Koelreuteria elegans
Sponsor
Kindly sponsored by
a member of the International Dendrology Society
Credits
Richard Moore (2024)
Recommended citation
Moore, R. (2024), 'Koelreuteria elegans' from the website Trees and Shrubs Online (treesandshrubsonline.
Genus
Common Names
- Flamegold rain tree
- Manawi
- Wiwi
- Lombolombo
Synonyms
- Azedarach elegans (Seem.) Kuntze
- Koelreuteria vitiensis A.C.Sm.
- Melia elegans Seem.
Tree 7–25 m tall, up to c. 50 cm diameter. Mature bark peeling in square plates, rough, corky, lightly fissured. Branchlets lenticelate, round to elliptic, pustulate, corky, cinnamon brown. Leaves bipinnate, terminal odd leaflet often absent or reduced, lower leaflets occasionaly pinnatisect, 25–60 cm long, 15–44 cm wide; rachis glabrous or shortly pubescent on upper surface, adaxially lightly canaliculate. Leaflets 8–17, sessile or with petiolules 1–2.5(–3) mm long, lanceolate to narrowly ovate to elliptic, 5.5–9.2 cm long, 1.3–3 cm wide, base strongly oblique, margins entire to irregular crenate-serrate, apex long-acuminate to caudate, lamina smooth and lustrous, veins faint from above, glabrous or lightly pubescent along veins and petiolule adaxially, midvein abaxially sparsely pubescent with tufts of hairs in axils, often with glands interspersed on midvein and side veins. Inflorescence 30–50 cm long, 20–25 cm wide, densely puberulent and glandular, especially at anthesis, less so in fruit; pedicels scattered-puberulent or glabrous. Flowers mildly sweetly fragrant; calyx lobes densely villous; petals usually 4 or 5, rarely 6, obtuse to rounded, 5.5–7 mm long, 2.7–3.5 mm wide, basal appendages (scale) lobulate-undulate, claw 2.5–5 mm long, up to twice as long as the calyx lobes. Stamens densely villous near base, filaments 8–10.1 mm long. Capsules ellipsoidal, valves rotund to suborbicular (placenta continuous and nonwinged), 3.4–5(–6) cm long, (2.5–)3.1–4.6 cm wide, inner side lustrous, reticulate-veined, lightly pubescent when young, glabrescent, deep rose-purple while young, maturing to brown; styles 4.5–7.5 mm long. Seeds pyriform to nearly spherical, 5.2–5.5 mm in diameter, black, slightly rugose or smooth. (Meyer 1976).
Distribution Fiji Vanau Levu, Viti Levu, Gau
Habitat Dry secondary forest used as pasture, rocky soils on ridges in open forest, clearings in low altitude forest edge, dry hillsides, slopes of escarpment, hillside thickets, and montane forest. Elevation 350–900 m. Flowering, March-May; fruiting, May-July.
USDA Hardiness Zone 12b
RHS Hardiness Rating H1a
Koelreuteria elegans has a very intriguing, disjunct distribution with Koelreuteria elegans subsp. elegans, the only taxon within the genus found in the southern hemisphere, endemic to three of the Fijian islands, Vanau Levu, Viti Levu and Gau, and its closest relative Koelreuteria elegans subsp. formosana, endemic to Taiwan. A distance of around 7200 km lies between these two subspecies in which no known living populations or evidence of fossilised ancestors have been found and the reasons for the presence of a Koelreuteria in Fiji remains a mystery (Meyer 1976). It has been hypothesised that Fiji may have once been linked with mainland Eurasia allowing for dispersal via land after which the two became separated stimulating speciation by vicariance; another possible cause is via a chance long distance dispersal event (Wu et al. 2007; Wang et al. 2013). Albert Charles Smith of the Arnold Arboretum who first came across specimens of this species in Fiji was equally surprised to find such a representative of this otherwise distant genus, noting that ‘From the size and habitat of these trees one must suppose that they are indigenous, although the record causes an extraordinary extension of the known range of Koelreuteria, otherwise limited to eastern Asia and Formosa’ (Smith 1950). A genetic investigation of the relationship between populations would be of great interest.
