Commercial citrus types differ in cold hardiness. Mandarin trees are most hardy followed by (in order from most hardy to least hardy) Sour Orange, Sweet Orange, Grapefruit, Pummelo (Shaddock), Lisbon lemon, Eureka lemon, Tahitian lime, Mexican lime, and citron. Trees of citron, lime, and lemon are least hardy because they tend to grow continuously and flower and fruit in cool weather. On the other hand, grapefruit, sweet orange, mandarin, and kumquats are more frost hardy because their growth stops in cold weather. Also, a tree's tolerance to frost is not the same as the fruit's tolerance to frost. For example, although satsuma mandarin trees are very cold hardy, their fruit are small and thin skinned, which makes them more susceptible to frost damage than grapefruit, which are large and have a firm rind. Another factor responsible for differences in frost tolerance is the maturity of the fruit. Fruit that is closer to maturity at the time of a freeze can withstand more cold than immature fruit. For example, although Valencia and navel trees are equally cold hardy, navel orange fruit are generally more cold tolerant than Valencia fruit because navel fruit mature earlier and have a higher sugar content during the coldest months. - Millet