Chinese elm bonsai

Placement

Full sun or part shade, ideally with strong morning light. In temperate climates, Chinese elm can stay outdoors all year, winter included, and that is where it grows most vigorously, thanks to the light and airflow. Its frost tolerance depends on where the tree came from: trees from northern China are hardier than those from the south. Depending on how harsh the winter is, it drops its leaves or keeps them until new shoots appear.

It is very often sold as an indoor bonsai. It adapts to life indoors as long as it gets plenty of light: place it at the brightest window in the house. Without enough light, shoots grow long and weak, leaves get larger and growth slows. A common approach is to keep it outdoors during the warm season and move it indoors in winter, to a cool but frost-free room.

Be wary of labels: in Europe, many trees sold as “zelkova” are actually Chinese elms. As it matures, Chinese elm develops bark that flakes in orange patches and has glossier leaves, while zelkova keeps smooth bark and has narrower, more pointed leaves. Siberian elm (Ulmus pumila), another look-alike, has larger, double-toothed leaves and deeply furrowed bark.

In dry air, especially indoors, spider mites and scale settle in easily. Frequent sprays of water help prevent them; if treatment becomes necessary, avoid diluted lime sulfur and systemic insecticides, which can make the tree drop all its leaves. The Chinese elm plant profile covers other diseases and the tree’s hardiness in the ground.

Soil mix

Chinese elm has no special soil needs, but the mix must drain well: a standard bonsai mix is enough. To make your own, combine two parts peat-free potting mix, two parts coarse sand or lime-free grit and one part peat-free, soil-based John Innes No. 3 compost.

Watering

The tree tolerates neither prolonged drought nor constantly soggy soil. Ignore labels that call for watering every day: water by observation. Check the pot daily; as soon as the top layer feels slightly dry, water generously until water runs out of the bottom and the whole root ball is wet. In hot weather that can mean every day, even twice a day in midsummer; in cool weather, much less often. Pot size, soil mix and sun exposure all change the rhythm considerably.

Fertilizer: Chinese elm grows fast and needs plenty of feeding during the growing season. A solid organic fertilizer combined with a balanced liquid fertilizer works well. Another option is to mix a balanced slow-release fertilizer into the soil at repotting, then apply a balanced liquid feed at a low dose every two weeks from spring to fall. No fertilizer is needed during dormancy in a cold winter.

Pruning

The trunk thickens fairly quickly, and dense branching is built through frequent trimming.

• Pinching and maintenance trimming: let each shoot extend three or four nodes, then cut it back to one or two leaves. Light, regular trimming works better than a few heavy cuts.
• Structural pruning: in early spring, before the strong flush of growth. Large branches are best removed in late fall. The tree buds back well from old wood after hard pruning.
• Caution: avoid heavy pruning during periods of weak growth or low light.

Styles: the broom style (hokidachi), whose straight trunk divides at about a third of its height into a rounded crown, suits deciduous trees with fine, abundant branching such as Chinese elm. It is also grown in informal upright and windswept styles, clinging to a rock, or in penjing landscapes that combine rocks, plants and figurines.

Wiring

Chinese elm takes well to both standard wiring and guy wires. Use anodized aluminum wire if possible, which suits deciduous trees better, about one third as thick as the branch and wrapped at roughly 45 degrees. Deciduous trees are easier to wire in late winter, when they have no leaves.

Wire bites into the bark as the branch thickens, which can happen within one to four months: check it regularly and remove it by cutting it at every turn rather than unwinding it. Protect thick branches with damp raffia before bending them; for branches that are too thick or brittle, use a guy wire anchored to a strong root, a sturdy branch or the pot.

Repotting

Repot a young tree every two years, in spring; older, larger trees can go longer between repottings. Elm roots tend to grow crooked and tangled: prune them carefully and precisely to develop an attractive root flare (nebari) at the base of the trunk. The free-draining mix described above works well, and a slow-release fertilizer can be added to it.

Propagation: cuttings are the easiest method and rarely cause problems; seed is possible but less recommended.