Distylium Siebold & Zucc.

As currently understood, Distylium is a group of evergreen small trees and bushes from eastern Asia. One species, D. racemosum Siebold & Zucc., has a wide distribution extending as far north as Korea and Japan; another, D. stellare Kuntze, is a tropical species from the rainforests of Indonesia and Malaysia; D. lepidotum Nakai is endemic to the Ogasawara Islands in the western Pacific, while most of the others are Chinese plants. Several of these are rare and threatened (Dong et al. 2021), with D. macrophyllum H.T. Chang and D. tsiangii Chun ex Walker assessed (according to Dong et al., although not listed as such in the IUCN Red List in 2023) as Critically Endangered and D. chungii (F.P. Metcalf) W.C. Chang and D. gracile Nakai also assessed as Endangered. An additional three species, from tropical Central America and from western Mexico, were transferred in a revision of the group by Peter Endress in 1969 into a new genus, Molinadendron. Fossilised pollen from Miocene deposits in Austria suggests that plants akin to Distylium were widespread through the forests that covered much of the northern hemisphere before the last – or current – Ice Age (Wikipedia 2023).

In spite of the obvious benefits in the ex-situ cultivation of threatened species, Distylium is not a familiar genus to most western gardeners. (Several vernacular names have been proposed for the group, but ‘Distylium’ seems likely to remain the default; ‘Isu Tree’ derives from the Japanese name for D. racemosum, Isunoki. The name Distylium itself references the (usually) two styles which continue to stand up like little horns from the ripening seed capsule, and was published by Siebold and Zuccarini in 1841, for. D. racemosum). None of them are spectacular plants: the flowers cluster in discreet bunches in spring among the foliage and the leaves themselves, although often cheerfully glossy, tend to conform to the general appearance of the vast array of unrelated ‘laurels’ – a peculiarly useful English word with absolutely no botanical foundation – from these south-east Asian forests. One tell-tale feature is that leaves of Distylium are usually held at an angle above the horizontal, rather than drooping uniformly to shed heavy rain, and this can make the plant appear less tidy and graceful. In common with many other evergreens from low latitudes and higher altitudes, the unfolding leaves can be red or purplish due to the presence of protective anthocyanins, and some garden cultivars have been selected for this attractive and rainbow coloration. The flowers share the often brilliant red stamens of another close but deciduous ally, Parrotia, and at close range are certainly attractive: to the western eye at least their colour is piquant, since the analogy of so many similar if unrelated garden plants (Ilex, Buxus, Sarcococca) leads to an expectation that they should be creamy-white. The contrast of red and glossy green is always a bold one.

This is also an exceptionally difficult group to identify, even to genus level. The small scale and clustered nature of the flowers makes it tricky, particularly in photographs, to spot the distinctive features, such as the absence of both sepals and petals, while another east Asian genus of very similar appearance, Distyliopsis, differs botanically only in the presence of a tiny ‘floral cup’ that encloses the base of the stamens and of the ripening seed-capsule. The genus Distyliopsis was described by Peter Endress in 1970 to define species which had previously been classified either in Distylium or (more usually) in Sycopsis; it seems likely than none of its representatives are cultivated as yet in the west, but the name does crop up frequently enough in horticultural literature for it to merit its own entry in Trees and Shrubs Online.

Within Distylium itself, there seem to be very few reliable differences in floral structure (Walker 1944), while habit and foliage can sometimes vary remarkably within the individual species. Brought together in cultivation, the species also seem to interbreed easily. Naming these ‘virtually… unidentifiable’ plants (Dirr 2015) therefore becomes as much an art as a science, demanding an intimate understanding of the various populations as they grow in the wild and an openness to the subtle ways in which habit and foliage texture combine to create a distinctive signature.

All of this makes it unusually hard to define with any confidence which Distylium species are indeed in cultivation. In 2023 seed of Distylium chinense (Franch. ex Hemsl.) Diels was advertised (online) but proof of any living plants in the west would seem lost within the cultivated population of the very similar D. buxifolium (Hance) Merr.. Meanwhile, seedlings from a 2016 introduction of D. buxifolium (SMH 16013), from a botanic garden in China where various Distylium taxa are cultivated, may include hybrids with D. dunnianum H. Lév. (Z. Hill pers. comm.). The endangered, little-understood and probably tender D. macrophyllum sometimes gains mention, on the basis of a single large-leaved and vigorous young tree at Iturraran Botanic Garden in northern Spain (e. g. De Langhe 2014), but this was obtained (as D. myricoides Hemsl.) from a plant sale in northern France (F. Garin pers. comm.), putting its authenticity very obviously in question.

This Distylium at Iturraran is one of several in cultivation which readily produce seedlings. One of these offspring is clearly a hybrid with the closely related but very different looking deciduous tree Parrotia persica (F. Garin pers. comm. 2023) The proven or suspected intergeneric hybrids which have arisen in cultivation between members of this group of Hamamelidaceae (Witch Hazel family) genera are discussed further in Trees and Shrubs Online under the entries for × Sycoparrotia, for Distylium × Parrotia, and for Distyliopsis.

Twenty years ago this would have been the end of the story. But various Distylium hybrids are now coming to prominence as evergreen bushes for mass landscaping, particularly in the south-eastern United States and thanks to the breeding efforts of the eminent Georgia-based horticulturalist Dr Michael Dirr; they luxuriate in the local climate, they seem almost free (for now) from pests and diseases, and they produce very little in the way of messy seeds or fruit. In this account, these new hybrids are listed together under the entry Distylium myricoides × racemosum.