The Genus Citrus belongs within the Family Rutaceae, a cosmopolitan grouping of plants that contains approximately 1650 species in 160 different genera. Many of them occur only in tropical areas but the following are frequently seen in British gardens and greenhouses.
Linnaeus, the ‘Father of Botany’ 
named this family after the herb ‘Rue’, Ruta graveolens, which he first described in 1753. Rue was formerly widely used in herbal medicine and as a flavouring herb, although it has a strong acrid taste and is liable to cause severe skin lesions in some people!
Close relatives of Citrus include the Kumquats in the genus Fortunella, species of which are often sold as houseplants and look attractive with their evergreen foliage festooned with lots of miniature oranges. The Kumquats have been hybridised with true Citrus and are considered to be relatively hardy having survived temperatures as low as -8ºC. Another related plant is the ‘Trifoliate Orange’, Poncirus trifoliata. (Illustration) This is a very spiny plant with a congested habit, green stems and small clusters of wrinkled, inedible yellowish fruit. It has been used as a hardy rootstock for grafting various orange cultivars but is not as popular as proper Citrus rootstocks.
Several of the genera contain familiar garden plants; Choisya ternata is an attractive evergreen shrub, about 1.2m high, with trifoliate leaves and clusters of sweetly scented white flowers that are borne in April and May. (Illustration) It is sometimes known as the ‘Mexican Orange’ although it does not bear edible fleshy fruits. There are various forms and hybrids of Choisya available, some having golden foliage, e.g. Choisya ternata ‘Sundance’, (Illustration) whilst others, such as Choisya ‘Aztec Pearl’ have 5 or more narrow leaflets. (Illustration)
Skimmia japonica is another popular evergreen shrub in this family, forming a low rounded bush rarely exceeding 700mm in height. Clusters of small red fruits are carried well into the winter following the attractive display of white flowers in spring. (Illustration)
Hailing from Australia, the evergreen shrubs in the genus Correa produce fascinating pendulous tubular flowers in a range of pastel colours. Although not fully hardy, some species may thrive in a sheltered spot near the sea or against a house wall but would probably prefer the warmth of a conservatory or greenhouse. (Illustration)
The ‘Burning Bush’, mentioned in the Bible, is thought to be Dictamnus albus, another member of this family. In hot sunny weather this plant will give off inflammable volatile oils which will flare up when ignited. (Illustration)