Meyer lemons turn yellow on the tree, because citrus fruits do not continue to ripen after harvest. Thus, they tend to become sweeter the longer they are left to hang.
Grown for their sweeter-than-average fruit, fragrant flowers and dark green leaves, Meyer lemons are thought to be a cross between a mandarin orange and a lemon. Since the 1970s, the only variety available in the United States has been the improved Meyer lemon, which offers better virus resistance. In addition, these hearty trees grow well in containers and can be kept inside as houseplants, because they remain small and are thornless, unlike other citrus.