Kalopanax septemlobus (Thunb.) Koidz.

Kalopanax septemlobus is a polymorphic yet distinctive tree of the ivy family distributed across a large swathe of east Asia, from hot sticky south-central China to cold harsh Sakhalin. For many years this tree was known as Kalopanax pictus – indeed this name is still frequently encountered – but for some time now K. septemlobus has been the accepted, correct name. The circumstances that prompted this change are improbably complicated (and are implicit in another complex and contentious problem concerning the correct name for Painted Maple, Acer pictum). See ‘taxonomic note’ above.

Within its native range Kalopanax has long been valued for its timber, medicinal properties, and ornamental qualities. A USDA forest service memo dated 1957 records that in Japan sen (the local name for Kalopanax timber) is used ‘in general construction and the manufacture of cabinets, furniture, interior trim, carved items, chests, handles, plywood, musical instruments, boats, and numerous miscellaneous articles. In the United States it is used primarily in the form of plywood for paneling [sic.] and doors’ (Kukachka 1957). More recent interest has focussed on its medicinal properties. It has been widely used in traditional medicine for its anti-diabetic, anti-inflammatory, antinociceptive, anti-rheumatoid and anti-fungal properties (Lee & Kang 2002). It contains a number of triterpenoid saponins, a group of bitter-tasting and usually toxic compounds involved in defending plants against pathogens and herbivores (Lee et al. 2020) and the leaves have been shown to contain high concentrations of compounds with potential application as a natural source of antioxidants (Han et al. 2012).

The first introduction to western cultivation was probably that of the Russian botanist Carl Maximowicz, who introduced seed to St Petersburg from his 1860–1864 visit to Japan. Maximowicz is unlikely to have encountered Kalopanax during his earlier work in the vicinity of the Amur River in Russia’s Far East, where he botanised c. 1854–1856, with much of the area he covered being well beyond the species’ known northern limit at c. 47°N in southern Sakhalin.

From St Petersburg material was soon sent to several other European countries, reaching the Belgian horticulturist Louis van Houtte in 1864 and probably arriving in England at about the same time (Elwes & Henry 1906–1913). Further seed collections soon followed, including from the Baltic German scientist Carl Friedrich Schmidt who collected seed on Sakhalin in September 1866 (Schmidt 1868). The French nurseryman Alphonse Lavallée sent a living plant to the Arnold Arboretum in January 1881 and the Arboretum’s Director Charles Sprague Sargent later collected seed on his first expedition to Japan in 1892 (Port 2014). By the late 19th century nurseries and seed merchants operating out of Japan for the benefit of western markets were regularly offering Kalopanax (Boehmer & Co. 1899).

Accounts of it are rare in 19th century literature, but after seeing it in Sakhalin Schmidt remarked that it was ‘a beautiful tree’ (Schmidt 1868) and the horticulturist Louis Böhmer later called it ‘a handsome tree, especially when in flower, and [it] has almost a tropical appearance’ (Böhmer 1876). Subsequent commentators have made much of this tropical appearance, including Wilson, who included it in his rollcall of Aristocrats of the Garden, observing: ‘The Kalopanax…is one of the noblest trees of the cool temperate regions [… ] The large and handsome palmate leaves give this tree a tropical appearance yet it is perfectly hardy and quick-growing […] As a lawn tree or as a specimen tree by side of water it is unsurpassed’ (Wilson 1917). Donlad Wyman, Head of Horticulture at the Arnold Arboretum, included it in his essay Forty-five of the Best Trees for Massachusetts Gardens but lamented that it remained ‘as yet very rare in American gardens’ (Wyman 1952). Indeed, the Arnold seems to have waged a one-arboretum crusade to popularise this tree in New England in the early to mid-20th century, culminating in 1972 when it was one of ten taxa offered as a ‘reverse birthday present’ to celebrate the arboretum’s centenary (Port 2014).

Yet for all these efforts, Kalopanax has remained stubbornly unpopular, planted only by the relatively few who seek out the unusual. An exasperated Wyman later attempts to understand the disinterest: ‘Still only listed by one nursery (as far as can be determined) this fine tree…has been distributed many times […] The seed takes two years to germinate, one of the reasons why many a plant grower becomes overly impatient and passes it up’ (Wyman 1956). In Europe, too, there is a relative paucity of specimens. Even in the UK and Ireland the Tree Register lists only 45 noteworthy individuals in its public-facing database. It is interesting to compare this tally with that of other ornamental Asian trees: it lists 152 Davidia involucrata; 117 Cercidiphyllum japonicum; 116 Acer davidii; 59 Liriodendron chinense; 52 Trochodendron aralioides; 44 Pseudolarix amabilis; 35 Emmenopterys henryi; 22 Tetracentron sinense and 21 Poliothyrsis sinensis (Tree Register 2022).

