Peach trees grow within a fairly limited range, since they have a chill-requirement that tropical areas cannot satisfy. The trees can usually tolerate temperatures to around -15 to -22 °F, though the following season's flower buds are killed at these temperatures, leading to no crop that summer. Flower-bud death occurs between 5 and -13 °F, depending on their particular cultivar (some are more cold-tolerant than others) and the timing of the cold, with the buds becoming less cold tolerant in late winter. Certain cultivars are more tender, and others can tolerate a few degrees colder. In addition, intense summer heat is required to mature the crop, with mean temperatures of the hottest month between 68 and 86 °F. A troublesome issue in many peach-growing areas is spring frost. The trees tend to flower fairly early in spring. The blossoms can be damaged or killed by freezes; if temperatures drop below 25 °F, most flowers will be killed. However, if the flowers are not fully open, they can tolerate a few degrees colder than that.