapa) to 11 °C (several species). As the temperature progressively increased, the percentage of seed that germinated rapidly increased to near 100%, whereas the duration for germination progressively decreased up to the thermal optimum. Above the thermal optimum, the duration for germination initially remained constant, and then rapidly increased until the thermal maximum was exceeded when no seed germinated. To assess the effect of temperature on germination rates, the reciprocal of the duration (1/duration) for 50% germination was calculated and regressed against temperature. For most species a linear regression accounted for >90% of the variation in germination rates for temperatures up to the thermal optimum. From the regressions, the base temperatures (Tb, below which no germination is expected) and the thermal constant (S, expressed in degree days) for 50% germination were calculated. The extremes were the alpine species Dryas octopetala, which had a high Tb (c. 12.0 °C) and low S (26 degree days), and Saxifraga tridactylites which had a low estimated Tb (c. −1.8 °C) and a high S (136 degree days). The estimated value of S increased with increasing percentage germination, and although S varied considerably between species with similar values of Tb, a weak inverse relationship between Tb and S was indicated. A thermal time approach was shown to be useful when analysing and comparing thermal requirements for germination of the seed of wild plant species. It showed, for example, that percentage germination often decreased as temperatures approached Tb, and that for some seed germination ceased at temperatures well above Tb.