The January mean minimum temperature values used in the current maps indicate a cold limitation of tall fescue suitability in the humid eastern United States extending across central Minnesota, Wisconsin, and Maine. The limit of adaptation in cold environments is defined by the ability of below-ground perennial plant parts, including crown buds and short rhizomes, to survive the subfreezing temperatures of winter. Thus, high latitude and altitude place geographic limits on tall fescue persistence. While tall fescue can survive quite cold conditions, it is less cold tolerant than Kentucky bluegrass (Poa pratensis L.), timothy (Phleum pratense L.), smooth bromegrass (Bromus inermis Leyss. subsp. inermis), or reed canarygrass (Phalaris arundinacea L.) (Balasko and Nelson, 2003). Desiccation of soil surrounding roots and crowns during winter reduces the temperature-buffering effect of soil water, thus reducing tall fescue survival in cold, dry climates such as in the prairies of the Dakotas, Canada, and central Asia. Genetic and paleo-climatic evidence suggests that one likely center of origin for tall fescue was the Mediterranean Basin, and that it survived in more northerly areas as the southern European and northern African regions became drier (Buckner et al., 1979).
Consistent winter snow cover enhances the probability of surviving the lower temperature extremes, owing to the protective insulation from extreme cold and the desiccating effects of wind provided by snow (Burns and Chamblee, 1979). With good management, tall fescue can withstand mean monthly minimum temperatures at least as low as -16°C (Burns and Chamblee, 1979). Thirty-eight percent survival was found during experiments in Brandon, Manitoba, Canada at this temperature, where the major factor associated with winter injury was lack of snow cover (Burns and Chamblee, 1979). They also reported moderate survival of tall fescue seedings near Regina, Saskatchewan, Canada (50°3' N). In northern areas, proper cultivar selection and cutting management (number of harvests per season and interval between harvests) are critical to minimize the risks of winter damage (Drapeau et al., 2005). Good management can help maintain persistence, but adequate snow cover to prevent winter injury is a defining factor for survival of perennial forages (Leep et al., 2001).
Currently winter indices are being determined to assess the impact of subfreezing air temperatures, loss of cold hardiness due to temperatures rising above 0°C, and the potential damage to the root system due to soil heaving and ice encasement (Bélanger et al., 2006). Winter survival is not assessed in the current iteration of the Mapserver application since suitability mapping is the goal; however, future iterations of this application could include the use of these winter indices to map winter survival.