Fertilizing
Feed every two weeks from spring until autumn with a balanced liquid fertilizer, or monthly with slow-release organic pellets. Always water first, so the fertilizer cannot burn dry roots.
Stop feeding in winter while the tree rests outdoors. A tree kept indoors that keeps growing through the winter can have a weak feed once a month.
Placement
Keep your Chinese elm outdoors in full sun from spring to autumn; in very hot climates, morning sun with light afternoon shade suits it best. Outdoors it grows compact, with small leaves and good ramification.
In winter, leave it outside but protect the roots once temperatures fall below about -5°C (23°F), for example in an unheated greenhouse, shed or cold frame. Trees that have spent a long time indoors are less hardy and should be kept frost-free in their first winter outside. If you keep it inside, choose the sunniest window and a cool room, ideally 5–15°C (41–59°F) in winter, away from radiators.
Pruning
Chinese elms grow vigorously and can be trimmed throughout the growing season. Let new shoots grow 3–4 leaves, then cut them back to 1–2 leaves.
- Prune larger branches for structure in late winter or early spring, before growth starts.
- Remove dead, crossing and inward-growing branches.
- Wire from spring to autumn, but check often, as the fast-thickening branches scar easily.
The species suits the broom style as well as informal upright and umbrella-shaped crowns.
Repotting
Repot every 2 years in early spring, just before the buds open; older trees every 3–5 years. Chinese elms tolerate root pruning well, so you can remove up to about a third of the root mass.
Use a free-draining bonsai mix, such as akadama with pumice and lava rock. After repotting, water thoroughly and keep the tree out of strong sun and wind for a few weeks while new roots establish.
Watering
Water thoroughly as soon as the topsoil feels slightly dry, but never let the soil dry out completely. Check daily; in summer, trees in small pots may need water every day.
Water less in winter, especially while the tree is leafless. Avoid permanently soggy soil, which leads to root rot, and remember that misting is no substitute for watering.
Seasonal care
- Spring: Repot before the buds open if needed and prune larger branches for structure. Start feeding as soon as the new leaves unfold.
- Summer: Trim new shoots back to 1–2 leaves once they have 3–4, and water daily in hot weather. Check any wire regularly, because branches thicken fast.
- Autumn: Keep feeding until mid-autumn, then stop, and reduce watering as growth slows. Yellowing and partial leaf drop are normal now.
- Winter: Let the tree rest outdoors and protect the roots below about -5°C (23°F). Trees kept indoors need the sunniest window, a cool room and less water.
Common problems
The Chinese elm is robust, and most problems come from too little light or the wrong amount of water.
- Leaf drop indoors: Too little light, warm dry air or irregular watering. Move the tree outdoors, or to the sunniest window in a cooler room, and it will releaf.
- Aphids and scale insects: Sticky leaves, curled shoot tips or small brown bumps on the twigs. Rinse off aphids, scrape off scale and treat with insecticidal soap or horticultural oil.
- Leaf spot (black spot): Dark spots on the leaves, mostly in damp spring or autumn weather. Remove affected leaves, keep the foliage dry and improve air circulation; use a fungicide if it spreads.
- Root rot: Yellowing leaves and dieback in constantly wet soil. Repot into a free-draining mix and water only when the topsoil starts to dry.
- Elm leaf beetles and caterpillars: Holes in the leaves or skeletonized foliage outdoors in summer. Pick off the pests by hand and remove eggs from leaf undersides.