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Green (2002) recognised 33 species of Olea in tropical and subtropical regions of the Old World. Olives are evergreen trees and shrubs, that may have peltate scales on young branchlets. The leaves are opposite, entire or serrated and leathery or papery. They may be glabrous or with peltate scales on both surfaces; venation is usually obscure. Inflorescences are terminal or axillary, racemose or paniculate. Olea species may have hermaphrodite flowers only or a mixture of staminate and hermaphrodite flowers. The calyx is small and tubular with four short lobes; the corolla is tubular with four lobes; stamens two or four. The fruit is a dark purple or black drupe, often with thick, oily flesh (Green 2002).
The only member of the genus familiar to gardeners is O. europaea, and in this we have another striking example of how the horticultural climate has changed. Where Bean (1976b) wrote of the ‘few fruits’ on the old tree in the Chelsea Physic Garden as exceptional, O. europaea is now a commonly planted species across England, and fruits even in tiny back gardens. A commercial olive grove has been planted in Devon (press reports 2007), and with the Mediterranean becoming increasingly arid a northwards trend in the area of production is inevitable. The range of O. europaea in western America has also expanded greatly, and it is planted now as a commercial crop into northern Oregon. With the exception of O. yuennanensis, described below, the other species of Olea are unknown to temperate horticulture. There are thirteen species in China, but all these seem to occur at low altitudes in southerly areas and may be too tender for our area.
A relatively limited number of small or medium-sized evergreen trees from tropical or warm temperate regions of the Old World. The leaves are simple and opposite. The flowers are white, borne in axillary, decussate, and opposite-flowered or paniculate inflorescences. The calyx and corolla are four-lobed, the lobes of the corolla valvate and rolled inwards in the bud stage. Stamens two. Fruit an ovoid or spheroid drupe, usually dark blue or black when ripe.
Of the members of the genus only O. europaea is hardy in Britain, and then only in the most favourable situations. The genus is closely allied to Osmanthus and Phillyrea, differing only in minor characters.
Notelaea ligustrina Vent. of Tasmania and southern Victoria, a privet-like shrub occasionally grown in Britain, is allied to Olea. It has little horticultural value. For the species once known as Olea excelsa, see Picconia.