Koelreuteria Laxm.

The genus Koelreuteria is justly admired for its handsome foliage, broad inflorescences of yellow flowers (whence the moniker ‘Goldenrain Tree’), and inflated fruits resembling lanterns. It is a small genus containing just three species: Koelreuteria paniculata and K. bipinnata native to central to eastern China, Korea and northern Vietnam, and Koelreuteria elegans comprising two disjunct subspecies, K. elegans subsp. elegans, endemic to Fiji in the Pacific and K. elegans subsp. formosana endemic to Taiwan (POWO 2024). The common name Pride of India is often associated with Koelreuteria, athough the origin of this name is unclear and utterly misleading as no members of the genus are naturally found there.

Koelreuteria was first described in 1772 by Erik Laxmann from specimens cultivated in the gardens of the Academy in St Petersburg, which first flowered in 1771, having been grown from seed sent from China by the French Jesuit missionary and amateur botanist Pierre d’Incarville in 1747 (Laxmann 1772; Meyer 1976; Dosmann, Whitlow & Ho-Duck 2006; Natural History Museum 2013). The species first collected by d’Incarville was named K. paniculata after its large pyramidal panicles of bright yellow flowers, and the genus itself named in honour of Joseph Gottlieb Kölreuter. Laxmann was so excited about the discovery of this new genus that he wrote an enthusiatic letter to Carl Linnaeus explaining that it had been growing in the Academy gardens for over 20 years before finally flowering and revealing itself as a new genus which he wished to name ‘to a man outstandingly and deservedly most distinguished, both in botany and in our botanic garden, Koelreuter’ (Laxmann 1772; Meyer 1976).

The flowers of Koelreuteria are particularly interesting, with trees that possess both bisexual and functionally unisexual flowers, with staminate and pistillate flowers occuring together on the same inflorescence. Staminate flowers contain functional stamens with a much reduced, under-developed pistil, whereas pistillate flowers contain non-functional, under-developed stamens with a fully developed gynoecium. Meyer (1976) notes that during the first flush of flowering a greater number of staminate flowers are usually produced at a ratio of two to one; however, later in the flowering season pistillate flowers may become dominant.

Koelreuteria belongs to the large and diverse family Sapindaceae. According to the most up to date phylogenetic studies Koelreuteria belongs to a small monophyletic clade (tribe Koelreuterieae) within subfamily Sapindoideae, most closely related to Erythrophysa E.Mey. ex Harv. & Sond., a genus found in Southern Africa, Ethiopia and Madagascar, and Stocksia Benth. containing a single species from Iran, Afghanistan and Pakistan (Cole & Ferrucci 2018; Buerki et al. 2011; Joyce et al. 2023; POWO 2024). The biogeographical orign of the genus remains unresolved, with phylogeographic and fossil data essentially pointing in different directions. According to molecular biogeographical analysis the ancestral origin of the Sapindaceae is Eurasia from where the earliest extant members of the family arose. A study by Buerki et al. (2011) suggests that the clade containing Koelreuteria diverged around 85 mya from continental Asia and Koelreuteria itself is estimated to have evolved as a distinct genus some 60 million years ago during the Paleocene (Buerki et al. 2011; Joyce et al. 2023); however, detailed studies of fossil specimens suggest that Koelreuteria may have originated in western North America before dispersing to Central and Eatsern Asia via the Bering Land Bridge in the early Paleogene (Chen et al. 2022).

Koelreuteria has an abundant fossil record of preserved pollen, leaves, wood and fruit capsules, the earliest of which date back to the late Cretaceous. The most reliable fossil data – a fruit capsule belonging to the extinct Koelreuteria allenii (Lesq.) W.N. Edwards – has been dated to the early Eocene, collected from the Eocene Green River Formation in Wyoming, USA, dated to c. 52 mya. A large number of beautifully preserved Koelreuteria fossils have been found across the Northern Hemisphere, mainly in western North America, Eastern Asia and Central and Eastern Europe, showing that the genus once had a much wider distribution, so far with eight extinct species having been described (Chen et al. 2022). On the basis of reliable fossil evidence it is suggested that Koelreuteria may have originated in western North America after which it was able to colonise Eurasia where the genus likely underwent rapid diversification. The current geographic distribution of Koelreuteria most likely reflects the fact that areas of Eastern Asia served as refugia during global cooling in the Pliocene and during the Pleistocene ice age during which populations in North America and Europe faced extinction (Lu et al. 2018; Chen et al. 2022). The interesting disjunct distribution of K. elegans subsp. elegans, endemic to Fiji and K. elegans subsp. formosana, endemic to Taiwan, remains a topic of debate as neither fossils nor extant species of Koelreuteria have been found between these two populations. One suggestion is 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).

