For a given soil texture, water availability to the plant is a function of the balance between precipitation and potential evapotranspiration. When summer evaporative demand greatly exceeds precipitation and soil water deficit is critical, tall fescue growth stops (Frank et al., 1996). When water is adequate, however, summer-active types remain green and continue some growth through warm weather (Norton et al., 2006). Endophyte [Neotyphodium coenophialum (Morgan-Jones and Gams) Glenn, Bacon, and Hanlin] symbiosis usually improves tall fescue survival in the warmer, drier climates; indeed, the naturally occurring wild endophyte extends the range of adaptation of summer-active types tall fescues further south in the humid transition zone (West and Waller, 2007). However, infection with wild-type endophytes frequently causes animal health problems, necessitating careful management of such tall fescue when used for livestock feed (see Chapter 12, Chapter 16, and Chapter 17). Tall fescue cultivars containing toxic endophytes are not recommended in areas having more favorable summer growing conditions. However, cultivars containing nontoxic novel endophytes that do not produce toxic ergot alkaloids have been developed and used effectively (see Chapter 20 and Chapter 25).
Arrested summer growth in summer-active tall fescue in response to water deficit is not true dormancy, but simply a symptom of drought stress. Tall fescue germplasm of Mediterranean origin can exhibit a degree of "summer dormancy" (i.e., arrested growth). This arrested growth can occur in response to postflowering ontogeny and summer conditions, such as long days or moisture deficits), and allows these tall fescues to maintain a low activity despite infrequent summer rain storms (Norton et al., 2006). True summer dormancy, although not necessarily complete, can allow tall fescue to survive long, rain-free summers and thus expand its adaptation zone toward the drier end of the subhumid zone (625 mm annual precipitation) of the southern Great Plains (Malinowski et al., 2005). Such tall fescue genotypes also exhibit a greater degree of winter growth than the summer-active genotypes.