KBG (Read et al., 1999). At present, the mechanism by which HBG resists heat stress, and whether it is an indication of drought resistance, is not known.

In the transition zone, where high summer temperatures are typically the most limiting factor to maintaining KBG, tall fescue (Festuca arundinacea Schreb.) is often recommended based on its better heat resistance (Duble, 1996; Christians, 1998). However, Wallner et al. (1982) reported no difference in the killing times and temperatures between these two grasses in vitro. This finding would suggest sustained transpirational cooling, via a deeper, more extensive root system, during periods of high temperature stress as the primary mechanism of heat avoidance in tall fescue. Strong positive correlations between possession of a deeper, more extensively developed root system with both heat and drought resistance have repeatedly been demonstrated in several turfgrass species (Lehman and Engelke, 1993; Qian et al., 1997; Ervin and Koski, 1998; Bonos and Murphy, 1999; Jiang and Huang, 2000). It is unknown if the improved heat resistance of HBG relative to Kentucky bluegrass is also associated with a deeper and more extensive root system.

In addition to deeper and greater root production and distribution throughout the soil profile, an inherently low rate of evapotranspiration (ET), as well the ability to maintain ET rate as the soil dries, have also been identified as mechanisms of drought avoidance (Beard, 1989; Carrow, 1996a). When soil is covered by a dense turfgrass stand, water loss is due mainly to transpiration. Low rates of transpiration may be achieved by inherent shoot characteristics such as leaf cuticle thickness, fewer exposed stomata, and slowed shoot growth rate (Beard, 1973). Research has indicated that KBG ET rate generally ranges from 3.6-6.3 mm d^-1 under varying climatic, edaphic, and