Koelreuteria bipinnata Seeds | Golden Rain Tree
LATIN NAME: Koelreuteria bipinnata
COMMON NAME: Golden Rain Tree
Plant Profile
The leaflets sit on secondary axes branching from the main leaf stalk, so the leaf is bipinnate rather than merely pinnate, and a single one may be half a metre long. That character separates it in practice from its better-known cousin Koelreuteria paniculata, whose leaves are usually divided once and whose flowers open earlier in the season.
Each capsule has three papery valves, and each valve holds one round black seed. The wall is thin, dry and full of air, and the valves flare as the fruit dries, so the whole structure acts as a small sail. Wind rolls fallen lanterns along the ground until they catch in ditches and verges, which is how the tree spreads beyond where it was planted.
Most yellow-flowered trees in this climate bloom in spring or early summer. This one waits, opening its panicles when little else is in flower, which makes it valuable to bees at a lean point in the season. The capsules colour after the flowers fade, so the display runs from late summer deep into winter.
Science & Biotechnology
Sapindaceae is the soapberry family, and its members are rich in triterpenoid saponins built on aglycones such as hederagenin and oleanolic acid, the same chemistry that makes soapnuts lather. Koelreuteria leaves add flavonoids of the quercetin and kaempferol families, and the pink of the capsule wall comes from anthocyanins laid down in the drying tissue.
The round black seeds are hard-coated and carry a fixed oil in which oleic and linoleic acids dominate, a profile typical of the family. Their more visible use is decorative: drilled and strung, they make beads, and the dry lanterns are sold for winter arrangements.
Because the species varies in capsule colour and hardiness, nurseries propagate chosen plants rather than sowing wild seed. Sapindaceae now has reference genomes for lychee and for soapnut, and those assemblies give breeders anchors for mapping flowering time and capsule colour in Koelreuteria.
Uses & Benefits
Three uses justify the space the tree takes.
The species tolerates poor soil, reflected heat and air pollution, and it transplants readily, all of which made it a street tree in warm cities far from China. Pruning is simple: train a single leader, lift the crown gradually, and remove dead wood in winter. In southern Chinese cities the species lines whole boulevards, and it fills the same role in warm regions of Europe and North America.
Cut the fruiting sprays as the capsules colour but before they shatter, and hang them to dry in an airy room. The seeds inside rattle, and once fully dry they can be extracted, drilled and strung. Neither capsules nor seeds hold their colour in strong sunlight.
Growing from Seed
The seed coat is thick, so break it before sowing by nicking the surface with a file or soaking the seed in warm water; untreated seed can sit for a long time. Sow one and a half to two centimetres deep in any free-draining mix. Seed gathered from a tree in your own district is worth more than stored seed, because the coat hardens further as it dries and old seed is slow to take up water.
Hardiness is the limiting factor away from the subtropics, and a late frost can cut the new growth. Otherwise the tree is seldom troubled; borers and scale appear on stressed plants, and branches break under snow, so plant it where falling limbs do no damage.
Our Take
Grow it beside its once-pinnate relative, goldenrain tree, which flowers earlier and takes colder winters, and compare the two lantern crops in the same autumn. Soapnut shares the family chemistry and a warm-climate habit, and yellowhorn brings soapberry foliage to a stony bank.
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