Cercidiphyllum japonicum Seeds | Katsura Tree
LATIN NAME: Cercidiphyllum japonicum
COMMON NAME: Katsura Tree
Plant Profile
In autumn a katsura does something no other common tree does, and it does it with litter rather than with flowers. The fallen leaves smell of brown sugar and burnt toffee, and the scent is strong enough to stop people on a path while they work out where it comes from. Nothing else in the temperate garden produces that warm, sweet, faintly scorched note, and it arrives not from a blossom but from dead foliage lying on damp ground.
The tree grows wild in two widely separated regions, the Japanese islands and the hill forests of central and western China. Japanese trees are the giants of the species and can reach thirty metres in sheltered valley forest; the Chinese population is generally smaller and more scattered. Fossil leaves close to the genus turn up right across the Northern Hemisphere from the Cretaceous onward, which is what earns katsura the label of a relict tree rather than merely an old one.
Katsura carries two kinds of shoot, and the difference explains the whole look of the tree. Long shoots extend the framework and carry opposite, heart-shaped, blue-green leaves with finely scalloped edges. Short spur shoots, packed along the older wood, carry a tighter set of smaller, rounder leaves that colour first and best. The young growth of a tree in its first years flushes red-purple, a screen of anthocyanin that protects tender tissue from strong light.
Only the female trees fruit, and they fruit on the same short spurs that carry the foliage: clusters of two to four curved pods that swell through summer, turn brown, and split along one side to release a scatter of tiny winged seeds. The seed is fine, light and short-lived, and it will not tolerate being dried out and stored on a shelf, which is the most important thing to know about handling katsura.
Science & Biotechnology
The molecule responsible is maltol, a small pyranone already familiar from the crust of toasted bread and from roasted malt. In senescing katsura tissue its concentration rises as the leaf dismantles itself, and the compound escapes readily with the moisture leaving the dying blade, which is why the scent is strongest on a still, damp morning after rain. Headspace profiling of autumn litter puts the burnt-sugar note at the top of the volatile list, with green leafy aldehydes giving the fragrance a slightly sappy edge.
Research on the species clusters around two problems. The first is the autumn volatiles, where the formation of maltol during leaf senescence has been followed through the season in an attempt to link it to the breakdown of starch in the dying blade. The second is propagation, because selected clones of a dioecious tree can only be multiplied vegetatively. Nodal culture of katsura is routine in woody-plant laboratories, with axillary shoots proliferating on a cytokinin-rich medium and rooting after an auxin dip, and somatic embryogenesis has been reported from juvenile tissue.
Uses & Benefits
Katsura is dioecious: male and female flowers sit on separate trees, and neither carries petals. Both sexes flower on the naked wood of the previous season’s spurs, before the leaves are fully open, and both are pollinated by wind rather than by insects. The genus has been on its own evolutionary side branch since the Cretaceous, and the pairing of wind pollination with unisexual flowers and a single relict genus is a fair summary of how a survivor looks after the rest of its relatives have gone.
Katsura wood is pale, fine-textured, easy to work and stable once dry, and Japanese cabinetmakers used it for the fitted chests that are now collectors’ pieces, as well as for joinery, internal doors, veneer and carving. It does not carry the durability of oak or larch, so it belongs indoors, in furniture and fittings rather than in ground contact. In the garden the tree is grown as a specimen on a lawn or at the woodland edge, where the layered crown can be seen from below.
Growing from Seed
Sow the seed the moment it is shed, or as soon after harvest as you can manage, on the surface of a moist mix of two parts peat-free compost and one part coarse sand, pressing it in and covering with barely two millimetres of grit. Fresh seed wants cool, damp, shaded conditions and a cold spell outdoors in a frame. Prick the seedlings out when the first true pair of heart-shaped leaves unfolds, pot them on singly at the four-leaf stage, and keep them shaded through their first summers. Plant out at fifty centimetres to a metre, six metres apart for specimens, in deep moisture-retentive soil with shelter from drying wind.
Our Take
Katsura has no close relative in ordinary cultivation, so the best companions are trees that share its appetite for cool roots and damp air. The Chinese spur-leaf tree is the botanical neighbour worth knowing, and Tetracentron sinense grows under the same conditions. For the autumn display on heavier soil, sweetgum is the obvious partner, since Liquidambar styraciflua colours over a longer window and will not resent the moisture. Where the ground is drier, plant paperbark maple instead, because Acer griseum gives the same seasonal show from a smaller crown.
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