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Owen Johnson (2024)
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Johnson, O. (2024), 'Ailanthus altissima' from the website Trees and Shrubs Online (treesandshrubsonline.
The International Dendrology Society does not condone the cultivation of Ailanthus altissima.
A tree to 30 m tall, with a rounded trunk lacking buttresses. Bark smooth, dark grey, eventually with very shallow paler vertical fissures. Buds small and largely hidden in summer behind the swollen base of the petiole. Twigs often with yellow or brownish pubescence at first and sometimes with soft spines when young. Leaves 40–60(–150) cm long; leaflets foetid, 13–27(–35), 8–15(–20) cm long, ovate-lanceolate, entire except for (1–)2–6(–8) large glandular teeth towards the base; dark green above and sometimes pubescent along the veins, rather glossy; paler green below and variably pubescent; petiolule 6–20 mm; petiole 7–13 cm; rachis sometimes red to purple. Flowers very numerous in terminal panicles 10–30 cm long; each flower light green with petals c. 2 mm long; male flowers malodorous. Wing of seed 30–70 × 7–18 mm, narrowly oblong, tapering and sharply twisted at each end, variably yellow to crimson, ultimately brownish, the seed central (Peng & Wayt Thomas 2008; Bean 1976).
Distribution China All areas except Hainan in the south, Heilongjiang and Jilin in the far north, and Ningxia and Qinghai in northern central Asia. Vietnam In the north Taiwan In mountains in the north (as var. tanakae)
Habitat Mountain forests and open woodlands in its native habitat. As an invasive it now occurs in many habitats around the world, especially in urban and peri-urban places.
USDA Hardiness Zone 5a
RHS Hardiness Rating H6
Awards AGM
Conservation status Not evaluated (NE)
Taxonomic note The made-up word Ailanthus is generally understood to be a latinate feminine noun with which the adjective altissima must agree, although in classical Latin female nouns ending in -us are greatly outnumbered by masculine and neuter nouns. A minority of authors, such as Peng & Wayt Thomas 2008, have treated Ailanthus as a masculine noun, so that the specific name becomes altissimus.
Ailanthus altissima is a common but nowhere abundant tree across most of China, and a rare one in Taiwan; its natural range arguably extends into the mountain forests of northern Vietnam (Kowarik & Säumel 2007). It seems to have been introduced to Korea and Japan at an early date, and it has long been planted all across these regions as an ornamental. The tree’s timber is pale yellow, close-grained and satiny, and is used for cabinet-making; being fast-growing it is also an important source of firewood. The plant has many uses in Chinese traditional medicine, primarily as an astringent, but western medical attention has focused on the species’ toxic and allergenic properties (Wikipedia 2024). A yellow dye is derived from the foliage, which can also be used as an insect repellant (Quantrell 2024). One of the species’ Chinese names translates as ‘Spring Tree’; being one of the last trees to come into leaf, its foliage is proof that spring has really arrived (Wikipedia 2024).
This is the host plant for the Ailanthus Silkmoth Samia cynthia. The silk produced by this moth’s larvae is stronger – and cheaper – but less smooth and shiny than the silk of Bombyx mori (which feeds on mulberry leaves), and does not take a dye. This fabric’s Chinese name translates as ‘peace silk’ since the fibres can be harvested after the larvae have flown, without harming them (Battles 2022).
Introduction to the west
Ailanthus altissima was one of the first Chinese trees to be brought to Europe; the Jesuit priest Pierre d’Incarville sent seeds from Beijing to Bernard de Jussieu in France in the 1740s, assuming that they were from the economically important Varnish Tree Toxicodendron vernicifluum. In 1751, Jussieu passed on some of the seeds to Philip Miller at the Chelsea Physic Garden in England, and to Philip Cartaret Webb, an amateur plant-collector at Busbridge Hall in Surrey. In 1768 Miller described the tree as a new species, Toxicodendron altissima – it makes a taller plant than the species still placed in Toxicodendron. In 1788, René Desfontaines observed the similarity of the fruit which trees in France were now producing to that of the tropical White Siris and created a new genus Ailanthus for these two trees, naming the Chinese tree A. glandulosa – the glandular teeth towards the base of each leaflet continue to serve as a distinguishing feature between A. altissima and its tropical allies. Although Miller’s description had precedence, the combination Ailanthus altissima was not made until 1916, by the American botanist Walter Swingle (Royal Botanic Gardens, Kew 2024).
