Citrus trees form a family of sub-tropical evergreens. Even in winter, they never become fully dormant and thus lack the deciduous tree's ability to protect itself against freezing weather. Most produce fruit with rinds of varying thickness and pulpy, juicy flesh. For the home landscaper, the most popular members of the citrus family are mandarins (satsumas, tangerines, etc.), oranges, grapefruits, kumquats, calamondins, lemons and limes. There are also varieties of tangerine and lime hybrids. Generally, kumquats and members of the mandarin group, including tangerines and satsumas, are considered to have the highest level of resistance to cold. Grapefruits and limes are considered to be the most cold-sensitive. All, however, require help in order to survive sharp or sustained freezes defined as periods when the temperature drops to the low 20s (°F) or below.