Hardy orange

Citrus trifoliata (syn. Poncirus trifoliata), the hardy or trifoliate orange, is a deciduous shrub or small tree in the family Rutaceae native to northern China and Korea. It grows to about 4–8 m and is distinguished within the citrus group by being deciduous, cold-hardy, and bearing conspicuous stout, flattened green thorns along its angular twigs. Unusually for a citrus, the leaves are trifoliate, with three leaflets, the central one largest. The fragrant white flowers, borne singly or in pairs in spring, have five petals and appear before or with the new leaves. The fruit is a small, rounded, downy yellow hesperidium resembling a tiny orange; it is intensely aromatic but sour, bitter and full of seeds, generally regarded as inedible fresh. The plant is widely used as a hardy rootstock for cultivated citrus and as a dense thorny hedge.