Ailanthus altissima was introduced to the United States by Wiliam Hamilton in 1784 (Jacobson 1996), and again by Prince’s Nursery in New York around 1820 (Wenning & Lewis 2014). The species had also reached Australia by the early date of 1845 (Constán-Nava 2014). The tree is now present as an ornamental (and usually as an invasive species) in most of the world’s temperate countries – younger trees in England deriving from other cultivated sources include KBE 304 at Hergest Croft (from seed collected in 1983 in the University Botanic Gardens at Srinagar, Kashmir, while material from Chollipo Arboretum in South Korea is also grown at Hergest Croft (R. Griffiths pers. comm.).
In the 1860s, there was much interest in establishing a silk industry in England, but the climate proved not to suit the moth Samia cynthia in these ‘ailantheries’ (Battles 2022).
Natural variation
Around the turn of the 19th and 20th centuries, wild Ailanthus was re-introduced to Europe from south-western China in some variety. Collections sent by Paul Farges, Giuseppe Giraldi and Francois Ducloux were described by the French botanist Louis-Albert Dode, respectively, as the new species and varieties, A. vilmoriniana (1904), A. sutchuenensis (1907), A. giraldii (1907) and A. giraldii var. duclouxii (1907; Royal Botanic Gardens, Kew 2024).
The type specimen for Dode’s Ailanthus vilmoriniana had been raised by the French nurseryman Maurice de Vilmorin from seed collected by Farges in Sichuan Province in 1897 (Bean 1976). The most obvious distinguishing feature of this population lies in the soft spines that clothe the shoots of young plants, but which tend not to be borne on mature trees. Other growers have also observed that the leaves can be even longer, the central leaf-stalk can be rich red, the leaflets are densely pubescent underneath, the keys, within longer panicles, can be as much as 6 cm long, the bark can be almost white and the habit can be very open (Bean 1976; Glyn Church pers. comm. to J. Grimshaw). Ernest Wilson sent seeds of similarly spiny plants from nearby Yunnan Province around 1908 (W 388), and more recent re-introductions of this form include KR 265, and a collection made in Yunnan by Gary Clapperton of Eastwoodhill Arboretum in the early 1990s (Tree Register 2024; Glyn Church pers. comm. to J. Grimshaw).
The type specimen for Dode’s Ailanthus giraldii had also been collected in Sichuan by Farges, four years earlier in 1893, while Giraldi himself had botanised in Shensi Province. This tree was described as resembling A. vilmoriniana in its large and very downy leaflets, its longer panicles with large keys, and its purplish leaf-stalks, although it lacked the distinctive soft spines of A. vilmoriniana. The var. duclouxii was described as differing in its less downy leaflets with green stalks, and its light orange rather than brown young twigs (Bean 1976).
Ailanthus sutchuenensis was also described in 1907 and from material collected by Farges in Sichuan (a word transliterated into French in Dode’s time as ‘Sutchuen’), differing from typical A. altissima in its hairless, shiny red-brown young twigs, its whitish bark, purplish leaf-stalks, leaflets with tapered bases and not fringed with fine hairs, and its particularly large keys (Bean 1976; Peng & Wayt Thomas 2008). The taxon was first treated as a variety of A. altissima (then known as A. glandulosa) by Alfred Rehder and Ernest Wilson in 1913, who also placed material collected by Augustine Henry in Hubei Province within this entity (Bean 1976).
Rather few subsequent botanists have accepted Dode’s Ailanthus giraldii, although the collection SICH 2400 was attributed to this species as recently as c. 2003. A. vilmoriniana, by contrast, is easily recognised – as a young plant at least – by its spiny twigs, making this the most distinctive hardy Ailanthus phenotype. However, most contemporary authorities follow Geerinck 1990 in treating this taxon as a synonym for A. altissima var. altissima (Royal Botanic Gardens, Kew 2024). Meanwhile, Plants of the World Online (Royal Botanic Gardens, Kew 2024) continues to recognise Rehder and Wilson’s var. sutchuenensis: an anomaly, in that Wilson – who knew most if not all of these variants in the wild – can be inferred to have felt that A. vilmoriniana (which he never renamed as a variety of A. altissima) differed more obviously from var. altissima than did his var. sutchuenensis.
