Although Citrus is a subtropical genus, northern gardeners can grow lemons, oranges, and other citrus trees in containers to enjoy fresh fruit. Standard-size orange and grapefruit trees can grow 18 to 22 feet tall, whereas dwarf varieties only grow 8 to 12 feet tall. Most citrus varieties are self-fertile, so only one tree is necessary. On average, fruit bearing begins when the trees are between 3 and 6 years old; however, exact timing will depend on the type of citrus (lemons, oranges, grapefruit, etc.), the cultivar, your climate, the health of the plant and its care, and other factors. Flowering is not seasonal, but occurs during warm weather and regular rainfall. Flowers and fruits may coincide.