Chapter 102 Koelreuteria spp.
102.1 Chinese lantern tree, golden rain tree
An ~30 year old Koelreuteria, showing how quickly they mature, and the overall growth pattern. Link to original image.
102.2 Soil: General Deciduous / Rich Conifer Mix
Ratio: 3 parts bark, 1 part grit
Add-ins: dolomitic limestone (~5 lbs/yd), Tree-Tone (15 lbs/yd)
pH: ~6.5 (slightly below neutral)
Moisture: Average
102.3 Light
Routine: full sun.
Summer: unknown whether it needs shade.
102.4 Cultivation Notes
These trees have multi-season highlights: flowers in early summer, lanterns in late summer that persist into winter, and colorful fall foliage.
Crown of a Koelreuteria in bloom. Link to original image.
Close-up view of flowers on one of the improved cultivars. Link to original image.
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102.4.1 Potting & Root Pruning
Schedule: uncertain. Avoid fall.
Notes: golden rain-tree’s root system is coarse, with fewer, larger roots. Be careful not to over-trim the roots.
102.4.1.1 Top & Branch Management
Bark on a mature Chinese lantern tree. Link to original image.
Heavy pruning: prune in winter when the tree is dormant.
A young golden rain tree is naturally thin with uneven growth. Rain-trees should be grown with a single leader. After establishing the scaffold, the only heavy pruning that should be done is removing any branches that cross other or are dead.
Cut the branches at the branch collar. Do not cut the branches flush with the tree because it will delay the healing process.
Fine pruning: the tree naturally develops into a rounded even shape if planted in full sun with plenty of room to spread out. Occasionally, smaller limbs inside the tree canopy will die out. Prune to keep the inside space more open and to maintain a neat appearance.
Leaf pruning: not appropriate for this species.
Wiring: golden rain-tree bark is thin and easily damaged. Check wires regularly. Limbs droop as the tree grows, so they will require pruning for vehicular or pedestrian clearance beneath the canopy.
102.4.2 Feeding
Young seedlings need extra phosphorus to encourage good root development. Look for a fertilizer that has extra phosphorus in it. Switch to balanced food mix for maintenance.
102.4.3 Pests & Problems
No serious insect or disease problems. Some susceptibility to root rot, leaf spot and canker.
102.4.4 Winter Storage
Fall foliage showing the leaf morphology, and the range of colors they turn, from yellow to deep red. Link to original image.
Sstore in mulch outdoors. Try to place in a shaded area so buds do not break in a mid-winter warm spell. No other special precautions are needed.
102.5 Propagation Notes
Partially mature seedpods. The pods are green when first inflating, but turn yellow, pink-brown, orange, then dry to brown. Link to original image.
I have tried to grow Koelrueteria from collected and commercially purchased seed, and despite the warnings about it being invasive, the seeds are exceptionally hard to get to germinate. The seeds have both a physical and second physiological dormancy barrier. This usually means the species has adapted to conditions where its seeds must survive a long time before they find the right conditions. While good for the species, it makes propagation difficult.
Protocols I have read for breaking seed dormancy include soaking them in sulfuric acid, sanding off the outer coat, multiple freeze-thaw cycles, and others. In the end I gave up and purchased liner trees from a nursery supplier. Unlike the seed, the liners proved quite hardy.
102.6 Artistic & Styling Notes
102.7 Additional Information
102.7.1 Natural History
Called golden rain tree or Chinese lantern, they are native to Northern China, Korea, Japan. Adapted to USDA Zones 5 to 9, they can reach a height and spread of 40 feet. They tolerate a variety of conditions and are low maintenance in general.
Left alone, it is a small, open-branching, irregularly-shaped, deciduous tree with a rounded crown. The feathery, compound leaves have 7-17 irregularly lobed leaflets.