In relation to its native tropical habitat Koelreuteria elegans subsp. elegans is certainly the least hardy member of the genus, rarely experiencing temperatures below 20°C in the wild, therefore requiring protection under glass in temperate regions (Fiji Meteorological Service 2006). As the name suggests K. elegans is a graceful species bearing exceptionally attractive evergreen bipinnate leaves, lustrous dark-green with long acuminate tips, producing a show of bright yellow flowers followed by beautiful deep rose-purple inflated ovoid fruits. As noted by Smith (1950) when documenting this species growing in habitat, ‘this tree is one of the most striking components of the vegetation during its short flowering season, which lasts for only about two weeks, in May or early June. From the high ridges one may observe large individuals, spectacularly covered with bright yellow flowers, in the forest on the inaccessible slopes.’ Morphologically this subspecies is very similar to K. elegans subsp. formosana but can most easily be distinguished by its slightly smaller, more slender leaflets with usually entire or more sparsely serrate margins, shorter petiolules, petals that are obtuse to rounded in shape with a longer claw, and both longer stamen filaments and styles (Smith 1950; Meyer 1976).
Koelreuteria elegans subsp. elegans was first described in 1865 under the name Melia elegans Seem. (from which the basionym is taken), based on a sterile specimen collected in Fiji in 1860 by German botanist Berthold Carl Seemann. Later in 1891 Carl Ernst Otto Kuntze reclassified Melia elegans under the genus Azedarach Mill. (Kuntze 1891). It wasn’t until Albert Smith (1950) witnessed living specimens of K. elegans subsp. elegans growing in the wild that he realised it was distinct, having initially identified a number of herbarium specimens collected from Fiji as K. formosana prior to his expedition, thereafter determining it as a separate taxon and giving it the name of Koelreuteria vitiensis A.C.Sm., referring to the Fijian island of Viti Levu. Meyer (1976), having carefully examined all available herbarium specimens at the time came to the conclusion that the most sensible treatment for this taxon was to name it as a subspecies of K. elegans rather than a separate species in order to emphasise the biologic affinities between both subspecific taxa identified whilst recognising their disjunct distribution.
In Fiji this species has a variety of traditional uses. The Kai Viti (indigenous people of the Fiji islands) use the leaves to extract a black hair dye, made by simply boiling the leaves after which the remaining liquid can be applied to the hair. It is also said to be a useful timber tree (Meyer 1976; Keppel & Ghazanfar 2011).
This subspecies is occasionally cultivated in Fiji but is rarely found growing elsewhere around the world despite being of such high horticultural merit and is perfectly suited to be grown as a street tree in tropical regions. Seeds were sent to botanical gardens in California, the Gulf Coast and Florida in 1974 (Meyer 1976); however, its close relative K. elegans subsp. formosana remains much more commonly planted in such areas probably due to its tolerance of slightly colder winter temperatures and its equally valuable ornamental worth, although despite these attributes this species has unfortunately been found to be particularly invasive in subtropical regions and as such it is unlikely to inspire further cultivation (NSW WeedWise 2024; UF|IFAS 2024). The only record of this rare subspecies growing in the UK is from RBG Kew, having been first accessioned in 1953 and and grown for some years in the Palm House from material originating from Suva on the Fijian island of Viti Levu, although it seems that after some time the specimen deteriorated and was not successfully propagated, being recorded as dead in 2013 (P. Rees pers. comm. 2024).
