Koelreuteria paniculata Laxm.

The Golden-rain Tree, Koelreuteria paniculata, is one of the most attractive ornamental trees to have been introduced to horticulture. In areas with reliably warm summers it forms an elegant, medium-sized tree producing a wonderful parade of yellow flowers during the middle of summer, followed by an exquisite display of lantern-like fruits, much in-keeping with its oriental origin, yellowish-green at first but soon becoming hued with orange and red as they mature. The show of colour continues even further into late autumn as the change in temperature sets the trees ablaze with the foliage turning incredible shades of yellow, orange and red. With such a rich set of attributes its not hard to see why this species has earned such a high reputation as an ornamental tree, yet it is surprising that they aren’t more commonly planted. Not only is K. paniculata of exceptional ornamental value, it is also extremely versatile and can grow in a range of different soil types, coping very well with drought and unaffected by urban pollution, making it well suited to use as a street tree; it is also tolerant of coastal conditions (Gilman & Watson 1993; Dosmann & Whitlow 2006).

Koelreuteria paniculata is native to Central and Eastern China as far north as Manchuria (Liaoning Province), northern Vietnam and the Korean Peninsula; however, there is some doubt as to whether it is truly a native of Korea, as clearly expressed by Meyer (1976). Numerous populations of K. paniculata are present on both the Korean peninsula and across Japan, although in the latter case it is highly likely that it was introduced from China some time ago. The fact that many old specimens in Japan are found planted around buddhist temples and shrines is one likely sign of its introduction for ornamental or religious reasons. It is also strongly believed by a number of Japanese botanists, such as Dr. Hiroo Kanai of the National Science Museum in Tokyo, that it was probably introduced to Japan following the introduction of Buddhism in 552 AD, later becoming naturalised (Meyer 1976). Meyer also points to a very specific description of the origin of this species in Japan written by Carl Ludwig Blume in 1849 stating that it was “first brought to Japan from a monastery in the Thien tai province of Tschekiang by a wandering monk, in the year 1220, and planted in the garden of a monastery in the city of Mijako [Kyoto]” (Blume 1847) although where Blume obtained this information is unknown (Meyer 1976). Whether K. paniculata is truly native to Korea is less clear, although again many can be found growing in the grounds of Buddhist temples and some populations resemble plantations originally managed by humans. Numerous populations are also found growing in coastal districts of Korea which is quite intriguing; perhaps having dispersed from nearby China across the Yellow Sea? (Dosmann, Whitlow & Ho-Duck 2006). The proximity of North Korean populations to those in Southern Manchuria (Liaoning Province) indicate that its status as a Korean native tree is certainly a possibility and that perhaps a mixture of native and cultivated populations can be found.

In the wild this species can be found growing in dry mountainous river valleys, open forests, rocky mountain slopes and in well-drained riparian soils (Meyer 1976). Although Meyer (1976) stated that Koelreuteria paniculata is found ‘from near sea level to 300 m’ in elevation, after examination of available herbarium specimens this is clearly a misprint and should read up to just over 3000 m, above which altitude George Forrest made a number of collections in 1922, including Forrest 22109 and 21330.

Koelreuteria paniculata was first introduced to Europe by Pierre d’Incarville in 1747 who sent seed of the then-unknown genus from China to St Petersburg where it flowered for the first time in 1771, after which the genus and species were described for the first time the following year (Laxmann 1772; Meyer 1976). The species was apparently first introduced to the UK by the 6th Earl of Coventry in 1763 and cultivated at Croome Court, as recorded by William Aiton (1789), seeds of which are thought to have been obtained from Bernard de Jussieu in Paris or indirectly via contacts at the Royal Botanic Gardens, Kew (Bean 1981; Meyer 1976; Stone et al. 2015). According to Meyer, K. paniculata has been in continuous cultivation at Kew since 1788 (Meyer 1976), with an earlier specimen identified that can be dated back to 1780 which was collected by a Mr Norman of Bromley, Kent. In France this species is likely to have been cultivated since the mid-1700s since D’Incarville had sent seeds to Bernard de Jussieu around the year 1750. The earliest recorded specimen in France was collected by Jean-Baptiste Lamarck and dates back to 1786. Carl Ludwig Willdenow records the species as being cultivated as early as 1796 in Germany (Willdenow 1796) and in the United States it is known to have been cultivated since 1809 when it was recorded by Thomas Jefferson who was sent seeds by Madame de Tesse of France (Meyer 1976).