Koelreuteria has a range of economic uses and has long been utilised for its good quality timber, making dyes, extraction of oil from the seeds as a lubricant and for soap and has a range of medicinal uses. Species native to China have also historically been cultivated around temples, palaces and gardens and have religious significance being used as official memorial tree which since the Chou Dynasty (1122–240 BC) have been planted on the tombs of high ministers; the seeds are also used in religious ceremonies (Meyer 1976; Wang et al. 2013). Another interesting property of Koelreuteria, specifically K. paniculata, is its ability to tolerate soils contaminated with heavy metals which may render it useful as an accumulator to remediate contaminated areas (Dalun et al. 2009).

Members of this genus have traditionally been considered free from significant pest and disease issues, and don’t often succumb to serious problems. Trees can however be infected by coral spot fungus, root rot and cankers. Verticillium wilt is one of the most serious diseases to affect Koelreuteria causing wilting of the foliage and eventual death of the tree. Recently a number of other fungal pathogens have been identified that threaten the health of Koelreuteria species both in the wild and in cultivation, such as a leaf spot disease (Juxtiphoma eupyrena), powdery mildews, and serious cankers and blight diseases that have been found both in nature and commercial horticulture (Pan et al. 2022). Pests that can be found on Koelreuteria include scale insects and boxelder bugs; however, they are unlikely to do any lasting damage. It is worth noting that the wood of Keolreuteria tends to be quite brittle and is therefore susceptible to damage during stormy weather (NC State University 2024).

All Koelreuteria species require full sun and should be planted in well drained soils. Whilst young they may require some formative pruning to ensure the development of a good branch structure and shape. As Koelreuteria has relatively weak wood trees should ideally be positioned in more sheltered areas to prevent wind damage; however, careful training of specimens should also help to minimise weaknesses. Their tolerance of poor soil conditions, alkaline soils, heat, drought and urban pollution means they are all well suited as street trees for providing light shade and, of course, high ornamental value (Gilman & Watson 1993a; 1993b; 1993c). In some climates Koelreuteria species can be invasive and are therefore not recommended for planting in certain locations (Stace 2010; Verloove 2018; NSW WeedWise 2024; NC State University 2024).

Koelreuteria are most easily grown from seed; however, the seeds exhibit both exogenous and endogenous dormancy so they require both scarification and cold stratification to induce germination. Additionally the seed coats of Koelreuteria are very hard making them impermeable to water, so scarification must be achieved either chemically, by leaving seeds in a bath of sulfuric acid for around one hour, or by carefully nicking the seed coat; the seed should then be given cold stratification for between one and three months at 2–5°C, after which germination should be relatively quick (Tawfik, Ibrahim & Taha 2020). In cold areas, seed can be sown fresh in the autumn soon after collection (and scarification) when seeds should be sown into pots with a lightly-moist, good quality multi-purpose compost. Sown approximately 15 mm deep with a light coverign of gravel, pots can then be left in cold frames over winter providing natural stratification and germination should begin in the spring; however, care should be taken to ensure pots do not become too moist during the winter (Michler & Rudolf 1974; Gonderman & O’Rourke 1961; Rehman & Park 2000). Seeds can also be sown later, in late winter or early spring after artificial cold-stratification, ideally in sealed plastic bags in lightly moist vermiculite in a fridge. Some sources suggest that after scarification seed should be soaked for 24 hours then sown into an appropriate medium. With spring-sown seeds the seed pot or container should be placed on a heat mat providing an ambient air temperature of at least 20°C; germination should then occur within nine weeks (Pascoe 2023).

Vegetative propagation of Koelreuteria paniculata is possible via root cuttings, layers and softwood cuttings; the same methods ought to be useful for other taxa, but evidence in literature is lacking and . experimentation to test whether these are viable methods of propagation for this other taxa would be worthwhile. For K. paniculata, softwood cuttings should be taken in spring from young vigourous shoots and rooted under mist (Hartmann et al. 2018). According to Dosmann & Whitlow (2006) the success of softwood cuttings can be increased by use of etiolated shoots. Layers should be prepared in the early autumn and can be harvested the following year once well rooted (Nicholson 1884). Root cuttings can also be taken in late winter (Hartmann et al. 2018). K. paniculata is the favoured rootstock for budding/grafting other Koelreuteria species and cultivars (Santamour Jr & Spongberg 1996; Carter 2007; Zahradnictví SAFRO 2012). A simple procedure for micropropagation of K. bipinnata has been successful using nodal stem segments rooting after two weeks on half-strength Murashige and Skoog (MS) medium supplemented with sucrose at 15–20 g/l (Tawfik, Ibrahim & Taha 2020).