subsp. formosana (Hayata) F.G. Meyer
Common Names
Flamegold rain tree
Taiwan mokugenji
Taiwan sendanbanobodaiju
Taiwan Luan Shu
Raarei (Paiwan)
Tamareku (Atayal)
Synonyms
Koelreuteria henryi Dummer
Koelreuteria formosana Hayata
Tree 7–25 m tall, up to c. 2 m in diameter. Mature bark peeling in square plates, rough, corky, lightly fissured. Branchlets lenticelate, round to elliptic, pustulate, corky, cinnamon brown. Leaves bipinnate, terminal odd leaflet often absent or reduced, 25–60 cm long, 15–44 cm wide; rachis glabrous or shortly pubescent on upper surface, adaxially lightly canaliculate. Leaflets (5)8–17, sessile or with petiolules 4–5(–10) mm long, lanceolate to narrowly ovate to elliptic, lower leaflets occassioinaly pinnatisect, (4.5–)6–10.2 cm long, 1.8–3(–4.2) cm wide, base strongly oblique, margins often coarsely serrate, apex long-acuminate to caudate, lamina smooth and lustrous, veins faint from above, glabrous or lightly pubescent along veins and petiolule adaxially, midvein abaxially sparsely pubescent with tufts of hairs in axils, often with glands interspersed on midvein and side veins. Inflorescence 30–50 cm long, 20–25 cm wide, densely puberulent and glandular, especially at anthesis, less so in fruit; pedicels scattered-puberulent or glabrous. Flowers mildly sweetly fragrant; calyx lobes densely villous; petals usually 5 or 4, acute, 6–7 mm long, 1.5–2.5 mm wide, basal appendages (scale) 2-lobed, lobulate-undulate, claw c.2 mm long, slighty longer than calyx lobes, sparsely villous. Stamens 7 or 8, densely villous near base, filaments 4.5–6 mm long. Capsules ellipsoidal, valves rotund to suborbicular (placenta continuous and non-winged), 3.4–5(–6) cm long, (2.5–)3.1–4.6 cm wide, inner side lustrous, reticulate-veined, lightly pubescent when young, glabrescent, deep rose-purple while young, maturing to brown; Styles 3–4 (–5) mm long. Seeds pyriform to nearly spherical, 5.2–5.5 mm in diameter, black, slightly rugose or smooth. Flowering September–October; fruiting November in situ. 2n = 32. (Meyer 1976; Urdampilleta, Ferrucci & Vanzela 2005).
Distribution
- Taiwan
RHS Hardiness Rating: H3
USDA Hardiness Zone: 9-10
Koelreuteria elegans subsp. formosana is a shapely upright tree with a rounded crown, bearing elegant evergreen bipinnate leaves and producing a wonderful display of yellow flowers late in the year in early autumn. These emit a delicious, mild, orange-blossom like fragrance, providing a splash of late-season interest with the addition of plentiful deep rose-purple coloured hanging lantern-like fruits in November. Unlike K. paniculata and K. bipinnata the autumn colour provided by this species is reportedly quite poor, which is no surprise considering this species is evergreen in its native habitat (Carter 2007). This subspecies is a close relative of Koelreuteria elegans subsp. elegans and the two do indeed share a number of morphological similarities, although they are widely separated in geographical terms with K. elegans subsp. elegans hailing from Fiji, around 7200 km away from Taiwan where K. elegans subsp. formosana can be found (Meyer 1976).
Koelreuteria elegans subsp. formosana is endemic to Taiwan. It is widespread across the island, most abundantly in lowland areas; however, despite claims of this species being found no higher than 1000 m in elevation (e.g. Wang et al. 2013), records from observations published on iNaturalist show that this subspecies has been sighted at relatively high altitudes up to around 2300 m (Liushengyu 2023). It is commonly found around forest edges, along roadsides and in forest openings on mountain slopes where a suitable amount of light is able to penetrate the tree canopy (iNaturalist 2024).
