Seed germinability at high temperature (25 °C) was found to differ among golf course populations of Poa annua L. located along a gradient of different intensities of turf irrigation and mowing practices. A seed germination index was calculated to express the effect of temperature on seed germination. The seed germination indices of the less managed populations from the golf roughs were uniformly low and less than 30%, with a mean index of 5%. Conversely, the germination indices of the golf green populations were high. Most seed families of the golf green populations had a germination index higher than 60%. A wide range of seed germination indices was found in seed families of the fairway populations, ranging from 10 to 90% with a mean index of 49%, and with a bimodal distribution. The results of this study indicate that rapid genetic differentiation in temperature-enforced seed dormancy among populations of Poa annua had taken place at the micro-ecological level.