Distylium racemosum Siebold & Zucc.

Distylium racemosum is the most widely distributed and hardiest member of its genus, and the first to be described, from Japan by Philipp Franz von Siebold and Joseph Zuccarini in 1835. (The specific name – ‘with flowers in racemes’ – is not particularly helpful since all the subsequently described species of Distylium carry their flowers in very similar heads, which are generally too congested for the branch structure to be clear.) For a long time, this was also the only species at all familiar to western gardens, and has been grown in Europe since 1876 (Edwards & Marshall 2019); the vernacular name Isu Tree derives from the Japanese Isunoki (Tozando 2017).

It is characteristic of the genus that, across its vast range, Distylium racemosum is a polymorphic plant, sometimes a rather untidy and upright shrub but sometimes a tree to 18 m tall which can live for a reported 600 years and whose dense, dark, heavy and durable timber is prized for construction and furniture making (Stewart 2012; Kennedy 2023); in Japan, it is often employed as a street tree (Tozando 2017).

In cultivation until very recently only bush forms were known; W.J. Bean, never a man to mince his words, defined the Isu Tree’s habit as ‘somewhat stiff’ and placed it firmly in the ‘the curious rather than… the beautiful class of shrubs’ (Bean 1981). Although the species is fairly hardy, it probably likes more summer heat than it gets in the UK where it tends to flower in late winter, with a second sparser display in midsummer (Edwards & Marshall 2019). However, an old plant in the much warmer climate of the University of Georgia’s Savannah research station, USA, is also only 3 m tall. In these conditions, with more defined hot and cold seasons, it tends to bloom in May but ‘one almost has to fall into the plant to notice the flowers’ (Dirr 2009); it is not troubled by intense sunlight but does seem to need a well-watered, slightly acidic soil rich in organic matter.

Like the more recently cultivated members of its genus, this is a plant as yet with very few pests or diseases, and one which is easy to propagate by cuttings, from May into winter (Dirr 2015); it is also easy to prune if it gets too stiff and untidy. It is hardy enough to be cultivated at the ELTE Botanical Garden in Hungary (Royal Botanic Garden Edinburgh 2023).

Recent introductions from the wild to the UK include K&K 88/51 at Howick Hall and the Royal Botanic Garden Edinburgh, and a plant from the Kew/Quarryhill Expedition to Southern Japan 371 at the Sir Harold Hillier Gardens (Royal Botanic Garden Edinburgh 2023). There are likely to be some arborescent forms among this increasingly diverse cultivated population; a 2000 planting at the J.C. Raulston Arboretum in particular looks as if it will become a substantial and shapely tree (J.C. Raulston Arboretum 2023). Another promising Distylium grows at the Iturraran Botanic Garden in northern Spain and is of mysterious provenance: it was obtained from a French plant fair as the shrubby and narrow-leaved D. myricoides, but its leaves are large and broad even for D. racemosum. It has been suggested that it may represent the more tender D. macrophyllum from the far south of China, but there is no good evidence that this endangered and little-understood species has even been brought into cultivation (F. Garin pers. comm.; De Langhe 2014). This specimen is the only Distylium that self-seeds at Iturraran, and one of the seedlings has proved to be an as yet unnamed intergeneric hybrid with Parrotia persica (F. Garin pers. comm.; such plants are discussed further under the genus entry for Distylium × Parrotia.

It is unsurprising that in a genus at once so uniform and so variable as Distylium, individual plants are likely to be mislabelled or to remain unidentified. This is true of the two largest Distylium in the UK, which are mature, 7 m bushes at the Sir Harold Hillier Gardens (Tree Register 2023); D. racemosum is the obvious identification here. Another cultivated population resembles D. racemosum in foliage and in floral details, but has long been grown as Distyliopsis tutcheri (syn. Sycopsis tutcheri). It seems likely that all of these plants – which account for a good proportion of the cultivated population and remain in commerce quite widely – derive vegetatively from a long-lost bush at the Royal Botanic Gardens, Kew, which in the middle 20th century was wrongly labelled as Sycopsis tutcheri (Bean 1981). This subject is further discussed under the genus entry for Distyliopsis.