The Type of Orange Tree Used for Grafting

An Early Favorite

• Sweet orange (Citrus sinensis) was historically one of the first rootstocks used in the U.S. There are many cultivars of sweet orange used for eating fresh and for juice. The varieties of sweet oranges grown for seedling rootstocks are generally those which have lots of seeds. The rootstock has moderate cold hardiness, growing in U.S. Department of Agriculture plant hardiness zones 9 through 11. Sweet orange rootstocks have good root growth in rich, sandy loam soil with abundant lateral root formation. However, it became less used because it is not resistant to the disease foot rot. Sweet orange rootstock produces citrus trees that grow large, have vigorous growth, give good yields of high-quality fruit, and are resistant to a number of citrus diseases.

Once Most Popular

• Sour orange (Citrus aurantium), also called bitter orange or Seville orange, mostly replaced sweet orange because it is resistant to foot rot. It is widely used in citrus-growing areas throughout the world except for areas where the tristeza, or quick-decline virus occurs. Sour orange is susceptible to tristeza. As tristeza virus spreads, other rootstocks resistant to the virus are used. It grows well on moist, fairly fertile soils and heavy adobe soils, developing a deep root system with multiple tap roots. Sour orange grows in USDA zones 9 through 11 and has uses in its own right as an ornamental tree and as the source of marmalade, liqueurs and essential oils from the fruit rind and flowers.

The Most Cold Hardy

• Trifoliate orange (Poncirus trifoliata) is hardy in USDA zones 5 through 9. This deciduous shrub to small tree has showy, fragrant, white spring flowers and small yellow fruits. Native to Asia, it is a citrus rootstock where cold-hardiness is important. The variety "Flying Dragon" (Poncirus trifoliata "Monstrosa") is frequently used for dwarfing citrus, which makes them easier to harvest and more suitable for home orchards where growing space is limited. Trifoliate orange isn't used in areas where the exocortis virus, sometimes called scalybutt, occurs, since it is susceptible to that disease.

Less Frequently Used

• "Cleopatra" mandarin orange (Citrus reticulata "Cleopatra," also called Citrus nobilis) had some use as a rootstock because of its resistance to the tristeza virus. Unfortunately, sweet oranges grafted onto "Cleopatra" have reduced size, lower yields than those on sour orange, and more acid fruits. It also has greater resistance to gummosis, or gum disease than sweet orange, and lemons and grapefruits were grafted to it. Mandarin hybrid rootstocks are possible candidates for tristeza virus-resistant rootstock. "Cleopatra" itself is grown for its juicy, sweet, easy-to-peel fruits. Mandarins are native to Asia and hardy to USDA zones 8b through 11, and are among the earliest-ripening citrus.