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Tetradium was revised by Hartley (1981), who expanded it to include all the hardy species of Euodia J.R. Forst. & G. Forst. (Horticultural references have generally used Euodia, or the orthographic variant Evodia – as reflected in the cross-references below.) It is now a genus of nine species, distributed from the Himalaya east to Japan and the Philippines, and south to Indochina, the Malay Peninsula, Sumatra and Java. They are evergreen or deciduous trees or shrubs with naked buds. The leaves are opposite, imparipinnate (rarely paripinnate) and are dotted with oil glands. Inflorescences are terminal only or terminal and in the axils of the uppermost pair of leaves; corymbose to paniculate with numerous flowers. The flowers are unisexual (rarely hermaphrodite) and the trees dioecious; flowers 5- or 4-merous, with valvate sepals, persistent in fruit, petals hooked at the apex, and the same number of stamens as petals. The fruit is a follicle containing one or two glossy seeds, the follicles in groups of one to four or one to five, the seeds remaining attached to the follicle (Hartley 1981, Flanagan 1988). A key to the cultivated species has been produced by Flanagan (1988).
Mark Flanagan’s article (1988) on species of Tetradium in cultivation gives a comprehensive picture of their status in British gardens during the 1980s, which can be updated here. Flanagan’s main point – applicable across the genus – is that the various species are very variable, so that it is wise to take the broad view and not attempt to split off local variants. The four species he described at the time appear to have remained a quartet, with no new additions. Two are described below, T. glabrifolium and T. ruticarpum; the plant of T. fraxinifolium (Hook.) T.G. Hartley at Benmore that he mentioned, of possibly dubious identity and of unknown origin, has since died. The fourth species, T. daniellii, has always been the most common, and can be distinguished from the other three by the absence of conspicuous oil glands on the non-glaucous leaf undersides (Flanagan 1988). As Flanagan put it, T. daniellii is ‘not in the first rank of ornamental trees [but is] a useful and subtly attractive plant’, and it is probably fair to say the same of the others. The foliage is handsome but the prime moment of all members of the genus comes when the fruits ripen to bright red, held in broad corymbs at the tips of the branches. For all species a good light open site should be chosen, but they are not fussy about soil types. As members of a largely tropical family, Tetradium would be expected to thrive best in warm conditions but they seem to be very adaptable. Propagation is by seed, or by semi-ripe cuttings in summer (Wharton et al. 2005).
Tetradium is very similar to Phellodendron, especially in vegetative characters, but has exposed axillary buds whereas in Phellodendron the buds are hidden at the base of the petiole and are only visible after leaf fall. In addition, the fruit of Tetradium is follicular, while that of Phellodendron is a drupe. Phellodendron has recently been revised (Ma et al. 2006) and only two species – P. amurense Rupr. and P. chinense C.K. Schneid. – are now recognised, without varieties. These are most easily distinguished by their fruiting panicles, those of P. chinense being compact with short branches while in P. amurense the branches are longer, making the panicles much more open in shape. The revision of Phellodendron did away with a complex set of varieties and ‘species’, including P. insulare Nakai, which referred to material from Korean islands distributed in recent years from Chollipo Arboretum (now placed in P. amurense).