Citrus fruits (Citrus spp.) range from small, sour Key limes (Citrus aurantifolia) through sweet oranges (Citrus sinensis) to large, acidic grapefruit (Citrus X paradisi), and include Asian citrus fruits such as pummelos (Citrus maxima). Citrus trees grow best in sunny spots and need frost protection. In areas that experience regular frosts and freezes, growing dwarf varieties in containers with drainage holes and overwintering them indoors in cool, bright rooms is the best method for growing citrus trees.
Limes produce fruit and leaves for drinks and cooking. Also called Persian lime and Mexican lime, Key lime is one of the most frost-sensitive citrus. Growing 20 feet tall, this evergreen tree is hardy in U.S. Department of Agriculture plant hardiness zones 9 through 11 and tolerates highly acidic and highly alkaline soil. Key limes' showy, fragrant, white flowers appear in spring, followed by yellow or green fruit 3 inches wide. Thai lime (Citrus hystrix, USDA zones 10 through 12) grows 6 to 25 feet tall. This tree provides strongly flavored leaves for cooking, but the rough, green fruits contain little juice, so often only the rind is used. Thai lime grows best in sandy, well-drained soil.
Lemon fruits (Citrus limon) can take a year to mature, but Meyer lemon (Citrus × meyeri) produces fruit year-round. Hardy in USDA zones 9 through 11, lemon trees grow 10 to 20 feet tall and carry sharp thorns. Fertile, well-drained, evenly moist soil produces the best fruit. Meyer lemons, which are lemon and mandarin orange hybrids, tolerate frost a little better than regular lemon trees. Growing 6 to 10 feet tall, Meyer lemons also bear fruits that are less acidic and more juicy than lemons. Meyer lemons are hardy in USDA zones 9 through 11 and grow best in neutral pH, freely draining, sandy soils.
Orange tree breeders have provided a wide range of varieties for commercial production and growing at home. Sweet oranges grow 20 to 30 feet tall and feature highly fragrant spring flowers and sweet juicy fruit. Hardy in USDA zones 9 through 11, sweet orange fruit remains fresh on the tree for months but doesn't ripen after picking. Mandarin oranges (Citrus reticulata) are hardy in USDA zones 8B through 11 and grow 15 to 20 feet tall. Mandarin varieties bear easy-to-peel fruit. Both sweet and Mandarin oranges produce the most fruit in evenly moist soil but tolerate dry soil.
Pummelos are Asian citrus fruits that are probably native to Thailand but may have come from Polynesia and the Malay Peninsula. Growing 15 to 20 feet tall, pummelos are hardy in USDA zones 9 through 11. Pummelo fruits are 6 to 9 inches wide, and weigh up to 4 pounds. Beneath thick rinds lies white, yellow or pink sweet, edible flesh. Sandy, freely draining, evenly moist soil produces the best crops.
Grapefruit are hybrids of sweet oranges and pummelos, yet their fruit is acidic. Highly fragrant, white flowers appear in early spring on grapefruit trees, which grow 20 to 50 feet tall. These trees are hardy in USDA zones 9 through 11, and they need freely draining soil.