The British dendrologist Alan Mitchell seems to have had mixed feelings about Kalopanax. The account in his book Trees of Britain uses such adjectives as ‘strange’, ‘gaunt’, ‘spiky’, ‘eccentric’ and ‘bizarre’ to describe it. It was for this publication that Mitchell coined the common name Prickly Castor Oil Tree, later expressing regret for having done so upon learning that the Americans were already using the ‘more civilised’ moniker Castor Aralia (Mitchell 1996). Regardless, this tree has always been too-seldom grown to warrant extensive league tables of the best and largest in different parts of the world. About 30 m is regularly cited as its climax height in native forests, with a dbh of up to 1 m, rarely more. The largest in cultivation are probably those grown in the UK and Ireland: 20 m × 65 cm dbh in Surrey, England; with others to 18 m tall in locations as disparate as Surrey and Worcestershire in England, Peeblesshire in Scotland, and Kildare in Ireland (Tree Register 2022). In North America the largest are probably those remaining from Sargent’s 1892 gathering in Japan at the Arnold Arboretum – 15.8 m with a canopy spread of over 23 m in 2014 (Port 2014) – although there have been bigger, to 17 m × 67 cm dbh c. 1980 (Jacobson 1996). In Belgium, a tree at Arboretum Tervuren had a dbh of 60 cm in 2020, but its height was not recorded; there are other fine examples at Kalmthout and Herkenrode which frequently produce a few seedlings (Arboretum Wespelaar 2023; pers. obs.).

A brief survey of cultivated trees quickly paints Castor Aralia as a hardy, adaptable, and thoroughly un-fussy tree. Examples in North America and northern Europe, including some very cold areas, quickly banish any question that it is anything other than bone hardy. It is also perfectly tolerant of heat: Philippe de Spoelberch successfully grows Kalopanax at Les Mourgues in the south of France where temperatures regularly reach 40°C and drought conditions are frequent (P. de Spoelberch pers. comm.; pers. obs.) but it remains to be seen whether the trade-off in such conditions is a reduced lifespan. In the UK mature trees may be found in collections on deep, fertile, humous-rich soils such as those at Dawyck Botanic Garden, Peeblesshire, and on shallow soils above solid chalk, such as at Pampisford, Cambridgeshire; it grows as cheerfully in areas with high rainfall as it does in the driest parts of these islands, and young plantings in several rather wet sites are thriving, too (pers. obs).

Nevertheless, in some soil types, Castor Aralia is possibly sensitive to rapid fluctuations in the water table. The tree from Nanjing, accessioned at the Royal Botanic Garden Edinburgh in 1934, had to be deaccessioned in March 2023: three successive spring-summer droughts had taken their toll and in its weakened, stressed state, the tree was rapidly succumbing to Armillaria (W. Hinchliffe pers. comm. 2023).

Castor Aralia is cultivated in Australia where there are multiple specimens in collections throughout the southeast, and where it is seemingly a more precocious flowerer than in the cooler climate of northwest Europe. It is very rare in New Zealand (G. Church pers. comm.).

Propagation by seed is straightforward, but without treatment seed will usually require two winters to germinate. Root cuttings are another easy means of propagation, as is the removal of suckers which mature trees will produce sporadically, and young trees in abundance in response to damage (which has proven a disadvantage in attempts to eradicate young trees from natural areas in the USA). Kalopanax has demonstrated a remarkable ability to recover from major arboricultural interventions: a specimen at Leckmelm, near Ullapool on the NW coast of Scotland, was pollarded in 2013 and has recovered beautifully (pers. obs.).

For all the efforts of their predecessors to popularise Kalopanax in North America, staff at the Arnold Arboretum today are probably quite grateful for their failure. Seedling recruitment has been known for some time but in recent years the species has shown a tendency to invasiveness in various parts of the northeast. A study at the University of New Hampshire found thousands of volunteer plants all probably traceable to a single individual planted in 1972: these levels of recruitment, taken with the species’ extraordinary shade tolerance as a seedling, potentially very rapid growth, and its ability to sucker, especially when damaged, all contribute to significant and legitimate concerns about its continued use as an ornamental, at least in the north-eastern USA (Lee et al. 2017).

A programme of deaccessioning Kalopanax at the Arnold Arboretum commenced in 2010, with seven accessioned individuals removed in the first phase to 2012 (Port 2014). In early 2023 three mature individuals remain at the Arnold, retained for now due to their historic and landscape value: two derived from Sargent’s 1892 collection in Japan (accessions 12453*A and *C); and another traceable to Nanjing Botanical Garden via the US National Arboretum (accession 841–81*A) (M.S. Dosmann pers. com. 2023). Dosmann, Keeper of the Living Collections at the Arnold, sums up his own mixed feelings for this species which will likely resonate with many others who grow or admire it: ‘On the one hand, I’m a big fan. And yet, because it is notable in its ability to wander and take root in natural areas, I’m wary of recommending it for widescale cultivation’ (M. Dosmann pers. comm.).

These North American experiences should sound a warning to those who grow this tree elsewhere, especially in Europe, where some seedling recruitment is regularly observed in Belgium (K. Camelbeke pers. comm.) and in warmer parts of the UK (pers. obs.). Nobody should plant one unless they are prepared to grub it out at a moment’s notice, but in areas where it hasn’t shown a tendency to invasiveness it remains thoroughly worthy of planting.