Fertilizing
Feed every two weeks with liquid fertilizer, or monthly with solid organic fertilizer, from the time the new leaves open until late summer. A balanced fertilizer works well; field maples need no special formula.
- On refined trees, use less nitrogen to keep internodes short.
- Do not feed during winter dormancy or a tree that is stressed or freshly repotted.
Placement
Keep the tree outdoors in full sun to partial shade, in an airy spot. In hot summers, afternoon shade protects the leaves from scorching.
- In the ground the field maple is hardy to about -25°C (-13°F), but a potted tree's roots need protection below about -10°C (14°F).
- Give winter protection by burying the pot in mulch, or using a cold frame or unheated shed; never bring it into a warm room, as it needs the cold to stay healthy.
Pruning
This species grows fast and coarse, with long internodes, so it needs regular trimming. During the growing season, let new shoots make four to six pairs of leaves, then cut back to one or two pairs.
- Do structural pruning and remove large branches in late winter, before the buds open.
- Healthy, vigorous trees can be partly or fully defoliated in early summer to reduce leaf size; do not do this every year.
- Wire in spring or summer and remove it within about six weeks, as the bark marks quickly.
Repotting
Repot young trees every two years and mature trees every three to four years, in early spring when the buds swell but before the leaves open. The field maple tolerates root pruning well.
Use a free-draining bonsai mix such as akadama with pumice or lava. The species is not fussy about pH and tolerates neutral to alkaline conditions.
Watering
Water thoroughly as soon as the top layer of soil begins to dry, without letting the root ball dry out. On hot summer days this can mean watering once or twice a day.
- Avoid waterlogged soil, which damages the roots.
- Rainwater is ideal, but this lime-tolerant species also handles hard tap water.
- In winter, water much less but keep the soil slightly moist.
Seasonal care
- Spring: Repot as the buds swell and start feeding once the new leaves open. Wire young branches and protect the tender new leaves from late frost.
- Summer: Trim shoots back to one or two pairs of leaves and water daily, more often in heat. Defoliate healthy trees in early summer if you want smaller leaves, and give afternoon shade if the leaf edges scorch.
- Autumn: Stop feeding at the end of summer and enjoy the yellow autumn color. Remove wire before it cuts in, as the branches thicken quickly.
- Winter: Keep the tree outdoors for its dormancy and protect the roots below about -10°C (14°F). Do structural pruning in late winter, before bud break.
Common problems
The field maple is tough, but a few problems come up regularly.
- Powdery mildew: A white coating on the leaves in warm, humid weather with poor air flow. Improve air circulation, avoid wetting the foliage and treat with a sulphur-based fungicide.
- Aphids: Clusters on new shoots and sticky leaves in spring. Rinse them off with water or use insecticidal soap.
- Leaf scorch: Brown, crispy leaf edges after hot sun, drying wind or a dry root ball. Give afternoon shade in summer and never let the soil dry out.
- Red leaf galls: Small red pimples on the leaves are caused by tiny gall mites. They are harmless and need no treatment.
- Coarse growth: Long internodes and large leaves come from too much nitrogen and too little trimming. Feed less nitrogen and cut back shoots regularly.