Growth of non-woody plants in the fall represents a tradeoff between an extended growing season and increased vulnerability to frost. Within species along latitudinal gradients, plants may exhibit ecotype-specific sensitivities to the temperature and photoperiod cues that influence cold acclimation. Therefore, it is unclear for a given latitude how climate warming will alter the timing and extent of cold acclimation, and thus vulnerability to frost events. We evaluated relative temperature and photoperiod sensitivities during the fall cold acclimation period for two northern (Sweden) and two southern (Italy) ecotypes of the common forage grass Arrhenatherum elatius. Three temperature levels (Templow: 3 °C, Temphigh+low: 8 °C followed by 3 °C, and Temphigh: 8 °C) were crossed with 2 photoperiod levels (Photoshort: 6 h, and Photolong: 12 h) and administered to the plants for a three week acclimation period. All plants were then frozen at -8 °C for 1 day, and post-frost growth was measured after 3 weeks. Temphigh and Photolong increased growth prior to frost, but resulted in decreased growth after frost. The effects of temperature on sugar concentration, biomass and flower presence depended on photoperiod, with temperature only influencing sugar concentration and flowering at Photoshort, while Photolong increased biomass only at the high temperature. The faster growth rate of southern A. elatius before frost, in combination with sufficient cold acclimation, resulted in higher biomass accumulation after frost. The faster growth habit of southern ecotypes may be advantageous in accumulating higher summer biomass even after moderate frost events in the fall.