Fertilizing
Feed from spring until early autumn with solid organic fertilizer every four weeks, or a liquid fertilizer every 1–2 weeks. Pause feeding while the tree is in flower so the blossoms are not shed, then resume once the fruit has set.
- Use a low-nitrogen fertilizer with more phosphorus and potassium while fruit is developing, so the tree puts energy into flowers and fruit rather than long shoots.
- Always apply liquid fertilizer to moist soil to avoid root burn.
- Stop feeding in autumn so new growth hardens off before winter.
Placement
Keep the tree outdoors all year in full sun, at least 6 hours a day, with good air circulation. Less light means few flowers, weak fruit and more fungal disease.
Crabapples are frost-hardy, but roots in a pot are far more exposed than in the ground. Protect the pot when temperatures fall below -10°C (14°F), for example by setting it on the ground in a sheltered spot or in an unheated shed. In hot summers, shade the pot itself so the roots do not overheat.
Pruning
Do structural pruning in late winter, just before the buds swell, when plump flower buds are easy to tell apart from slim leaf buds. Flower buds form on short spurs during summer for the following spring, so avoid cutting those spurs off.
- After flowering, let new shoots grow out, then cut them back to 2–3 leaves once they harden.
- Thin the fruit to one fruit per cluster so the tree is not exhausted.
- Keep the centre open for air flow and remove crossing or inward-growing shoots.
Young branches can be wired in spring or summer; older branches are stiff and brittle, so bend them gradually with guy wires.
Repotting
Repot young trees every 1–2 years and older trees every 2–3 years, in early spring just before bud break. Prune the roots moderately and use a free-draining soil that still holds plenty of moisture, slightly acidic to neutral (pH 6.0–7.0).
Do not repot while the tree is flowering or fruiting, and do not repot a weak tree. Let it recover with good light and care first.
Watering
Apple and crabapple bonsai need a lot of water: check the soil daily and water thoroughly as soon as the surface starts to dry. In summer this can mean daily watering, sometimes twice a day, especially while fruit is developing.
Water stress during flowering or fruiting makes the tree drop its fruit. In winter, keep the soil slightly moist but never waterlogged.
Seasonal care
- Spring: Repot just before bud break, then enjoy the blossom and pause feeding while it lasts. For reliable fruit, hand-pollinate with a brush using pollen from a different Malus variety.
- Summer: Water daily, thin the fruit and cut back hardened shoots to 2–3 leaves. Watch for aphids and mildew.
- Autumn: Stop feeding and enjoy the ripening fruit. Rake up fallen leaves and fruit to reduce fungal disease next year.
- Winter: Keep the tree outdoors so it gets the cold dormancy it needs to flower. Protect the pot below -10°C (14°F) and do structural pruning at the end of winter.
Common problems
Most trouble with apple and crabapple bonsai comes from fungal disease and pests that love its soft new growth.
- Aphids and woolly aphids: They cluster on shoot tips and bark wounds. Wash them off or use an insecticidal soap or a suitable insecticide early.
- Scab and powdery mildew: Dark spots or white powder on the leaves come from damp, stagnant air. Improve sun and air flow, remove fallen leaves and spray a fungicide if needed.
- Rust: Orange spots on the leaves often come from nearby junipers, which host the fungus. Keep the trees apart and remove affected leaves.
- No flowers or fruit: Usually too little sun, no cold winter, or the flower spurs were pruned off. Many varieties also need pollen from another Malus to set fruit.
- Fruit drop: Caused by drought, too much fruit on a small tree, or feeding during flowering. Water consistently and thin the crop.