Hovenia Thunb.

Despite the breadth of their natural distribution – one species is from Mesoamerica, the others from eastern Asia from the Himalaya south to Thailand and east to Japan – the five Hovenia species recognised by Royal Botanic Gardens, Kew 2024 are extraordinarily uniform in appearance, allowing for the detailed description at genus level (above) and the relatively brief descriptons of each of the species treated here. Within the Buckthorn Family (Rhamnaceae), the genus seems most closely allied to Ziziphus and Paliurus (Nesom 2023).

The most memorable feature unifying Hovenia is the way that, as the fruit ripens, the stalks of the inflorescence swell and contort and become sweet and succulent, providing the vernacular name ‘raisin trees’. The rather large and glossy heart-shaped to ovate leaves are handsome if slightly anonymous, but, in each of the species, about half the leaves typically show a unique ‘tell’ at the base, which is difficult to describe but easy to spot: the leaf has three main veins which spread neatly at 45° to one another, and the lower edge of the blade tends to form a straightish line, but this meets the outer pair of veins about 5 mm above the point at which they diverge from the midrib, so that there is a small pyramid-shaped promontory or bracket to the leaf above the petiole. This ‘bracket’ is a great help in distinguishing the foliage of Hovenia from that of sundry trees such as Carrierea calycina, Mallotus japonicus or Alangium chinensis, which, like Hovenia, are not encountered often enough in temperate gardens to become really familiar. Poliothyrsis sinensis (Chinese Pearlbloom) is another rather scarce lookalike: in this case the similarities extend to the bark, habit and flowers, even though these species are only very distantly related (Poliothyrsis belonging in the Willow Family, Salicaceae). The Pearlbloom tends to be a more compact tree, sometimes bushy; its flowerheads are a bit broader and showier, and in a garden or a street planting Poliothyrsis has the additional advantages of good autumn colour, a colourful flush to its foliage in spring, and dry fruit which will not make a mess after it falls.

The features used by botanists to differentiate the species of Hovenia are subtle: H. celtidifolia (Schltdl. & Cham.) G.L. Nesom probably has the biggest seed-capsules (to 12 mm wide) but the smallest leaves (6–14 cm long, with stalks descibed as less than 2 cm long). Only H. tomentella (Makino) Nakai bears dense rusty hairs on its sepals and fruit; H. pubescens Sweet has soft wool on its seed-capsules and the bottom part of its style, which forks below halfway; H. acerba Lindl. shares the deeply forked style, but this is glabrous; H. dulcis Thunb. also has a glabrous style, which forks near the tip or not at all. The seed-capsule is described as yellow to brown in H. pubescens and H. acerba, pale brown to blackish in H. dulcis, and reddish in H. celtidifolia. H. dulcis tends to bear slightly more sharply and coarsely serrated leaves than the other species, but this feature can differ depending on the tree’s maturity and can vary from plant to plant (Chen & Schirarend 2007; Nesom 2023).

Given the obscurity of these distinguishing features, there has inevitably been considerable disagreement about how many species of Hovenia should be recognised – to say nothing of how many are cultivated in the west. The commonest species – both in the wild and in cultivation – and probably the most versatile is H. dulcis, which was the first to be described, by Carl Thunberg in 1781; Thunberg named the genus after David ten Hoven, a senator of Amsterdam who had helped to finance Thunberg’s botanical exploration of South Africa (Jacobson 1996). Both H. acerba and H. pubescens were described from specimens cultivated in England (under glass) in the first decades of the 19th century; these descriptions were too brief – and confused – to detail the floral features which are now used to distinguish these taxa, and many subsequent authorities, including W.J. Bean, preferred to lump them within H. dulcis. H. acerba was revived as an entity by the the Japanese botanist Yojiro Kimura in 1939 and is now widely understood to describe a population of Chinese trees which, although broadly sympatric with H. dulcis, seem to maintain their differentiating features without any interbreeding (Bean 1981; Royal Botanic Gardens, Kew 2024; Nesom 2023). Some modern authorities, such as Chen & Schirarend (2007), treat H. pubescens as a variety (var. kiukiangensis (Hu & W.C. Cheng) C.R. Wu) of H. acerba. The fourth Asiatic species, H. tomentella, was first described (as a variety of H. dulcis) by Tomitaro Makino in 1914 (Royal Botanic Gardens, Kew 2024).