It seems very likely that future authorities will modify the nomenclature recommended by (Royal Botanic Gardens, Kew 2024) as it relates to var. sutchuenensis. If this variety is to be understood to be representative of the variants of Ailanthus altissima throughout the mountain forests of the southern and western end of the species’ range, which are generally characterised by larger fruit and a whitish bark, it makes very little sense to treat Dode’s A. vilmoriniana and A. giraldii – which derive from the same general area and share the large keys and sometimes the pale bark – as typical of A. altissima var altissima instead, or even as cultivar Groups within that entity; if A. vilmoriniana were to be treated in synonymy with var. sutchuenensis, its name would probably have precedence, both because it was published first and because its spiny shoots, while not common to all of these south-western trees, do at least seem to offer the most recognisable feature within these sub-populations. In this account, these variants are treated together under the heading ‘Forms from south-west China’.
Another variety of A. altissima, var. tanakae, was described by Bunzō Hayata in 1914 from the mountain forests of northern Taiwan; Hayata named it in honour of a fellow Japanese botanist, Chōzaburō Tanaka. This variety is also still recognised by Royal Botanic Gardens, Kew 2024, and can be told by its smooth, yellowish-grey twigs, the shorter stalks to each leaflet, the curving shape of the leaflets, and its small keys which are only 7–8 mm wide (Peng & Wayt Thomas 2008). Var. altissima does not grow wild in Taiwan (Li 1993).
Tree of Heaven or tree of Hell
Ailanthus altissima gained popularity as an ornamental tree in Europe and North America through the 19th century, wherever summers were long and hot enough for it to thrive. More recently, the species’ invasive or weedy qualities has led many commentators to emphasise its less ornamental features – its somewhat disagreeable aroma, and its gauntness through a long season without foliage – but it has to be admitted that Trees of Heaven can make enticingly beautiful specimens. The spectacular length of the leaves, particularly on vigorous sucker growth, hints exhilaratingly at this tree’s tropical affiliations, as perhaps does their deep green and rather glossy finish. The leaflets can be more numerous than in any similar hardy tree and are set in particularly neat parallel pairs, like the rungs of a remarkably long ladder. For a few days, these new leaves are a rich mahogany brown or even purplish. The bole’s bark remains quite smooth to a great size, while the surprising lack of any taper or buttressing at the base may suggest the foot of a stupendous elephant. Male trees were selected against – their smelly flowers and pollen that can also be allergenic (Enescu, Houston Durrant & Caudullo 2016), but they do need to be within insect pollinating distance for female plants to produce their key-like fruit (Wenning & Lewis 2014). As they ripen, these keys can be among the showiest fruit of any tree, shining yellow, amber, scarlet or even crimson in great drooping masses amid the rich green foliage. If they ripen fully these keys turn a duller brown, outlasting the leaves which tend to fall with little colour. Because seedlings are too large to transplant by the time they flower and indicate their gender, it became conventional to sell female Ailanthus as transplanted root suckers (Bean 1976); sometimes, a graft at the base seems to indicate nothing more than a guaranteed female planting.
Despite these qualities, by the 21st century the conversation around this species had begun to focus almost exclusively on its invasive tendencies – something not seen in its native China, where numerous insect species have evolved to exploit its foliage and plenty of other plants are able to compete with it. (In Italy, an eriophyid mite, Aculus taihangensis, which feeds on Ailanthus in China, is being investigated as a potential biological control; PBA Solutions 2024).
Ailanthus altissima is certainly a tree with an impressive repertoire of competitive strategies. The aerodynamic wing, with its distinctive twist at each end, allows each of a large female tree’s hundreds of thousands of keys to spiral rapidly in the air and fall some distance from the parent. Saplings can grow two or three metres in a year and, while most liable to germinate in sunny urban situations, they can also invade woodlands and form dense single-species stands. They can cope with almost any soil pH and are very drought-tolerant, and are notably resistant to salt in the soil and to pollutants such as heavy metals, though they are not so tolerant of waterlogging or of dense shade. The roots secrete an allelopathic chemical, ailanthone, which prevents the establishment of seedlings of other plants, and similar chemicals leach into the ground from the fallen leaves through winter. Damage to the tree or its roots provokes vigorous suckering as well as coppice regrowth, meaning that year-on-year effort is needed to begin to eradicate any population. (Healthy, undamaged trees tend not to sucker, leading to occasional reports of non-suckering strains or varieties.) The foul-smelling foliage is toxic to many insects and browsing animals (Constán-Nava et al. 2010; Wenning & Lewis 2014).
