Fertilizing
Feed every 2–3 weeks from the start of spring growth until autumn with a balanced fertilizer, organic or liquid. Steady feeding supports the fast growth that builds fine twigs.
- Reduce nitrogen on refined trees in midsummer to keep leaves and internodes small.
- Do not feed a freshly repotted tree for about four weeks.
- Stop feeding when the leaves start to colour in autumn.
Placement
Keep the hackberry outdoors all year in full sun, with good air circulation. Indoors it weakens quickly and loses its leaves.
In very hot weather the pot dries out fast, so give some afternoon shade and check the soil more often. In winter, Chinese and American hackberries are hardy, but protect the potted roots below -5°C (23°F). Celtis africana is less cold-hardy and should be kept frost-free.
Pruning
Hackberries are pruned all season to build their dense ramification. Let new shoots extend to 4–6 leaves, then cut them back to 1–2 leaves.
- Do structural pruning and remove large branches in late winter or early spring, just before the buds open.
- Hackberry sometimes dies back behind a cut, so leave a short stub on big cuts and tidy it later.
- Partial defoliation in early summer helps refine a healthy, mature tree; never do it on a weak tree.
In development, let branches grow freely until they reach the thickness you want before cutting back. Wire young shoots in the growing season and check often, because the bark marks easily.
Repotting
Repot young trees every 1–2 years and mature trees every 3–4 years, in early spring as the buds begin to swell. The roots grow fast and form dense mats of fine roots, so moderate root pruning is well tolerated.
Use a well-draining mix such as akadama with pumice and lava rock. If drainage gets poor between repottings, replace the top layer of soil with fresh substrate.
Watering
Hackberry is thirsty during the growing season but dislikes waterlogged soil. Water thoroughly as soon as the top layer of soil begins to dry, and never let the root ball dry out completely.
In hot weather a well-rooted tree may need watering more than once a day. In winter, keep the soil only slightly moist.
Seasonal care
- Spring: Repot as the buds swell and do structural pruning just before they open. Start feeding once new leaves appear.
- Summer: Water often, trim new shoots back to 1–2 leaves and partially defoliate strong trees if needed. Watch for aphids and spider mites.
- Autumn: Stop feeding as the leaves colour, and enjoy the small fruits that ripen this season. Let the tree drop its leaves and go dormant.
- Winter: Keep the tree outdoors and dormant, protecting the pot below -5°C (23°F) and keeping Celtis africana frost-free. Clean up the branch structure while the tree is bare.
Common problems
Hackberry bonsai are robust, and most problems come from pests or the wrong spot.
- Aphids and scale: They suck sap from new shoots and leave sticky honeydew. Wash them off and treat with insecticidal soap or a suitable insecticide.
- Spider mites: Speckled, dull leaves in hot, dry weather. Rinse the foliage regularly and treat severe infestations with a miticide.
- Branch dieback: Branches can die back after heavy cuts or when the tree is weakened. Leave stubs on big cuts and keep the tree healthy in full sun.
- Leaf drop in summer: Usually the root ball dried out once, or the tree is kept indoors. Water consistently and keep it outside.
- Galls on the leaves: Small bumps on Celtis occidentalis are caused by tiny psyllid insects. They look bad but are harmless, so simply remove the worst leaves.