Sour Orange (Citrus Aurantium), also known as bitter orange, Seville orange, bigarade orange or marmalade orange, is an evergreen, spiny tree that grows up to 9 m high by 6 m wide, having a compact, rounded top. It is hardy to zone 9 and is frost tender, yet it can stand several degrees of frost for short periods. Dormant plants can withstand temperatures down to about -6 Celsius degrees (21 F) so long as this is preceded by cool weather in order to harden off the plant. The young growth in spring, even on mature plants, is frost-tender and so it is best to grow the plants in a position sheltered from the early morning sun.
It has smooth, brown bark, green twigs, angular when young and flexible, not very sharp and thorns of 2.5-8 cm long. It is in leaf all year, bearing ovate leaves that are fragrant when crushed. It flowers from mid spring to early summer. The hermaphrodite flowers are highly fragrant, borne singly or in small clusters in the leaf axils. The plant is self-fertile. The fruits are round, oblate or oblong-oval, orange or reddish-orange with a rind that is rough, strong scented and bitter. The fruits have 10-12 sections and the pulp is acid, containing from a few to numerous seeds.
Sour orange is used mainly as root stock for other citrus trees. The fruits are too acid to be edible, but they are used in the manufacture of marmalade, the peel oil is used in the liqueur curaçao, the peel and flower oils and the oils from the green twigs are used in perfumery.
The plant prefers a moist but well-drained soil, moderately heavy loam with a generous amount of compost and sand added, acid or neutral, and a place in full sun as it cannot grow in the shade. They are also intolerant of water logging.
If you live in a cold area you can still grow sour orange in a pot that is placed outdoors in the summer and brought into a greenhouse during the winter. Plants dislike root disturbance and so should be placed into their permanent positions when young. If you grow them in pots, take care when potting them on into larger containers. When growing sour orange in pots use a compost comprising equal quantities of loam and leaf mold plus a little charcoal for good results. It is important to neither over-water or underwater plants in pots since the plant will soon complain by turning yellow and dying. Water only when the compost is almost dry, but do not allow it to become completely dry.
Propagate sour orange by sowing seeds in a greenhouse as soon as the fruits are ripe after thoroughly rinsing them. Sow stored seed in early spring in a greenhouse. Germination usually takes place within 2-3 weeks at 13 Celsius degrees (55.5 F). Seedlings are liable to damp off so they must be watered with care and kept well ventilated. When they are large enough to handle, prick the seedlings out into individual pots and grow them on in the greenhouse for at least three growing seasons before trying them outdoors. Plant them out in the summer and offer them some protection from the cold for their first few winters outdoors. You can also try to propagate them by taking cuttings of half-ripe wood in mid to late summer, in a frame or by layering in mid autumn.