Each leaflet is also equipped with nectaries, structures which in Prunus and Populus serve to attract ants into the tree, which will in turn deter insects that might eat the tree’s foliage; a similar relationship may not yet have been observed within Ailanthus, but the alternative explanation of Kowarik & Säumel (2007), that these nectaries simply get rid of excess sugars from the foliage, may appear less convincing.
Yet another of this tree’s demerits is that the flavour of its pollen can taint honey: in 1944, Ronald Melville – the botanist who compiled the world’s first Red Data Book – was alerted to the honey produced by a beekeeper in Kensington (London), which was ‘greenish-brown’ with an ‘after-taste reminiscent of cats’; Melville’s analysis revealed that 44% of the pollen it contained was from local Ailanthus (with much of the rest coming from Sweet Chestnut Castanea sativa, another tree with dodgy-smelling flowers; Melville 1944).
As early as 1886 an ordinance in Atlanta, Georgia, USA, instituted a $50 fine for any landowner on whose property Ailanthus grew (Wikipedia 2024), and by the late 19th century the species, which had been planted in America to beautify and shade roadsides, was also noted as widely naturalised in Tennessee (Constán-Nava 2014). Except in the northern Prairie states where winters are too harsh for the species to persist, Ailanthus is now known in the United States more or less exclusively as a widespread, troublesome and difficult-to-eradicate invasive weed (Wenning & Lewis 2014; Constán-Nava et al. 2010). It is is also the host plant of the Spotted Lanternfly, Lycorma delicatula, which introduced to the eastern states is itself becoming an invasive pest, feeding on and damaging many native American plants and fruit trees (The Nature Conservancy 2024). Ailanthus is showing similarly invasive tendencies in southern Ontario and British Columbia (Canada), Mexico, some warm-temperate parts of South America, Australia and New Zealand, South Africa, the Atlas Mountains, south-west Asia and the Himalayas, Japan, and across central and Mediterranean Europe, where its ability to invade open natural woodlands is now of especial concern (Constán-Nava 2014; Enescu, Houston Durrant & Caudullo 2016). Attempts to eradicate these invasive stands are costly and generally depend on large quantities of chemical herbicides, whose long-term effects on the natural environment and on human health remain far from clear. The least environmentally damaging control method may be to ring-bark the tree and wait until it dies; suckering seems not to be provoked by this treatment (Polcin 2023).
In 2019, Tree of Heaven was added to the European Union’s list of ‘Invasive Alien Species’ which cannot be sold, planted, or allowed to grow (European Union 2019). The UK’s departure from the EU the following year did not result in a reprieve: Ailanthus altissima remains a ‘Species of Special Concern’ which in England and Wales can only be cultivated as a botanical specimen, where it must be clearly labelled as an ‘invasive alien species’ (Department for Environment, Food and Rural Affairs 2019).
Many users of Trees and Shrubs Online will live and garden in the UK and, being familiar with Ailanthus altissima only as a rather scarce and often languid garden tree, may be surprised by this legislature, or even inclined to dismiss it as alarmist, or a bureaucratic irrelevance. (The only other woody species in the 2019 schedule of Species of Special Concern – derived from the same EU directive and for similar reasons – are Acacia saligna (Golden Wreath Wattle) and Triadica sebifera (Chinese Tallow Tree), which, so far from being invasive in Britain, are hard to keep alive here, as least for now.) But the behaviour of Ailanthus altissima in areas with summers a few degrees warmer than most of Britain’s needs to be seen by local gardeners as a timely warning, particularly in the face of ongoing Climate Change; it makes sense to remove the plant before, rather than after, it becomes an aggressive weed.
In Greater London – Britain’s hottest area, and also the place where Ailanthus has been most widely cultivated – this tree is already starting to misbehave. Suckers – and possibly seedlings; these can be hard to distinguish – spring up beside pavements, in development sites and along railways; like self-seeding Buddleja davidii, they are able to thrive in places where there is little native competition but, unlike Buddleja, they can quickly reach tree size and cause extensive damage to drains and foundations. Trees of Heaven along the line may not yet have been presented as an excuse for a late-running train, but anyone using the District Line around Earls Court may feel that this is merely a matter of time. Naturalising Ailanthus seedlings are now being recorded in other parts of southern England (Stroh et al. 2024). One unusual opportunity to study the behaviour of Ailanthus altissima within semi-natural woodland, rather than disturbed urban sites, is afforded by the long-neglected Victorian arboretum at Beauport Park in the far south-east of England; mature suckers reveal the sites of a couple of long-lost specimen Ailanthus, but here they have so far tended to lose out to the more vigorous regeneration of native species, plus invasive Sycamore, Turkey Oak, Cherry Laurel and rhododendron (pers. obs.).
