The trifoliate orange, Citrus trifoliata or Poncirus trifoliata, is a member of the Rutaceae family and described as a distant relative of the other well known Citrus species. Believed to be native to central or northern China, this plant has a rough passing resemblance to other Citrus but plays an important role in cultivation as a rootstock.
The trifoliate orange tends to grow into a large shrub or small tree, with large 1-2 inch thorns. It has deciduous leaves with typically three leaflets as its name implies, and this can be contrasted with the largely unifoliate leaves of cultivated Citrus. Like true citrus the leaves give off a distinctive, almost spicy or zesty scent when crushed. The flowers roughly resemble citrus blooms, being white, slightly larger, and with a less pronounced scent. The round fruits tend to reach about 1.5 inches in diameter, ripen to a dull lemon yellow, have a soft downy surface, and tends to be filled with large amounts of seeds and a slimy pulp with an unpleasant bitter flavor that is nearly inedible. However with a bit of processing the fruits can be made into marmalade, the juice into flavoring syrup, the peel candied, used as a source of pectin, or used medicinally in China.
What makes the trifoliate orange especially important is its cold hardiness and tolerance to frost and snow, unlike many cultivated Citrus it can survive winter temperatures well below freezing to −22 °F (−30 °C). This, combined with its resistance to many Citrus diseases, and tolerance to waterlogged soil, exemplifies its value as a rootstock.
By grafting budwood of a desired Citrus fruit variety to the roots of trifoliate orange, the resulting tree can benefit from these resistances and can likely lead to healthier trees and better fruit yields than if it were grown on its own roots. Certain varieties of the trifoliate orange can impart a different habit to the plant, such as “Flying Dragon” which can be used to maintain the plant in a smaller “dwarf” state for container growing at home.
Given its relation to true Citrus, the trifoliate orange has been hybridized with other species, in particular by the citrus breeding program of the U.S. Department of Agriculture. These hybrids, known by their portmanteaus (e.g. citranges, citrumelos) were hoped to gain frost tolerance while still producing edible and commercially viable fruit. That could not be achieved, with the fruits still possessing an unpleasant flavor. Some of these hybrids still had some potential use as important rootstocks and decorative plants kept as interesting specimens.
References:
Trifoliate orange and rootstock
Dwarfing and Freeze Hardiness Potential of Trifoliate Orange Rootstocks