Fertilizing
Start feeding once the first leaves have fully opened in spring and continue until late summer, then stop so the shoots can harden before winter. Slow-release organic fertilizer gives steady growth and supplies the trace elements maples need.
- Solid organic fertilizer: every 4–6 weeks during the growing season.
- Liquid fertilizer: every 2 weeks at the dose on the label.
- Use strong mineral fertilizers sparingly on refined trees, as they push long shoots and large leaves.
- Do not feed newly repotted trees for about 4 weeks.
Placement
Keep the tree outdoors all year in full sun or light partial shade. In hot summers, shade the pot from the midday sun so the roots do not overheat and the leaves do not scorch.
The species is extremely hardy in the ground, but roots in a pot are more exposed. Protect the root ball below -10°C (14°F) in an unheated greenhouse or cold frame, or by sinking the pot into the ground, and shield it from drying winter winds. Never overwinter it indoors: it needs the cold to go dormant.
Pruning
Amur maple tolerates pruning very well and buds back readily on old wood. Do structural pruning in late winter or early spring before bud break, and trim new growth from spring to midsummer.
- Let shoots grow freely when you want a branch or the trunk to thicken.
- For refinement, cut new shoots back to 1–2 leaf pairs once they have 4–5 pairs.
- On healthy trees, partial or full leaf pruning in early summer reduces leaf size and increases ramification.
- Stop pruning by late summer so the last shoots can harden before frost.
Wire young shoots in spring or early summer with aluminium wire. The smooth bark marks easily, so check the wire often and remove it as soon as the branch has set, usually after 4–6 weeks.
Repotting
Repot every 2–3 years in early spring, as the buds begin to swell; older trees can wait 3–5 years. Use a free-draining bonsai mix such as akadama with pumice or lava rock.
The vigorous root system tolerates firm root pruning: remove up to about a third of the roots, a little more on vigorous young trees. New roots form quickly, which makes this species ideal for developing a broad nebari. Keep the freshly repotted tree out of strong wind and frost until it has leafed out.
Watering
Amur maple grows vigorously and uses a lot of water in summer. Water thoroughly as soon as the surface of the soil starts to dry, which can mean daily in hot, sunny weather.
It is more drought-tolerant and less prone to leaf-tip burn than Japanese maple, but do not let the root ball dry out completely. In winter, water only when the soil is drying, and never leave the roots waterlogged.
Seasonal care
- Spring: Repot as the buds swell and do structural pruning before bud break. Start feeding once the leaves are fully open.
- Summer: Shade the pot from the midday sun and water as soon as the surface dries. Trim new shoots, and leaf-prune healthy trees in early summer if you want finer twigs.
- Autumn: Stop feeding and enjoy the scarlet autumn colour. After leaf fall, check the wire and remove any that is starting to bite.
- Winter: Keep the tree outdoors and dormant, protecting the roots below -10°C (14°F). Water only when the soil is drying.
Common problems
Amur maple is tough, but a few problems come up regularly.
- Aphids: Clusters on soft new shoots in spring. Rinse them off with water or use insecticidal soap.
- Powdery mildew and tar spot: A white coating or black blotches on the leaves in humid, still weather. Improve air circulation, clear away fallen leaves and avoid wetting the foliage in the evening.
- Leaf scorch: Brown, crispy leaf edges after the root ball dried out or in hot afternoon sun. Water more consistently and shade the pot during heat waves.
- Wire scars: The smooth bark marks within weeks. Check wire often and remove it as soon as the branch holds its position.
- Sudden dieback of a branch: Can be verticillium wilt, a soil-borne fungus. Cut back to healthy wood, disinfect your tools and do not reuse the old soil.