This subspecies was first identified and described under the name Koelreuteria henryi by Richard Arnold Dümmer in 1912 after the author noticed a number of morphological differences between a specimen collected by Augustine Henry in 1895 in Taiwan, originally identified as Koelreuteria bipinnata Franch., and the type specimen of K. bipinnata, observing that they were quite distinct from each other. Dümmer recorded differences in the leaves which are narrowly ovate with a more strongly acuminate to caudate apex and the fruit capsules as being more rounded and smaller in size (Dümmer 1912). The following year, in ignorance of Dümmer’s publication, Bunzō Hayata also described a new species based on a specimen collected in Taiwan by Ushinosuke Mori in 1906, naming it Koelreuteria formosana, noting that it differs from K. bipinnata by its subentire leaflets (Hayata 1913). In Meyer’s revision of Koelreuteria (1976) this taxon was described in a new combination as K. elegans subsp. formosana to account for the close affinity this taxon has with K. elegans subsp. elegans; (it is worth noting that botanical names do not have priority outside the rank at which they were originally published, so Meyer’s choice of ‘formosana’ for the subspecific epithet was perfectly legitimate, despite the fact that this taxon was originally described as K. henryi – R. Govaerts pers. comm. 2024). The close relationship between K. elegans subsp. formosana and K. bipinnata has been confirmed via cytological studies showing similarities between their two karyotypes whilst supporting their status as separate taxa (Urdampilleta, Ferrucci & Vanzela 2005).
Koelreuteria elegans subsp. formosana can be most easily distinguished from K. elegans subsp. elegans by its slightly larger leaflets, often coarsely serrate, and usually in greater number with longer petiolules, flowers more commonly with 5 petals, acute in shape with a shorter claw, and with shorter stamen filaments and styles (Meyer 1976). It is probable that K. elegans subsp. formosana has (like its Chinese relatives) been utilised traditionally in Taiwan for its valuable timber, production of dyes and medicinal properties; however, there is little evidence of this particular species being used by the indigenous people of Taiwan. In the west where this species has been cultivated the ornamental inflated fruits which retain some of their colour once dried can used in flower arrangements, to which they make a fine addition (Gilman & Watson 1993).
Koelreuteria elegans subsp. formosana was first introduced to the United States from Taipei, Taiwan in 1915 by the US Department of Agriculture, Office of Foreign Seed and Plant Introduction as seed presented by Mr Genjiro Takata, chief of the Bureau of Productive Industry (No. 41679) (USDA 1918). It has proved to be extremely successful in the southern states enjoying the subtropical climate and is widely grown in Hawaii, Florida, and southern California, where its rounded crown, evergreen leaves, spectacular golden-yellow flowers and inflated rose-purple fruits make it a popular street and specimen tree (Meyer 1976; Gilman & Watson 1993). Despite its early introduction to the US this subspecies didn’t become widely utilised until the 1950s (Carroll & Boyd 1992). In the UK seeds were sent to Kew in 1976 from Sydney Botanic Garden, though Clarke (1988) reports it as being recieved from the Taiwan Foresty Institute. Kew’s accession was grown outside near to where the Pavilion Restaurant now stands, though due its subtropical origins, possibily exacerbated by the provenance of the seed if originally collected at low elevation, it did not last long and was recorded as dead in 1987 (P. Rees pers. comm. 2024). A recent introduction by Bleddyn and Sue Wynn-Jones of seed from the Institute of Taiwan Endemic Species in 1999 (BSWJ 7038) was distributed from Crûg Farm, Gwynedd; however, these too have proven unsuccessful in UK cultivation. The only survivors traced are a couple of specimens struggling along at Tregrehan where they spent many years dying back each winter before regaining no more than a metre in height, year-in-year-out, at least until very recently, but now Tom Hudson reports that at least one of them has ‘powered away and is now a mighty 2 metres or so’ (T. Hudson, pers. comm. 2024).
Koelreuteria elegans subsp. formosana has been successfully cultivated in southern Japan having been introduced in the 1980s to Okinawa prefecture for use as a street tree and was introduced to Australia around the year 2000 (Wang et al. 2013; Suehiro 2024). Unfortunately, its reproductive prowess and ability to proficiently self-seed has enabled this tree to become quickly naturalised and it is now classed as an invasive species in southern Florida, Hawaii, Australia and in southern Japan where the long hot summers and mild winters have enabled it to thrive (Meyer 1976; Wang et al. 2013; NSW WeedWise 2024; UF|IFAS 2024).
