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, 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 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. 
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. 
|Wikimedia Commons has media related to Citrus trifoliata.|
- The Plant List: A Working List of All Plant Species, retrieved 26 March 2016
- Dianxiang Zhang & David J. Mabberley, "Citrus Linnaeus, Sp. Pl. 2: 782. 1753", Flora of China online, 11
- Dianxiang Zhang & David J. Mabberley, "Citrus trifoliata Linnaeus, Sp. Pl., ed. 2. 2: 1101. 1763", Flora of China online, 11
- Garcia-Lor, Andres; Curk, Franck; Snoussi-Trifa, Hager; Morillon, Raphael; Ancillo, Gema; Luro, François; Navarro, Luis; Ollitrault, Patrick (2011). "A nuclear phylogenetic analysis: SNPs, indels and SSRs deliver new insights into the relationships in the 'true citrus fruit trees' group (Citrinae, Rutaceae) and the origin of cultivated species". Annals of Botany. 111: 1–19. doi: 10.1093/aob/mcs227. PMC 3523644.
- "BSBI List 2007". Botanical Society of Britain and Ireland. Archived from the original (xls) on 2015-01-25. Retrieved 2014-10-17.
- "Poncirus trifoliata". Natural Resources Conservation Service PLANTS Database. USDA. Retrieved 31 January 2016.
- "Notice to Fruit Growers and Nurseymen Related to the Naming and Release of the US-942 Citrus Rootstock" (PDF). Agricultural Research Service, USDA. 22 October 2010. pp. 1–2. Retrieved 23 October 2017.
- Gerald Klingaman. "Plant of the Week. Hardy Orange or Trifoliate Orange. Latin: Poncirus trifoliat". University of Arkansas. Division of Agriculture.
- Green Deane Hardy. "Hardy Orange".
- Citrus rootstock
- Ohta, Satoshi; Endo, Tomoko; Shimada, Takehiko; Fujii, Hiroshi (2011). "Karatachi no kankitsu torisuteza wīrusu teikōsei to rensa suru DNA mākā" [PCR Primers for Marker Assisted Backcrossing to Introduce a CTV Resistance Gene from Poncirus trifoliata (L.) Raf. into Citrus]. Shimizu, Tokuro; Kuniga, Takeshi; Yoshioka, Terutaka; Nesumi, Hirohisa; Yoshida, Toshio; Omura, Mitsuo ( University of Shizuoka). Japanese Society for Horticultural Science. pp. 295–307. Retrieved 23 October 2017.
- Zhou H.Y.; Shin E.M.; Guo L.Y.; Zou L.B.; Xu G.H.; Lee S.-H.; Ze K.R.; Kim E.-K.; Kang S.S.; Kim Y.S. (2007), "Anti-inflammatory activity of 21(alpha, beta)-methylmelianodiols, novel compounds from Poncirus trifoliata Rafinesque.", European Journal of Pharmacology, 572 (2–3): 239–248, doi: 10.1016/j.ejphar.2007.07.005, PMID 17662711