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Pittosporum includes about 150 species, restricted to the tropics and subtropics of the Old World. They are slender trees or shrubs with evergreen leaves. The leaves are alternate, though may appear opposite or whorled; they are leathery or membranous, entire and petiolate. Pittosporum flowers may be hermaphrodite or unisexual; the inflorescences are terminal or axillary, paniculate, corymbose or umbellate, or rarely the flowers are solitary. The individual flowers are 5-merous; sepals free, but rather small; petals free or partially connate. The fruit is a woody or leathery dehiscent capsule with two to five valves. The seeds are brightly coloured and immersed in sticky resin (Cooper 1956, Allan 1961, Cayzer et al. 2000, Zhang et al. 2003c).
Pittosporum is a popular genus in areas with mild winters (Zone 8 and above), P. tenuifolium and P. tobira being particularly important for their excellent foliage qualities, and the latter for its fragrant flowers as well. For some of us brought up to believe that P. tobira, in particular, is tender in most of the British Isles (see, for example, Bean 1976b), it still comes as a pleasant surprise to see it thriving in many unlikely garden sites in central and eastern England – another indicator of how the gardening climate has changed in recent years. For others its appeal may be less: Dirr (1998) employs the word ‘overused’ twice in his account of the species and its cultivars. Despite such examples of wider growability, however, members of this largely tropical or subtropical genus are most likely to thrive in the mildest parts of our area. Pittosporum is one of those genera where the distinction between tree and shrub is difficult to make, even in the wild, the growth-form often varying with habitat and conditions. For example, in Africa P. viridiflorum can be a large shrub, or a single-stemmed tree, 20–30 m tall (Coates Palgrave 1990, Beentje 1994). In horticulture, species growing in marginal conditions are more likely to be shrubby than otherwise.
Bean (1976b) reviewed 17 species – a somewhat surprisingly large range – but the number of taxa available to gardeners has greatly increased since then. The RHS Plant Finder 2007–2008 listed 21 species, often with several cultivars, available from British nurseries, and a number of other species are cultivated in the British Isles and elsewhere but are not commercially available. Those not mentioned by Bean and not described here include P. anomalum Laing & Gourlay (New Zealand; divaricate shrub), P. hetero phyllum Franch. (China; shrub, with linear leaves and strongly scented yellow flowers: see Hogan 2008), P. illicioides Makino (China, Japan; large-leaved shrub: see Hogan 2008), P. lineare Laing & Gourlay (New Zealand; divaricate shrub), P. michiei Allan (New Zealand; shrub), P. obcordatum Raoul (New Zealand; small tree, with divaricating branches and tiny leaves), P. omeiense H.T. Chang & S.Z. Yan (China; low shrub, with long narrow leaves) (but note that some in cultivation under this name may be P. crispulum Gagnep.), P. pimeleoides A. Cunn. ex Putt. (New Zealand; shrub), P. rhombifolium Hook. (Australia; rainforest tree to 25 m, with abundant white flowers and orange fruits, grown on Tresco: see Johnson 2007), P. truncatum E. Pritz ex Diels (China; shrub, with small stiff leaves: see Wharton et al. 2005) and P. viridiflorum Sims (montane Africa; medium-sized tree, well established in California).
A further assortment is cultivated in western North America – giving an indication of the range of species currently available in horticulture – including P. erioloma C. Moore & F. Muell. (Lord Howe Island, Australia; shrub or small tree to 8 m, with small leaves), P. kirkii Hook. f. ex J. Kirk (New Zealand; shrub, with elongate leaves), P. napaulense (DC.) Rehder & E.H. Wilson (Sino-Himalaya, from Pakistan to Yunnan; shrub or tree, medicinal, with large leathery leaves: see Hogan 2008), P. parvilimbum H.T. Chang & S.Z. Yan (China; shrub or small tree, with small leaves, white flowers, bluish fruits) and P. subulisepalum Hu & F.T. Wang (China; shrub, with medium-sized leaves). Many are of interest to gardeners not only for their evergreen foliage but also for their good scented flowers, especially the Asian species with white or yellow flowers. The Australasian species tend to have dark red, pinkish or dark yellow flowers.
The majority of the Asian and Australian species like warm to hot conditions, whereas the hardier New Zealand species prefer cooler summers. They should not be allowed to become too dry in summer, but poor drainage should also be avoided. Most Pittosporum root easily from cuttings, either from semi-ripe growth in summer with mist and bottom heat, or from mature wood in autumn or winter, rooted in a cold frame; a few, however, including the beautiful P. dallii, are difficult (Huxley et al. 1992, Hogan 2008). Seed is another option, with the New Zealand species often requiring a cool stratification (Hogan 2008). Several members of the genus have become weedy in various parts of the world – notably P. undulatum, in Australia and elsewhere.
An interesting genus of evergreen shrubs and small trees whose headquarters are in Australia and New Zealand, whence come most of the species cultivated in the open air in the British Isles. The others described here are from Japan and China, but the genus is also represented in the Himalaya, the Malaysian and Pacific regions, and in Africa. Remarkably there is one species – P. coriaceum Ait. – on the island of Madeira, which has become exceedingly rare in the wild state, and one in the Canary Islands. The leading characters of the genus are: Leaves alternate. Flowers with five sepals, five petals, and five stamens alternating with the petals. Ovary superior, developing into a capsule with two to five leathery or woody valves. The generic name refers to the resinous or viscid substance by which the seeds are surrounded.
The pittosporums are essentially shrubs for the milder parts of the British Isles. At Kew they can only be grown against a wall. Several of the species are very handsome evergreens, and all here mentioned are charmingly fragrant when in flower. They are easily cultivated and thrive in a light loamy soil. Cuttings taken from the half-ripened wood will root in gentle heat. Seeds ripen in favourable localities, and may also be used.
The flowers in general must be regarded as more notable for their fragrance than their beauty.