Although the UK champion Tree of Heaven, at Canford School in Dorset near England’s south coast, had a trunk 1.47 m thick in 2017, comparing quite well with the largest ornamental survivors in central Europe and the eastern United States, the species remains a rare plants of modest stature towards the north of England, and in Scotland. A street tree in St Leonards Road in St Andrews, Fife, probably selected by University botanists, was 12 m × 55 cm dbh in 2022, while in Aberdeen’s Duthie Park one of 11 m × 42 cm in 2018 was the northernmost surviving specimen on the Tree Register 2024; there are no records at all from the milder but much wetter north-western side of Scotland. (One interesting report in Loudon’s Arboretum et Fruticetum Britannicum (Loudon 1838) concerns a tree 13 m × 46 cm dbh at Dunrobin Castle, remarkably close to the northern tip of Scotland where very few tree species grow at all well; although from a distance of nearly two centuries this is easy to forget, Loudon was writing during an interlude of warm weather, when trees as tender as Acacia dealbata also appear to have thrived as far north as Dundee.) Like Paulownia tomentosa, another large-leaved warmth-loving and potentially very invasive tree from the same parts of China, it used to be recommended to coppice Ailanthus each winter in northerly gardens, and to treat the annual regrowth like a herbaceous plant (Bean 1976).
The Tree of Heaven’s designation in Britain as a ‘Species of Special Concern’ has not, yet, resulted in the widescale removal of existing specimens. (Some of these are much-loved and, it could be argued, occupy sites than would otherwise remain void of any tree.) Ailanthus altissima is not, however, particulary long-lived as an individual tree; if suckers and seedlings fail to establish or are successfully controlled, the species seems bound to dwindle, eventually, this far north, into a rarity in cultivation, perpetuated as new generations of suckers within a few arboreta. Online sources frequently cite a lifespan of only 50 years (Wikipedia 2024), which presumably derives from the species’ behaviour in warmer temperate regions. A Tree of Heaven at Yewden Manor in Buckinghamshire, dating from around 1830, stood dead when it was measured by Alan Mitchell 133 years later; a thriving tree in South Park, Ilford (east London) probably dates back to the park’s layout in 1902, while the oldest var. altissima at Kew, accessioned in 1907, was also flourishing in 2022 (Tree Register 2022).
There are, of course, hundreds of beautiful ornamental tree species, scarcely grown in Europe or North America as yet, which could be planted to fill most of spaces left by Ailanthus in temperate gardens, and which show few if any signs of ever becoming invasive.
The existence of other specific names for hardy Ailanthus (A. vilmoriniana, A. giraldii and A. sutchuenensis) is occasionally presented as a loophole allowing European gardeners to continue to grow the genus (Scirpidiella’s Plants 2018). Shaky though the botanical foundations for these alternative species may be, one significant issue is raised here: these species were described from the mountain forests of south and west China, whose rainy but seldom hot summers differ significantly from the growing conditions around Beijing. The Scottish champion Ailanthus is a specimen grown as A. vilmoriniana from W 388 in the Royal Botanic Garden Edinburgh (Tree Register 2024), while Ireland’s largest, at the Glasnevin National Botanic Gardens in Dublin, was grown as A. giraldii var. duclouxii; both these variants seem better able to tolerate cool and cloudy summers. Plants from the western Chinese populations have yet to be observed behaving invasively, but as single specimens in a few collections they are perhaps unlikely to have had much opportunity to pollinate each other. If these forms were ever to be cultivated as widely as var. altissima has been, it is not impossible that they could prove comparably invasive – but in cooler and wetter places.
