Grafting combines two different plants to make a single, better variety. After hundreds of years of study, botanists have concluded that plants of different species, but within the same genus, are compatible. You may not be able to graft apples to oranges, but you can make a lemon-lime tree.
Citrus are an acidic fruit variety that produce small fragrant flowers and juicy fruit high in vitamin C. Citrus trees fair best in tropical and subtropical climates. Species in the genus Citrus include lime, orange, lemon, citron, grapefruit, mandarin and tangelo.
The genus Prunus encompasses plants typically considered summer pitted fruit. Fruit trees in this genus require a cold winter and do best in USDA Hardiness Zones 3 to 8. Fruit trees in this genus include plum, almond, apricot, cherry, almond, peach and nectarine.
Malus encompasses the apple species. Malus trees prefer cold winters and thrive in USDA Hardiness Zones 4 to 9. Fruit trees in this genus include apples and crab apples.