One of the most notable specimens growing in the UK can be found in the grounds of Syon Park, London. It is the UK and Ireland champion tree for height and girth, at 17 m tall × 81 cm dbh in 2023, 3 m taller than the next tallest known which grows in Abbey Park, Leicester (Tree Register 2024). When in flower, the Syon tree’s golden crown can be seen clearly from outside the gardens, standing proudly next to the glass dome of the Great Conservatory (pictured below). Many other notable specimens are in southern Britain where the summers are warm enough for the species to form substantial trees, but lack of reliable summer heat is clearly limiting. The largest example in Scotland, growing at the Royal Botanic Garden Edinburgh, is an old tree raised from Rock 14994; just 5 m × 41 cm dbh in 2015, despite its age (Tree Register 2024). The Syon tree is on par with some of the largest growing elsewhere, including a tree in the garden at Vigneaux, Bagnères-de-Bigorre, France, 18 m × 78 cm dbh in 2018, and one in the churchyard in Cösitz, Zörbig, Germany, 12 m × 68 cm in 2019.

In China Koelreuteria paniculata enjoys great economic and cultural importance with a wide range of uses. Various parts of the plant have been used in China for religious ceremonies and the species can often be found planted around temples, palaces and in gardens. It was traditionally planted on the graves of high ministers, a relatively high position historically within Chinese society (Meyer 1976; Wang et al. 2013). The species has been used in Chinese Traditional Medicine for the treatment of eye related diseases (Kumar et al. 2011) and recent research has revealed antitumor and antibacterial properties discovered in dry ethanol extracts from the flowers, leaves and stems, with the potential to treat a wide variety of health problems including alzheimer’s disease and cancer. The biologically active substances found within the species also have the potential to be used in food production, providing natural preservatives to extend food shelf-life (Lu et al. 2019; Andonova et al. 2021). In China the flowers of K. paniculata are highly regarded not just for their beauty but as a valuable source of nectar for bees and the buds have traditionally been used as a food source (Lu et al. 2019). The wood is valued for making furniture and as a source of fuel, and dyes can be obtained different parts of the plant with the leaves producing a black dye and the flowers producing a yellow dye. The attractive bead-like seeds are also used to make rosaries for religious purposes and contain oils traditionally used as a lubricant and for soap. An interesting study by Tomić et al. (2020) has shown that the oil-rich seeds of K. paniculata could be used as an effective feedstock for biodiesel production.

It has long been known from experience in the wild and in cultivation that Koelreuteria paniculata is very tolerant of poor soils and of drought conditions; however, a recent study has revealed that K. paniculata is also able to grow in soils contaminated with a range of heavy metals such as Zinc, Manganese, Nickel and Cadmium, and is able to act effectively as an accumulator playing a potentially important role in areas where phytoremediation is required not only by helping to decontaminate soils but also to beautify abandoned mines and other such facilities (Dalun et al. 2009).

Despite its many merits, Koelreuteria paniculata may be viewed as a weed in many areas of Europe and North America where it has become invasive, requiring careful management. In the US this species is currently listed on the National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant list (Andonova et al. 2021; NC State University 2024). In Europe this species has been recorded as seeding abundantly with emerging saplings damaging hardstanding and walls and in areas where saplings are not removed they can cause significant problems and are able reach maturity reasonably quickly, setting further seed. It is a consequence of the species’ ability to tolerate such poor, drought prone growing conditions that it has become a problem in some such areas (Stace 2010; Verloove 2018).

For propagation protocols see the genus article.

The vernacular name Pride of India has long been associated with this tree, despite its absence from that country, but in the course of research for TSO we have been unable to discover when or why this name was bestowed. The earliest record found dates to 1948, demonstrating its use at least in the UK by the mid-twentieth century (Makins 1948 – second edition: the first edition of 1936 has not been seen. If any reader has this available, please check!).