Citrus trifoliata or Poncirus trifoliata
|A fruiting tree in Jardin des Plantes, Paris|
Citrus or Poncirus
C. trifoliata or P. trifoliata
|Citrus trifoliata or Poncirus trifoliata|
The trifoliate orange, Poncirus trifoliata or Citrus trifoliata, is a member of the family Rutaceae. 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 among citrus for having deciduous, compound leaves and pubescent (downy) fruit.   The trifolate orange was long viewed as the sole member of Poncirus, until the discovery of a second species, Poncirus polyandra, in Yunnan ( China) in the 1980s. 
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. 
The trifoliate orange 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 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.
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 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 Trifoliate Orange specimens can be seen in the gardens of St Paul's Cathedral.
The fruits are very bitter, due in part to their poncirin content. Most people consider them inedible fresh, but they can be made into marmalade.  When dried and powdered, they can be used as a condiment.
The fruits of the trifoliate orange are widely used in Oriental medicine as a treatment for allergic inflammation. 
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