Kousa dogwood

Identification and origin

Cornus kousa belongs to the dogwood family, Cornaceae. Its native range covers temperate East Asia: north-central, south-central and southeastern China, Inner Mongolia, the eastern Himalaya, Korea, Japan, the Ryukyu Islands and Taiwan.

It is a small, slow-growing deciduous tree: it gains about 10 ft in fifteen years and reaches 19.5–29.5 ft tall at maturity, with a similar spread. Young trees are upright, conical or vase-shaped; with age, the branches spread horizontally in tiers and the crown becomes rounded. The leaves are opposite, ovate to elliptic, with smooth margins, and measure 2–4 in; they are glossy dark green above and paler below, with small tufts of brown hairs where the veins meet.

What look like flowers are actually four pointed, creamy white bracts, 2–3 in long, surrounding a small ball of inconspicuous greenish flowers. They open in late spring or early summer, after the leaves emerge and several weeks after those of the native flowering dogwood (Cornus florida), and they last for several weeks; some turn pinkish as they age. From late summer to early fall come pinkish-red fruits 1–2 in across, held on long stalks and shaped like a raspberry. In fall, the foliage turns reddish-purple to scarlet for three to five weeks. The bark, smooth and light brown on young trees, later peels in patches of tan, brown and gray.

Light

It grows in full sun or partial shade. The ideal site is sheltered, with morning sun and light afternoon shade, especially in regions with hot summers; even so, it handles afternoon sun better than flowering dogwood and does as well in the middle of a lawn as at the edge of a woodland. The more sun it gets, the more intense its fall color.

Watering

Its roots are shallow, so they suffer quickly when the soil dries out. Although it is more drought-tolerant than Cornus florida, it does not like to go short of water. During hot, dry spells, water deeply at the base rather than lightly and often, and check the soil a few inches down before watering again.

On the other hand, it will not survive in waterlogged soil: too much water suffocates the roots just as surely as drought exhausts them. Always check the soil before watering. A layer of mulch 2–3 in deep, spread in a ring 8–10 ft across and kept away from the trunk, keeps the roots cool, reduces evaporation and prevents mower and string-trimmer damage.

Potting mix

It prefers fertile, humus-rich, moist, well-drained soil on the acidic side, and tolerates a pH of roughly 5.0 to 7.4. It adapts to clay, loam or sand and will accept neutral or even alkaline soil, but too high a pH turns the leaves yellow between the veins (chlorosis). Heavy, poorly drained clay encourages root rot.

In spring, after the last frost, a light application of an acid-forming, slow-release azalea-type fertilizer is enough; a second feeding about six weeks later is optional. Do not fertilize a tree that is drought-stressed in summer.

Temperature

It is a hardy tree, grown in USDA zones 5a to 8b, and withstands winters dropping to around -4–5 °F. It is more cold-hardy than flowering dogwood. The evergreen var. angustata is more tender, although it has survived 9 °F in South Carolina.

Repotting

This is a tree for the open ground. Choose a spot large enough for its shallow roots to spread freely, allowing for its mature width. Fall is the best planting season; container-grown trees establish faster than balled-and-burlapped ones. The shallow roots make transplanting tricky: if a move is unavoidable, do it in early spring. Buy nursery-grown plants rather than digging trees from the wild, which often carry diseases and take longer to recover.

Pruning

It needs very little pruning, as its tiered silhouette develops naturally. Simply remove dead wood, damaged branches or limbs that get in the way of a path. Because it blooms on old wood, prune right after flowering so you can enjoy the next season’s bloom.

Propagation

It can be propagated by cuttings, layering, grafting or seed. Cuttings are taken from semi-hardwood or hardwood shoots. Almost all dogwoods sold are propagated vegetatively, which lets them bloom at a young age.

Seed also works: many seedlings sprout under a mature tree, to the point of becoming weedy.

Common cultivars include ‘Satomi’ and Heart Throb (‘Schmred’), with pink bracts; Scarlet Fire (‘Rutpink’), deep pink to fuchsia; ‘Milky Way’, a heavy bloomer; ‘Moonbeam’, with very large flowers; ‘Wolf Eyes’ and ‘Summer Fun’, with white-edged variegated leaves; ‘Gold Star’, with a yellow blotch on each leaf; and ‘Lustgarden Weeping’, with a weeping habit.

Common problems

• Leaves yellow between the veins: soil too alkaline (chlorosis).
• Decline, brown roots: heavy or poorly drained soil, repeated flooding.
• Tree struggling in summer: lack of water during hot, dry spells, to which its shallow roots make it sensitive, or, conversely, soggy soil.
• Diseases: it resists dogwood anthracnose and powdery mildew far better than Cornus florida, without being completely immune; botrytis blight of leaves and bracts, leaf spots, crown canker, bacterial leaf scorch and honey fungus can also occur.
• Pests: it is rarely troubled. Watch for scale insects, leafhoppers, dogwood sawfly larvae and wood-boring insects.
• Fallen fruit: it litters paths and, as it ferments, attracts yellow jackets; plant the tree away from patios if this bothers you.

The same symptom can have several causes: look at the soil, watering, exposure and the condition of the roots before drawing conclusions. It is fairly resistant to deer damage, and its fruit feeds birds and squirrels.

Toxicity

No toxicity has been reported for dogs, cats or people. The fruit is even edible, with slightly mealy, yellow-orange flesh, though it is usually left for the birds. As with any plant, contact a veterinarian if an animal eats an unusual amount of it.