In the days when Trees of Heaven were still sold by nurseries, rather few ornamental selections were made; these seem likely to lapse into extinction. In addition to the variants listed below, two variegated clones, ‘Aucubifolia’ and ‘Tricolor’, were described from the United States by Dr Michael Dirr in 1989, but seem not to have been particularly attractive plants and are probably already lost to cultivation (Dirr 2009); a variegated foliis-variegatis had first been described in Europe by Cornelius Oudemans in 1866 (Royal Botanic Gardens, Kew 2024).
Common Names
Red-fruited Tree of Heaven
Synonyms / alternative names
Ailanthus erythrocarpus Carrière
Ailanthus altissima f. erythrocarpa (Carrière) Rehder
Ailanthus erythrocarpus was described by Élie-Abel Carrière from cultivation in Europe in 1867 as a tree with darker green leaves than A. altissima, and red fruit (Bean 1976; IPNI 2024). Although the fruit can be particularly showy, there is no longer any presumption that trees showing this feature are any more than a minor variant of A. altissima. One such plant still grown at the Arnold Arboretum, Massachusetts, was selected as a seedling in the United States, as f. erythrocarpa, by Peter del Tredici (Battles 2022). At the Royal Botanic Gardens, Kew, a Tree of Heaven of rather poor growth (10 m × 34 cm dbh in 2022) was labelled as a cultivar ‘Erythrocarpa’ and might have represented a clone once distributed in Britain (Tree Register 2024). It is tempting to assume that the occasional surviving grafted specimen Ailanthus, with ordinary foliage, represents some such selection, but it may simply have been that the nursery was ensuring its plants were females by grafting them.
Selected for its compact habit by W. Wendell in Urbana, Illinois (Jacobson 1996); as a male plant, ‘Metro’ also did not produce any seedlings (although it would have been able to fertilise any nearby females).
Synonyms / alternative names
Ailanthus altissima f. pendulifolia (Dippel) Rehder
A slower-growing selection whose leaves drooped instead of being held more or less horizontally, first described from cultivation in Europe 1889 (Bean 1976); ‘Pendulifolia’ was introduced to the Arnold Arboretum in the United States in 1925 (Jacobson 1996). In 2024, it was still grown at the Sir Harold Hillier Gardens in Britain, where it had attained the relatively modest dimensions of 17.7 m × 43 cm dbh (Tree Register 2024). In 2023, an old municipal planting of Ailanthus in Canons Park in north London was not grafted, but was unusually petite, with rather drooping leaves, and may have represented the same clone; this tree was also distinguished by its unusually roughly fissured bark (pers. obs.).
Synonyms / alternative names
Ailanthus altissima 'Hongye'
Ailanthus altissima 'Bíborsárkány'
Selected by the Faculty of Horticulture at Corvinus University in Hungary for the rich purplish-red of its unfolding leaves, this colour fades through red to green into summer. This is a female plant of good habit, less vigorous than typical Ailanthus altissima (Edwards & Marshall 2019); its cultivar names mean ‘purple dragon’ in Hungarian and in Mandarin. ‘Purple Dragon’ became commercially available just in time for the sale of its species to be outlawed under European Union legislation (European Union 2019), but five years later it was still advertised online by a few nurseries (perhaps using the cultivar name as a legal loophole). A young standard at the Bluebell Arboretum in Derbyshire, UK, was 7 m tall by 2017, but has since been removed, while at Hergest Croft the gorgeous foliage display is managed by annual coppicing (Tree Register 2024).
A Chinese selection with a compact habit; a male plant which consequently does not produce seedlings (Dirr & Zhang 2004; Wikipedia 2024).
Synonyms
Ailanthus glandulosa var. tanakae Hayata
Trees from mountain forests in northern Taiwan. Twigs yellowish grey; leaves with reddish rachis and shortly-stalked falcate leaflets, tardily deciduous in mild climates. Fruit small, the wing c. 7–8 mm wide (Peng & Wayt Thomas 2008; Edwards & Marshall 2019).
Distribution
RHS Hardiness Rating: H5
USDA Hardiness Zone: 7
First described by Bunzō Hayata in 1914, the Taiwanese race of Ailanthus altissima differs very subtly from typical, Chinese trees; its fruits are unusually small, and other features can include a yellowish tinge to the twigs and a more curved (falcate) shape to the leaflets, which are shorter stalked and whose common stalk can be red (Peng & Wayt Thomas 2008; Edwards & Marshall 2019). The variety occupies a restricted area of mountain forest towards the north of the island, where its numbers are in steady decline; in 1998 it became the first Ailanthus taxon to be assessed by the IUCN as Endangered (IUCN 2024), in common with all too many Taiwanese trees.