The Mesoamerican Hovenia celtidifoia was first described by Diederich von Schlechtendal and Adelbert von Chamisso in 1830 as a Ceanothus – the specific name, meanwhile, means ‘with leaves like a nettle tree’ – and it was transferred by Schlechtendal eleven years later to the tropical genus Colubrina (Royal Botanic Gardens, Kew 2019), an entity which is nowadays felt to be something of a dumping ground for various unrelated plants. As a minor player within the extraordinarily biodiverse mountain forests of southern Mexico, Guatemala, Hondurus and Nicaragua, this tree attracted very little botanical interest until 2023, when Guy Nesom recognised its remarkable similarities to the Asiatic species of Hovenia (Nesom 2023). Nesom’s paper relied on herberium specimens rather than modern phylogenetics, but rapidly gained acceptance. Meliosma alba and M. beaniana provide a comparable instance of a pair of trees from these same two geographically remote biomes which have scarcely evolved any macroscopic distinguishing features; the ranges of Meliosma and of Hovenia seem to have contracted drastically in the face of Ice Age-related climate change over the last few million years, and fossil Hovenia species from the Oligocene Epoch have been described from present-day Oregon and Colorado (Nesom 2023).

Hovenia celtidifolia is probably tender, and is not known to be cultivated in temperate North America or Europe. The four Asiatic species have each been given their own species entry in this account, athough the combination across the genus of nomenclatural instability and uniform appearance makes it particularly hard to confirm which of them are really in cultivation.

Fruits in which it is the stalk rather than the seed-coating which is edible have independently arisen many times. Among trees cultivated in temperate regions, the conifer genus Podocarpus offers one such instance. The bright red colour of Podocarpus fruit is an indication that these have evolved to be eaten – off the tree – by birds, which have good polychromatic vision but generally lack a sense of smell. In contrast, the fleshy stalks of Hovenia fruit are dully coloured but richly flavoured and aromatic, implying that they are eaten by mammals, foraging mostly at night. In the parts of China where Hovenia grow wild, fruit bats are said to be absent, but wild goats, civets, badgers, bears and martens have all been observed feeding on the fruit after it falls in winter (Zhou et al. 2013). As they eat, these animals spread the seedheads around, or accidentally ingest the seeds, which may pass through their guts undigested and be deposited elsewhere.

The taste of these fleshy stalks has been compared to raisins or bergamot pears; they are eaten raw across eastern Asia but can also be cooked, or dried, or brewed into a wine. They are of little commercial importance, since they do not lend themselves to mechanised harvesting; cultivated trees have probably scarcely been selected for larger or sweeter fruit, although the species which is most often planted, Hovenia dulcis, may also be the tastiest (Koller & Alexander 1979; Chen & Schirarend 2007). Individual trees are self-fertile, so one in a garden on its own will still ripen fruit, given long enough summers (Penny 2024).

The seeds contain the flavanonol ampelopsin, and have been used in Chinese herbal medicine to treat fevers and infection by parasites, and as a laxative. Ampelopsin acts upon the liver in particular, and Hovenia seeds are still widely used as a hangover cure, and also show promise in treating some liver cancers (Sferrazza et al. 2021; Penny 2024).

Hovenia timber is hard and durable and is used in east Asia for construction and furniture making, and trees are planted there as much for their timber as for their fruit (Chen & Schirarend 2007; Wikipedia 2024); in the subtropical climate of northern Thailand, Hovenia plantations can grow at a rate of two metres a year. The scented blossom is highly attractive to bees (Dirr 2009), making the tree a useful one for honey production; an extact from the seeds, leaves and young bark can also been used as a substitute for honey (Koller & Alexander 1979; Penny 2024).

In cultivation, Hovenia prefer a continental climate, growing comparatively slowly in England and seldom ripening their fruit (historically at least). But they remain well worth cultivating as foliage – and flowering – trees, although one minor demerit is that they show little autumn colour. Their rather glossy foliage helps them withstand urban pollution, the grime washing off in rain – apart from the mess from falling fruit, they can make a good street tree – and they are also quite tolerant of salt in the soil. Even in the wild they often prefer full sun, and they tend to grow in moist sandy loams (Chen & Schirarend 2007; Wikipedia 2024), but they can cope with most garden soils. In Europe and temperate North America at least they currently have no significant pests or diseases (Dirr 2009), but saplings can be quite trickly to establish.

In several areas, including Texas, Tanzania, and subtropical eastern South America, Hovenia dulcis at least behaves invasively (Penny 2024) and modelling suggests it has the potential to become a significant pest in many parts of the world (Bergamin et al. 2022).

The genus is one of rather few in which no ornamental sports ever seem to have been selected; in the west at least, there are not even any named fruiting selections (Koller & Alexander 1979).