The trifoliate orange, Citrus trifoliata or Poncirus trifoliata, is a member of the family Rutaceae. Whether the trifoliate oranges should be considered to belong to their own genus, Poncirus, or be included in the genus Citrus is debated. The species is unusual among citrus for having deciduous, compound leaves and pubescent (downy) fruit. It is native to northern China and Korea, and is also known as the Japanese bitter-orange, hardy orange or Chinese bitter orange. 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 tall. Because of its relative hardiness, citrus grafted onto Citrus trifoliata are usually hardier than when grown on their own roots. Description The trifoliate orange is recognizable by the large thorns on the shoots, and its deciduous leaves with three (or rarely, five) leaflets, typically with the middle leaflet long, and the two side leaflets long. The flowers are white, with pink stamens, 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 in diameter similar in size to a lime and resembling a small orange, but with a finely downy surface and having a fuzzy texture similar to a peach. The fruits also have distinctive smell from other citrus varieties and often contain a high concentration of seeds. Uses Cultivation The cultivar 'Flying Dragon' is dwarfed in size, has highly twisted, contorted stems, and has even stronger thorns than the type. It makes an excellent barrier hedge due to its density and strong curved thorns. Such hedges have been grown for over 50 years at Oklahoma State University in Stillwater, and are highly student-proof. The plant is also highly deer-resistant. In central London, mature specimens of the trifoliate orange can be seen in the gardens of St Paul's Cathedral. Trifoliate orange and various hybrids of this plant are widely used as citrus rootstock, and valued for their resistance to cold, the tristeza virus, and the fungus Phytophthora parasitica (root rot). Recent studies have revealed that the trifoliate orange contains auraptene at a high concentration, which is one of the functional components having immunity against citrus tristeza virus (CTV). As food The fruits are very