The variety seems not to have been introduced to the west until 1993, when a Kew-led expedition collected seed (ETOT 51). Planted at Kew, seedlings have thrived and the largest specimen in 2022, in the Berberis Dell, was 16 m × 31 cm dbh. ETOT 51 has also flourished in the slightly cooler microclimate of the Westonbirt National Arboretum, where one seedling was 13.7 m × 24 cm dbh in 2024 (Tree Register 2024). The female trees in the group at Kew fruit freely, while the dark green leaflets, on their red stalks, persist into early winter (Grimshaw & Bayton 2009). Another collection was RWJ 9906 in 2003, distributed by Crûg Farm Plants and represented by a young tree at Caerhays Castle in Cornwall; Matthew Ellis has also succeeded with the variety in the heavy soils and more continental microclimate of his Grange Farm Arboretum in the Lincolnshire fens (Tree Register 2008).
Blanket European Union legislation in 2019 put a stop to the sale or distribution of var. tanakae in our region, blacklisting this Endangered tree as potentially invasive (European Union 2019), although the very different microclimate to which it is native might make it seem unlikely that its behaviour in cultivation should resemble that of var. altissima from the Beijing area. In particular, it is likely to be much less frost tolerant, though the hardiness data suggested here is a matter of guesswork.
Synonyms
Ailanthus sutchuenensis Dode
Ailanthus altissima var. sutchuenensis (Dode) Rehd. & Wils.
Ailanthus giraldii Dode
Ailanthus giraldii var. duclouxii Dode
Ailanthus vilmoriniana Dode
Ailanthus vilmoriniana var. henanensis Jian Y.Chen & L.Y.Jin
Trees from southern and western China, with large fruit (wing 50–70 × 14–18 mm); bark sometimes paler grey to whitish; leaves sometimes longer (Peng & Wayt Thomas 2008; Bean 1976).
[PLEASE ADD THE NAMES MENTIONED BELOW, ALONG WITH VILMORINIANA GROUP, TO THE LIST OF SYNONYMS]
Distribution
RHS Hardiness Rating: H5
USDA Hardiness Zone: 6
As discussed within the species article, the widespread tendency among modern taxonomists to submerge within Ailanthus altissima the variants from southern and western China which were described by Louis-Albert Dode in the first years of the 20th century as A. vilmoriniana, A. giraldii and A. giraldii var. duclouxii has had the side effect of leaving var. sutchuenensis as the only name available to serve as a portmanteau for these populations, although this was certainly the intention neither of Dode when he published the name A. sutchuenensis in 1907, nor of Alfred Rehder and Ernest Wilson when they described that taxon as a variety of A. altissima in 1913.
It is not at all clear how genetically diverse these populations from south-western China really are; common traits include a significantly larger wing to the fruit, variously accompanied by a paler or even whitish bark, and even larger leaves with red or purplish common stalks, while the most distinctive feature of all – soft spines that cover the young twigs of immature trees – appears confined to the population described by Dode as Ailanthus vilmoriniana (Peng & Wayt Thomas 2008; Bean 1976). A few other named variants, such as Ding and Chao’s white-barked var. leucoxyla, are placed by Royal Botanic Gardens, Kew 2024 in synonymy with var. altissima – presumably for convenience – but also seem more likely to describe parts of this south-western population. Plants known to derive from a variety of these populations are still grown, sparingly, in western Europe and in New Zealand; for convenience, this account treats them all together here.
Given the invasive tendencies of var. altissima (sourced largely or even exclusively from around Beijing) in temperate parts of the world with sufficiently warm summers, an even more significant difference may lie in these south-westerly variants’ adaption to climates where summers are relatively cool but reliably wet; there is not yet any evidence that any of these forms can become invasive in cultivation, either through seedlings or suckers, although this could well simply be because they remain too scarce to pollinate one another, and as valued botanical specimens are not treated in ways likely to provoke aggressive suckering. Such plants are also likely to be slightly less hardy than var. altissima from northern China, though the hardiness data presented above is a matter of guesswork.
