The trifoliate orange, Citrus trifoliata or Poncirus trifoliata, is a member of the family Rutaceae in the Citrus genus. Whether the species should be considered to belong to its own genus, Poncirus or included in the genus Citrus is debated. The species is unusual in Citrus for having deciduous, compound leaves and pubescent (downy) fruit.
The plant is a fairly cold-hardy citrus (USDA zone 6) and will tolerate moderate frost and snow, making a large shrub or small tree 4–8 m tall. Because of its relative hardiness, citrus grafted onto Citrus trifoliata are usually hardier than when grown on their own roots.
Citrus trifoliata is recognizable by the large 3–5 cm (1.2–2.0 in) thorns on the shoots, and its deciduous leaves with three (or rarely, five) leaflets, typically with the middle leaflet 3–5 cm (1.2–2.0 in) long, and the two side leaflets 2–3 cm (0.79–1.18 in) long. The flowers are white, with pink stamens, 3–5 cm (1.2–2.0 in) in diameter, larger than those of true citrus but otherwise closely resembling them, except that the scent is much less pronounced than with true citrus. As with true citrus, the leaves give off a spicy smell when crushed.
The fruits are green, ripening to yellow, and 3–4 cm (1.2–1.6 in) in diameter, resembling a small orange, but with a finely downy surface.
The cultivar "Flying Dragon" is dwarfed in size and has highly twisted, contorted stems. It makes an excellent barrier hedge due to its density and strong curved thorns. Such a hedge had been grown for over 50 years at Oklahoma State University in Stillwater, and is highly student-proof. The plant is also highly deer resistant
The fruits of Citrus trifoliata are widely used in Oriental medicine as a treatment for allergic inflammation, although there is no evidence such treatments are effective.
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