Hovenia dulcis Thunb.

Hovenia dulcis has stood as the representative or default species for its tightly-knit little genus ever since Carl Peter Thunberg first described it in 1781. By this time it was already widely cultivated across temperate and subtropical east Asia, making it harder to ascertain its original natural distribution; even within China, it is strongly associated with disturbed habitats and is not a feature of primary forests (Chen & Schirarend 2007). The edible ‘raisins’, the durable timber, the sweet young bark and the medicinal seeds were all traditionally utilised. The species can be recognised botanically by the general absence of hairs on the inflorescence in particular, and by the flower’s style which is only forked shallowly and towards the tip; the serrations around the leaf tend to be coarser and less regular than in the other species.

The earliest attempts to grow this tree in the west are poorly documented. 1912 is sometimes listed for its introduction to Britain (Edwards & Marshall 2019) but this is clearly too recent; the date in fact seems to derive from a misprint, in early editions of The Hillier Manual of Trees and Shrubs, for 1812, which was the year cited by Robert Sweet for the raisin tree’s cultivation by Aylmer Bourke Lambert at Boyton Manor in Wiltshire (Sweet 1826). Lambert’s plant had come from China but was almost certainly being grown in a greenhouse, and the first evidence for one out of doors in Britain may be W.J. Bean’s description of a plant at Kew which in the first years of the 20th century was a ‘rather ungainly shrub with erect branches, growing very vigorously in the summer, but cut back more or less in the winter’ (Bean 1914). In its later years this example seems to have made better progress, and was 10 m tall when it finally died in 1954 (Bean 1981).

Hovenia dulcis is better adapted to the continental climate and hot summers of the eastern United States; it may have first been introduced to the New World by George Rogers Hall, who gave material to the S.B. Parson and Co. nursery in New York in 1862 on his return from seven years’ work as a doctor in Japan (Howe 1923). A tree from this source was planted at the Arnold Arboretum in 1880, and in the same year the Arboretum received seed from the Agricultural College in Sapporo, Japan (Koller & Alexander 1979). A tree at the Morris Arboretum in Pennsylvania is also sometimes dated to 1880 (Dirr 2009), although the collection here was not actually begun until 1887–8 (Morris Arboretum 2024). Surprisingly, this tree reached 27 m (Dirr 2009), almost three times the stature indicated in China by Chen & Schirarend 2007; even more remarkably, it was killed to within a few feet of the ground by about 30°C of frost in 1933–4, and quickly regrew with two trunks (Koller & Alexander 1979). At the Morton Arboretum in Illinois (North American hardiness zone 5b–6) plants used at least to get cut to ground level by winter cold, and the species proved impossible to grow in Orono, Maine (zone 5a; Dirr 2009). Although Hovenia in the wild prefer an acidic loam (Chen & Schirarend 2007; Wikipedia 2024), thriving Japanese Raisin Trees can be found in all kinds of locations in the eastern United States (Koller & Alexander 1979) and they thrive on chalk in England (T. Christian pers. comm. 2024). In the softer conditions of the west coast, there is a good example at the Hoyt Arboretum, Portland, Oregon (Oregon State University 2024).

Raisin trees were also being grown out of doors in Australia at a comparatively early date; Hovenia dulcis was first listed in Sir William MacArthur’s catalogue for Camden Park near Sydney in 1850 (Mills 2012).

The predicted failure to ripen the ‘raisins’ in chilly climates has probably served to distract some more northerly gardeners from experimenting with a handsome foliage tree; even in subtropical conditions the fruit crop is not especially significant, and certainly not commercially so, although it has been widely planted in some regions and has become an invasive species in regions including Texas, southern Brazil, northern Argentina and parts of Paraguay (Bergamin et al. 2022).

Although cool conditions will limit the tree’s stature – and probably its lifespan – Hovenia dulcis remains worth growing for its bold leaves and its pretty if less than spectacular display of off-white sweetly scented flowers in high summer; it is certainly a tree that should tolerate, and indeed enjoy, a warming climate in northern Europe. (Since Hovenia is such a uniform genus, most notes on cultivating H. dulcis probably apply equally to the other species, and are expanded here in the generic introduction.)

In 20th century Britain the best example of Hovenia dulcis was one planted by Daniel Bliss in Singleton Park in Swansea, Wales, some time after 1919 when this estate had been obtained by the City Council; Bliss, the park’s first Superintendent, had trained at Kew, which provides a likely source for this tree (Johnson 2015). The climate of Swansea is more Atlantic than Kew’s, but its springs are mild and early and contribute to its region’s high score in terms of degrees of heat per annum. The raisin tree, in a sheltered spot just east of the greenhouse range in the Botanic Garden, had a single trunk which was 46 cm thick in 2013, three years before its death (Tree Register 2024). With its demise, the UK champion becomes a multistemmed plant about 18 m tall in the Sir Harold Hillier Gardens. Raisin trees luxuriate in the warmth and the fertile soils of Oxford, with a grafted specimen dating from around 1974 in the order beds of the Botanical Garden having plenty of room to spread; another mature tree stands hidden from the view of most visitors at University College, under the south-facing wall of the university’s Examination Schools. In the eastern, more ‘continental’ side of Ireland, a mature tree has reached 16 m at Malahide Castle near Dublin (Tree Register 2024).

A tree grown from KFBX 147, collected in South Korea in 1989, was already 14 m tall in 2024 in a pocket of acidic soil within Westonbirt National Arboretum, Gloucestershire, UK, and KFBX 169 (from the same expedition) was 9 m in 2022 at Marks Hall Arboretum in Essex, where the annual rainfall is scarcely more than 600 mm and where summers can be particularly dry; specimens grown from these two collections also thrive at Kew and at Wakehurst Place. These plants seem likely to represent an exceptionally hardy provenance, while there has probably been some unconscious selection for hardiness within the population cultivated in the west as a whole; there seem to be no recent reports of cold damage to established plants in England. In the much cooler summers of Howick Hall in Northumberland, a tree grown from a collection made by Motowo Kobayashi in central Japan (MK 91/19) was 9 m tall in 2019 (Tree Register 2024; Howick Hall Arboretum 2024). Plants from seed collected in 2005 in Iwate, near the northern end of the species’ Japanese range (BBJMT 88) are growing at Logan Botanic Garden in Scotland, and also further north at the Benmore Botanic Garden, along with BCJMMT 102 from 2007 (Royal Botanic Garden Edinburgh 2024). Wild collected plants from BSWJ 11024 and NJM 11.003 are or have been commercially available in the UK (Royal Horticultural Society 2024).

In the possibly more congenial conditions of continental central Europe, a good but short-boled mature raisin tree grows at Insel Mainau in southern Germany (Baumkunde 2022).