Ailanthus vilmoriniana was described by Dode from a young tree at Les Barres in France which the nurseryman Maurice de Vilmorin had raised from seed sent from Sichuan by Paul Farges in 1897 (Bean 1976). Seed and rooted cuttings were also sent to the Royal Botanic Gardens at Kew, where three examples continued to thrive in 2022, the largest of them 23.8 m × 85 cm dbh near the Broadwalk (Tree Register 2024), suggesting that this may prove an unusually long-lived Ailanthus variant within the local conditions. With age, the young twigs no longer carry the soft spines, and the appearance of these trees becomes more typical for A. altissima (Geerinck 1990). Seed from a plant identified by Ernest Wilson as A. vilmoriniana reached the Royal Botanic Garden Edinburgh in 1908 (W 388); in 2014 the resulting tree was 16 m × 66 cm dbh and had slightly outgrown the very few Ailanthus in Scotland known or assumed to represent var. altissima from northern or eastern China (Tree Register 2024). A rather slender tree in the slightly more continental climate of Delft Botanical Garden in the Netherlands, dating from around 1935, was 18.4 m × 64 cm dbh in 2024 (Monumental Trees 2024), while the tallest, planted around this time and drawn up in close competition at the Thorp Perrow Arboretum in North Yorkshire, had reached 25 m in 2022 (Tree Register 2024).
Keith Rushforth’s more recent collection KR 265 has thrived in the Valley Gardens in Windsor Great Park, UK, where it was accessioned in 1999 and was 19 m × 60 cm in 2022 (Tree Register 2022). Near New Plymouth in the humid, warm-temperate to subtropical climate of New Zealand’s North Island, a collection made by Eastwoodhill Arboretum from Yunnan Province in the early 1990s has grown even faster, reaching 24.4 m × 75 cm dbh in 2024 (Glyn Church pers. comm. to J. Grimshaw); this plant suffered from wind damage at first but has since developed a sturdy but open crown. A tree from this same collection planted in 1998 at Tregrehan, in the cooler and very Atlantic climate of Cornwall, UK, was 17 m × 31 cm in 2024 (Tree Register 2024). None of these specimens have been observed to self-seed; whether or not the Chinese populations of Ailanthus are genetically uniform enough for a Vilmorin’s Tree of Heaven to be pollinated by a nearby var. altissima remains to be discovered. Glyn Church in New Zealand observes that his tree does not sucker (pers. comm. to J. Grimshaw), but suckers are not perhaps to be expected to arise from the roots of an unstressed and undamaged young specimen tree. Church also remarks upon the ornamental quality of his tree’s ‘lovely white trunks’.
Dode’s Ailanthus giraldii reached France around the same time as his A. vilmoriniana, but is less readily distinguishable from typical A. altissima. The taxon reached Kew in 1907, via Chenault’s nursery in France (Bean 1976). Partly because of the ongoing nomenclatural instability, it is difficult to trace the various introductions that have been made under the names A. giraldii and A. giraldii var. duclouxii, but it seems probable that any trees labelled as such will ultimately derive from material sent to France around this time. These include a 1984 repropagation at Kew, north of the Waterlily House, which is already 20 m tall with a trunk 1.09 m broad below a low fork. At the Royal Botanic Garden Edinburgh, a 1924 accession was 15 m × 61 cm in 2014, seeming just as happy in this garden’s cool and cloudy summers as the nearby tree grown as A. vilmoriniana. At the National Botanic Garden, Glasnevin, Ireland, an example grown as A. giraldii var. duclouxii is the largest Ailanthus in Ireland, 17 m tall and with a trunk 1.17 m thick at one metre up. One more recent introduction grown as A. giraldii is SICH 2400, from which a tree accessioned at Kew in 2011 was already 12 m × 33 cm dbh in 2022 (Tree Register 2008).
Dode’s Ailanthus sutchuenensis once again reached Europe in the last years of the 19th century (Bean 1976), but the loss over the years of all the original examples at Kew might be taken to imply that this variant is less well adapted to the local microclimate. Young plants are grown as A. altissima var. sutchuenensis at the National Botanic Garden at Glasevin in Ireland and at John Ravencroft’s Cherry Tree Arboretum in Shropshire, UK (Tree Register 2024).
These variants may be less well adapted to the continental climates of the eastern United States, but examples grown as Ailanthus giraldii, A. altissima var. sutchuenensis and A. vilmoriniana continue to thrive at the Arnold Arboretum in Massachusetts (Arnold Arboretum 2024); they have been grown in America since the middle 20th century (Jacobson 1996).