Rockspray cotoneaster

Identification and origin

Cotoneaster horizontalis belongs to the rose family, Rosaceae, like apples and hawthorns. It is native to central and southern China and to Taiwan.

It is a low, dense, slow-growing shrub that usually reaches 2–3 ft tall and 5–8 ft wide. Its most recognizable trait is the way its branches are arranged: stiff, flattened and tiered, they carry side branchlets alternating on each side, like the bones of a fish. Young stems, green to red-brown, are hairy at first and become smooth with age.

The leaves, round to elliptic, are barely 0 in long, glossy and dark green. The shrub is deciduous in cold regions and semi-evergreen to evergreen in mild climates. In fall, the foliage turns red, orange or reddish purple.

In late spring, small flowers about 0 in across appear, with five white petals tinged pink, borne singly or in pairs along the branches; bees visit them eagerly. They are followed by small bright red fruits about 0 in across that ripen from late summer into fall, hang on for a long time and feed the birds.

The flip side of all that fruit is that birds spread the seeds. In Great Britain, the species is listed as an invasive non-native plant, and it is an offense to plant it or allow it to grow in the wild there. Pull up stray seedlings regularly.

Light

It grows in full sun or part shade, but flowers most freely and sets the most berries in full sun. In shade, it stays sparser and less ornamental.

It does prefer cooler climates, though: in very hot summers it struggles and becomes more prone to spider mites.

Watering

Water thoroughly at planting, then during dry spells for the first two years while the roots establish. Aim for soil that stays moist but never soggy.

Once established, rockspray cotoneaster tolerates drought well and needs extra water only during prolonged dry spells.

In containers, the mix dries out much faster: check it every day during hot, dry weather by pushing a finger into the soil. In winter, its needs drop sharply, but check from time to time that the root ball has not dried out completely.

Potting mix

It likes well-drained, loose, moist, loamy soil, but it tolerates poor, clay, sandy or chalky soils, and it is almost indifferent to pH, acidic or alkaline. Only very wet or waterlogged soils do not suit it. It also stands up to wind and urban pollution.

In very poor or very heavy soil, dig in a bucketful of organic matter per square meter. Each late winter or early spring, when the soil is moist, a mulch of compost or well-rotted organic matter 2–3 in deep is all it needs; leave a clear circle of about 4 in around the stems so they do not rot.

In containers, use a peat-free, soil-based potting mix.

Temperature

Very cold hardy, it suits USDA zones 5 to 7 and comes through severe winters, below -4 °F, without protection in the ground.

Heat is its real limit: south of zone 7, hot summers wear it down.

Repotting

Plant ideally in early to mid fall or in spring, when warm, moist soil encourages rooting; winter planting is possible if the ground is neither frozen nor waterlogged. Against a wall or fence, set the root ball at least 18 in out from the base so rain can reach the roots.

In containers, choose one at least 12 in wide and deep. Repot every three to five years in spring, as the plant resumes growth; avoid doing it late in the season, because the excess mix would stay too wet over winter. In other years, replace the top 1–1 in of mix with fresh compost in spring and give a liquid feed once a month during the growing season.

Pruning

It shapes itself and needs little pruning. If needed, prune in spring or after flowering, removing overlong shoots to keep the shrub in bounds and preserve its tiered outline. Before pruning, check for nesting birds.

Against a wall, train it on wires or a trellis: tie in branches where you want growth and remove stray shoots growing outward or back toward the wall. After that, a light clipping in spring and removing overlong shoots in summer are enough.

An old, overgrown plant can be renovated with hard pruning in early spring, but it will not flower the following year. Avoid hard pruning in late summer, which removes wood that would have flowered the next spring.

Propagation

Semi-ripe cuttings are taken in mid to late summer from this season’s shoots, when the base is firm and the tip is still soft.

Layering is very easy with this low-branched species: bend a branch down to the soil, make a small cut at a leaf bud, bury the wounded section and pin it down, keeping the soil moist. After a few months, once roots have formed, sever the layer from the parent and replant it.

Seeds need a warm period followed by a cold one before they germinate: sow them in fall in trays placed outdoors in a cold frame, and be patient, since germination takes anywhere from a few weeks to a few months. Self-sown seedlings can be transplanted at 6 in tall, but cotoneasters hybridize easily, so seed-grown plants might not match the parent.

As for cultivars, ‘Robusta’ stands out for its distinct fishbone pattern and heavy berry crop, while ‘Variegatus’ has variegated foliage.

Common problems

• Branches that suddenly turn brown and die back: fire blight.
• Pale, stippled leaves: lace bugs or spider mites, the latter especially in hot, dry sites.
• Webbing and chewed leaves: cotoneaster webworm caterpillars.
• White, cottony tufts on branches: woolly aphids; small brown shells: scale insects.
• Spotted leaves, sunken wounds on stems: leaf spot diseases and cankers.
• Whole-plant decline: honey fungus root rot.

A symptom can have several causes, and soggy soil or excessive heat weakens the plant: look over all its growing conditions before treating anything.

Toxicity

The berries are ornamental and must not be eaten, by people or by pets: in quantity they are toxic and cause stomach upset. Keep them out of reach of young children. Wearing gloves is advised when handling the plant. If berries are swallowed, contact a poison control center or a veterinarian.