Cornus kousa Seeds | Kousa Dogwood
LATIN NAME: Cornus kousa
COMMON NAME: Kousa Dogwood, Japanese Dogwood
Plant Profile
What looks like a flower on a kousa dogwood is a piece of theatre. The four white blades that draw the eye are bracts, modified leaves that surround a tight central cluster of small greenish flowers with no beauty of their own, and the bracts are there to advertise the whole cluster to insects. Each bract ends in a drawn-out point, which is the quickest way to tell this species from the American flowering dogwood, whose white blades are notched at the tip as though a thumbnail had been pressed into them.
Dogwood anthracnose, a fungal disease that spread through eastern North America from the 1970s, killed flowering dogwoods across whole woodlands, and the search for a resistant replacement turned to Asia. The kousa dogwood resists the fungus far better, so breeders crossed the two species, and the resulting hybrids, selected in New Jersey and released as a named group in the 1990s, combine the disease resistance of the Asian parent with the flowering habit of the American one.
The hybrid selections carry their own names, several of them in a series borrowed from the stars, and they vary in habit and in the depth of the pink that appears in the bracts. Gardeners who want the pure species should look for the Chinese variety, which carries larger bracts than the Japanese form and shows greater resistance to both anthracnose and powdery mildew, a disease that marks the foliage in humid summers.
The natural use for this tree is as a specimen on a lawn or at the edge of woodland, where its layered, horizontal branches can be seen in profile and the fruit falls onto clean grass. It also takes the conditions of a town garden, tolerating air pollution and a restricted root run better than most flowering trees. Give it room: a well-grown tree spreads as wide as it is tall, and the branches sweep low, so underplanting with bulbs and low perennials works better than a shrub layer.
Science & Biotechnology
Unlike the American species, which opens its bracts on bare branches, this dogwood leafs out first and flowers afterwards, so the display arrives later in the season and the tree is in full leaf behind the bracts. That sequence protects the flowers from late frost and is the main reason the species carries a crop more reliably in a cold climate. The leaves are opposite and oval, with a glossy surface, and the veins curve toward the tip in the pattern that runs through the genus, and they colour to crimson and purple in autumn.
The ripe syncarp is a genuine fruit crop in a modest way. It is sweet, low in acid and full of small stones, so it is eaten fresh over the tree, pressed into a juice, or fermented into a country wine in its native range. The stones and the bark are a different matter, being rich in the tannins that run through the family, and the wood is dense and hard enough that it has been used for handles and small turned work. The chemistry that matters most to the plant is defensive: the tannin load in the bark and young shoots makes the foliage unpalatable to most browsing animals.
Uses & Benefits
The fruit is stranger still. Dozens of individual drupes ripen pressed together into one knobbled red head, a syncarp, so the autumn crop looks like a raspberry the size of a golf ball. Each of the fused units holds one hard stone, and the whole structure ripens from green through pink to a deep red as the bracts fall. The flesh is sweet and edible when fully ripe, and it is eaten fresh or fermented in the countries where the tree is native.
Growing from Seed
Clean the pulp from the stones and sow them in autumn, since the fruit ripens late and the stones carry a hard coat that needs a moist cool period to soften. Use a mix of two parts loam-based compost, one part coarse sand and one part leaf mould, sow the stones about fifteen millimetres deep in a deep pot, and stand the pot in a cold frame outdoors through the winter, protected from mice. Some stones will not break dormancy in the first spring, so keep the pot for a second season. Seedlings grow unevenly and a good proportion will lean toward red bracts rather than the deep scarlet of the parent, so pot each one separately once it carries four true leaves.
Space permanent trees at least four metres apart and water them well for their first three summers; after that the tree is reasonably drought-tolerant, though the foliage scorches in a long dry spell on thin soil. Choose the strongest shoot as the leader when the seedling is small and remove competing shoots below it, then allow the crown to build naturally, cutting only dead wood and crossing branches in late winter. Mulch with composted bark over a wide circle rather than banking material against the trunk, and feed lightly in the second spring after planting. The main disease risk is powdery mildew on the foliage in humid weather and, in poorly drained soil, a root rot; the chief pest is the dogwood borer, which attacks weak or sunburned trunks, so keep the bark shaded by allowing low growth to remain for the first seasons.
Our Take
If you are replacing a dogwood lost to disease, plant this one and let it establish slowly in deep, lime-free soil. The American species, Cornus florida, is still worth growing where anthracnose has not arrived, and the giant dogwoods such as Cornus controversa give the same horizontal layering on a much larger frame. Where the flower is wanted in the depths of winter instead, the Japanese cornel, Cornus officinalis, carries small yellow blooms on bare wood well before this tree has woken up.
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