Rockspray cotoneaster bonsai

Placement

Rockspray cotoneaster is an outdoor tree. During the growing season, give it full sun: that is where it flowers and fruits best. On the hottest summer days, though, give it partial shade in the afternoon. The species prefers cool climates and struggles more in scorching summers, when it also becomes more prone to spider mites.

In the ground, it is very frost hardy. In a small pot, however, its roots are exposed: protect them from freezing. Wait for the first frost before moving it to shelter, so that it goes fully dormant, and never bring it into a heated room. In a mild climate, simply set the pot on the ground, out of the wind, perhaps wrapped in insulation. Where temperatures often drop below 14 °F, overwinter it in a cold frame or an unheated greenhouse, vented on sunny days so it does not start growing too early.

Soil mix

Cotoneaster is not fussy: a standard, free-draining bonsai mix suits it. Equal parts akadama, pumice and lava rock work well. It tolerates a wide pH range, from acid to alkaline. Sift the akadama to remove the dust, which would eventually clog the pot. If you cannot check the tree often, add a little more akadama to hold more water; in a rainy climate, add more lava rock to improve drainage.

Watering

In summer, cotoneaster drinks a lot. It survives short dry spells; a tree that has lost its leaves from lack of water can even recover and leaf out again. Do not count on that: water as soon as the soil feels slightly dry when you push a finger about 0 in into it. Then soak the whole root ball until water runs out of the drainage holes, and repeat a few minutes later. Overwatering is even more dangerous: it causes root rot. In winter, keep the roots only slightly moist. It is not particular about water quality: tap water is fine.

Fertilizer: throughout the growing season, apply a liquid fertilizer every week, or a solid organic fertilizer on the soil surface every four weeks. Choose a balanced product with enough phosphorus, potassium and micronutrients to support flowers and berries; a fertilizer higher in phosphorus helps flowering. Reduce, then stop feeding at the end of the season.

Pruning

Cotoneaster tolerates constant trimming. Throughout the growing season, regularly cut back young shoots that grow beyond the intended outline: this repeated trimming builds dense ramification and keeps the tree compact. Older branches are best pruned in spring. Take inspiration from its natural habit, spreading horizontally, with branchlets arranged like a fishbone.

For structural pruning, do not remove more than a third of the foliage at once, and do not combine heavy pruning with repotting in the same year. A concave cutter leaves a hollow wound that closes better; seal large cuts with cut paste.

The species is especially well suited to small bonsai sizes, shohin, kifu or chuhin. Good starting material can often be found in gardens and garden centers.

Wiring

Cotoneaster can be wired at any time of year. Young branches are flexible; older ones become stiff and can snap if bent hard. For those, use a guy wire instead: a thin wire anchored to a sturdy point, such as a thick root, another branch or the pot, with a sleeve to protect the bark.

Choose wire about a third as thick as the branch, preferably aluminum, which suits broadleaf trees and is easier to handle, and wind it at 45° from base to tip. Bend the branch by pressing your thumb on the inside of the curve, just once. It takes a few months to hold its new position. During the growing season, check often: wire can bite into the bark within one to four months. Remove it by cutting it at every turn.

Repotting

Repot young trees every year and older trees every two or three years, in early spring as the buds swell. The roots tolerate heavy pruning. Tease out the old soil with a chopstick, shorten roots that are too long, cover the drainage holes with mesh, anchor the tree with tie-down wires and work soil into the gaps. Water thoroughly, then keep the tree out of full sun, wind and temperature swings while it recovers, with no fertilizer and no heavy pruning.

Propagation: seed in spring, cuttings in midsummer, or air layering. Birds readily spread the seeds: pull up volunteer seedlings in the garden.

Pests and diseases: aphids, scale, caterpillars, borers, lace bugs, cotoneaster webworm and spider mites, especially in hot, dry weather; among diseases, powdery mildew, leaf spots, cankers, bacterial blight, root rot and above all fire blight. Rockspray cotoneaster is less prone to it than many others, but stay alert: wilting flowers, shoots that shrivel and die, reddish-brown staining in the wood under the bark. Cut well below the affected area, disinfect your tool between cuts, burn the debris and remove late flowers; there is no chemical control. The same symptom can have several causes: a dead twig can also come from lack of water or wire that is too tight. The ornamental berries are not edible: eaten, they can cause stomach upset.