Serissa bonsai

Placement

Serissa japonica is a subtropical shrub that cannot take frost. In a temperate climate it spends the warm season outdoors, in a sunny spot sheltered from wind, as long as nights stay above 50 °F. Morning sun suits it especially well; shield it from the scorching afternoon sun, which dries out the foliage.

From fall to spring it must come into the house or a heated sunroom, between 50 °F and 68 °F. The warmer the room, the more light it needs: set it right next to a bright window and add a grow light if the days are short. High humidity is good for it, especially when heating dries the air.

Its great weakness is its sensitivity to change. A sudden drop in temperature, a move or a change in light is often enough to make it shed leaves. Choose a stable spot for each season and limit moves to the two that cannot be avoided, bringing it out in spring and in again in fall, making each transition gently.

Soil mix

A standard bonsai mix suits it. It must hold enough water, since serissa cannot stand drying out, and let the excess drain away immediately, since its roots rot in soggy soil. An akadama-based mix lightened with pumice and lava rock meets both needs; sift it to remove the dust. The pot must have drainage holes.

Watering

Keep the root ball evenly moist at all times: it must never dry out completely. Water thoroughly as soon as the surface of the mix starts to dry, until water runs out of the holes, then empty the saucer. Do not water on a fixed schedule: feel the mix, because needs change a great deal between a summer day outdoors and a winter day indoors. Too much water is as dangerous as too little: it causes root rot.

Serissa prefers water with a neutral pH, so avoid very hard water. Outside the flowering periods, mist the foliage often with lime-free water to raise the humidity. If the tree drops its leaves after a change in conditions, keep watering, but a little less, while it recovers.

Fertilizer: during the growing season, give a small amount of solid organic fertilizer every four weeks, or a low-strength liquid fertilizer every week, with a balanced nitrogen-phosphorus-potassium ratio. In winter, if the tree is kept warm and keeps growing, diluted liquid fertilizer once a month is enough. Never feed a serissa that is sick or not growing.

Pruning

Serissa tolerates hard pruning, done in early spring if it is needed. On a young tree in training, let each shoot form four or five leaves, then cut it back to two. This repeated trimming makes the tree branch out and builds the very fine ramification that gives it its value. An older tree is pruned less while it is in bloom, so you can enjoy the flowers, then more firmly once flowering is over. Every two or three years, shorten the branches back to old wood to keep a compact outline.

Prune the top and outer parts of the crown harder, since they are naturally more vigorous, to push strength toward the inside and lower branches. A healthy tree copes with losing about a third of its foliage. For thick branches, use a concave cutter and cover the wound with cut paste. Crushed leaves and wood give off an unpleasant smell: this is normal.

Wiring

Branches and shoots can be wired in any season, but they are fragile: apply the wire very gently and bend gradually. Aluminum wire, soft and easy to handle, works well; choose one about a third as thick as the branch and wind it at about 45°. Remove it after about six months, before it marks the bark, by cutting it at every turn rather than unwinding it.

Its rough gray trunk, fine branching and flowers also make it a favorite in the style of Chinese penjing.

Repotting

Repot every two years, in early spring, in a standard bonsai mix. Prune the roots only moderately, never more than about a third of them. When cut they give off an unpleasant smell, which is no sign of trouble. After repotting, keep the tree in a stable spot, out of harsh sun and wind, and wait for it to recover before any major pruning or feeding.

Serissa is easily propagated from semi-hardwood cuttings; root cuttings also work. Watch for aphids, root aphids, woolly aphids, spider mites, which thrive in dry air, scale and powdery mildew. Against spider mites, raise the humidity and use a suitable product. In case of root rot, unpot the tree, remove the damaged roots and replant it in fresh, free-draining mix, then correct the growing conditions.