Chinese rain tree is a problem weed in Japan and the USA, where it has naturalised across five states. Due to its rapid growth habit and high seed viability in warmer climates, Chinese rain tree may crowd out native plant populations. It was first recognised as a naturalised environmental weed in the 1990s in the Brisbane City Council area where it has invaded urban parks, bushland, gullies and streets.
Chinese rain tree has naturalised in Brisbane and near Noosa in Queensland and near Lismore in NSW. Although only in the early stages of establishment, this weed has the potential to seriously degrade Australia’s ecosystems.
Chinese rain tree reproduces by seed, which can germinate within 6–8 days. Recorded infestations of the plant in Australia may be traced back to escapees from gardens, urban parks and street trees. It is not known exactly how the seeds are dispersed, but it is possible that water may contribute.
Chinese rain tree is a small, deciduous tree. It has been recorded in Australia as growing to approximately 5 m tall, and up to 18 m under ideal conditions and may achieve a spread of 10-15 m when mature.
It has compound leaves (ie made up of several distinct leaflets) that are dark green above and paler green below and of variable size and shape. The leaves are hairless, often appear feathery and measure up to 90 mm in length. The leaflets are narrow, ovate (pointed at one end and rounded at the other), and have irregularly toothed edges and a long, tapering point. Each leaflet is approximately 90 mm in length.
Flowers are small, to 20 mm in length, and occur in branched clusters at the stem tips. They are butter-yellow with five petals that vary in length until opening. Each flower contains seven to eight pale yellow stamens with hairy white filaments.
The fruit/seed pod is an inflated papery capsule that splits into three parts and is light pink to deep rose in colour. Fruits are up to 50 mm long and appear in large drooping clusters. The seeds are small, black and round and about 5 mm in diameter.
The trunk is light brown and smooth when young, but may develop ridges later.
Chinese rain tree is native to Taiwan and thrives in temperate climates. It tolerates full sun, partial shade, drought, frost, heat, well drained to wet soils, extended flooding and air pollution. It grows on a wide range of soil types and may be tolerant of light salt spray, but not of saline conditions.
CRC for Australian Weed Management: Hillary Cherry (University of Florida, Center for Aquatic and Invasive Plants), Rod Wood (Brisbane City Council, Environment Department), John Hosking (NSW DPI).
Because there are relatively few Chinese rain tree infestations, and it can potentially be eradicated before it becomes established, any new outbreaks should be reported immediately to your local council's weed officer. Do not try to control it without their expert assistance. Control effort that is poorly performed or not followed up can actually help spread the weed and worsen the problem.
Chinese rain tree (Koelreuteria elegans subsp. formosana) is not declared in NSW under the Noxious Weeds Act 1993.