Pterocarya rhoifolia Seeds | Japanese Wingnut
LATIN NAME: Pterocarya rhoifolia
COMMON NAME: Japanese Wingnut
Plant Profile
Most broadleaved trees of the temperate wood hide their fruit among the leaves. The Japanese wingnut does the reverse. From midsummer until the leaves fall it carries dozens of narrow, pendulous chains, each strung with small green nutlets, hanging clear of the canopy like a curtain of beads. A tree on a Honshu stream bank can be picked out at a distance by those chains alone, and they are the reason the genus is called Pterocarya, the winged nut.
It is, in other words, a tree for the difficult bottom of a garden or a park: wet in spring, dry in late summer, occasionally flooded, never quite good soil. That tolerance is the whole point of planting it.
Wingnuts separate their sexes on the same tree, and on different wood. The male catkins emerge from lateral buds on wood formed the previous season, and they shed pollen as the new leaves unfold. The female catkins sit at the very tip of the current season’s shoot, so the fruit that follows is carried beyond the foliage instead of inside it. The two phases overlap only partly, which pushes pollen from one tree toward another. Each fertilised flower becomes a small nutlet wearing two papery wings, and the chains lengthen through the summer.
Like its walnut relatives, the wingnut makes juglone, the naphthoquinone that gives the family its reputation for poisoning the ground beneath it. Living tissue stores the harmless glucoside, which is released when cells are damaged, so the effect builds as leaves and fine roots senesce. In practice a stand of wingnut keeps the understorey thin, and tomato, pepper and other sensitive plants sulk within the root run.
The genus matters commercially as a rootstock. Walnut growers in China graft named cultivars onto wingnut stock, which tolerates wet, heavy ground far better than a walnut seedling does, and the same approach has been tested in Europe. A rootstock has to be uniform, so laboratories have worked on clonal systems for the genus: nodal segments taken from young shoots, and somatic embryos from immature tissue, for multiplication and for the earlier stages of genetic work.
Science & Biotechnology
The leaf is compound and large, often between twenty and forty centimetres long, built on a central stalk that carries eleven to nineteen lance-shaped leaflets with finely saw-toothed edges. The stalk itself is smooth and green, and that detail separates the Japanese wingnut from its Chinese cousin, whose leaf stalk carries a papery wing between the leaflets. The epithet rhoifolia records the resemblance of the foliage to that of a sumac.
Uses & Benefits
The timber is pale, of moderate weight and easy to work by hand, with none of the hardness of the related hickories. It has gone mostly into light interior work, plywood, packing and small turned articles, and the offcuts make a charcoal that burns cleanly. Nobody plants the Japanese wingnut for its lumber, but a tree removed from a boundary or a river bank pays for its own removal in useful boards, and while it stands the spreading root plate keeps the bank from slumping after flood. Plant it where the water lies, give it room, and keep it clear of drains, where the questing roots will find moisture.
Growing from Seed
This is a tree of moving ground. It follows gravel bars, the inside of river bends, seepage slopes and the damp floors of mountain valleys, where the water table stays high and the soil is young. Growth is quick on such sites: a seedling that finds an open bank can build a straight trunk and reach the lower canopy of a riverside grove while slower broadleaves are still bushy. Old specimens pass thirty metres, with a columnar trunk that forks high up and a crown of long, drooping shoots.
Two wings make the nutlet a slow, spinning faller, so wind carries it a short way downslope into the nearest gap in the canopy. Water does more: the nutlets float, and spate water lifts them off gravel and tows them to the next open bar, which is why wingnut seedlings appear on freshly scoured banks well below the parents.
Wingnut seed is best handled quickly, because the kernel inside the nutlet loses vigour once it has dried hard. Rub the wings off, then press each nutlet about a centimetre deep into a mix of two parts loamy topsoil, two parts coarse grit and one part leaf-mould, firmed and kept damp but never sodden. Work in rows twenty-five centimetres apart, and shade the frame lightly through the first summer, because young leaves scorch on an open bed.
Keep the nursery bed evenly moist in the first season, then give deep soakings rather than frequent sprinkling, which encourages surface rooting. Train a single leader up a cane and rub out any shoot that competes with it, since the species forks early when left alone. Aphids gather on young shoots and powdery mildew appears in still, humid air; free air through the rows does more good than any spray, and the one serious danger is waterlogged compost, where root-rotting organisms finish a container plant within a season.
Our Take
If you have a damp hollow, a ditch edge or a soggy corner that defeats everything else, this is a tree worth trying. Sow the nutlets the week they come off the tree, keep them damp all winter, and move the strongest seedling to its final place while it is still small enough to lift with a spadeful of soil. Growers who want the smaller Chinese species for the same wet ground should look at Pterocarya stenoptera, which carries the winged leaf stalk and suits a tighter space, while anyone planting the walnut family as a crop will find the practical notes in Juglans regia. The winged-fruit clan has a third and stranger member in the wheel-wingnut Cyclocarya paliurus, whose nuts sit inside a circular disc. Plant all three on wet ground and the family resemblance shows at once.
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