Japanese maple

Acer palmatum is a small tree of the woodland edges and mountain valleys of Japan, Korea and eastern China, where it grows under taller trees in dappled light and damp, well-drained soil. That origin explains almost everything about keeping it anywhere. It does not want full sun through a July afternoon, it does not want its leaves shredded by a dry east wind, and it does not want its roots sitting in water through a wet winter. Give it morning sun, afternoon shade and shelter, and it is one of the most rewarding trees a bonsai grower in this country can own.

It suits bonsai because it does, in miniature, what the big trees do. The leaves are small to begin with and reduce well under pot culture. The twigs ramify finely, so a tree that has been pruned carefully for a few seasons carries a dense, even canopy of fine shoots and reads as a tree even in winter, when there is nothing on it. The bark is smooth and grey, the trunk flares gently at the base, and the whole plant has an elegance that conifers can only approximate. The species itself, the plain green Acer palmatum grown from seed, is usually the best starting point. The named cultivars sold as garden plants are mostly grafted, and a graft union low on the trunk is a lump that never goes away.

Beginners go wrong in three ways. They put the tree in full sun and watch the leaf edges scorch brown in the first warm spell; the tree lives, but the summer is spoiled. They let it dry out once, in May, when a maple in fresh leaf can drink a shallow pot dry between breakfast and tea; the leaves wilt, and the fine shoot tips that would have become next year's ramification die back. And they bring it indoors for the winter to keep it safe, where it either breaks dormancy in January under a radiator or simply desiccates. A Japanese maple is a hardy tree. It needs the cold. What it needs protecting from is the pot freezing solid for days at a time and a wind that dries the twigs while the roots are frozen and cannot replace the water.

Through the year, then: in spring and summer it lives outside in a bright, sheltered spot with shade from about eleven until three. In autumn it can take more sun, which deepens the colour. In winter it stays outside, but in a hard frost the pot goes on the ground against a wall, under a bench, or into an unheated shed or cold frame for the worst of it, out again when the weather softens. A tree that spends the whole winter in a shed will not harden properly and will leaf out early; a few days at a time is the idea, not the whole season.

The maple grows in a flush. Buds swell in late March or April, the leaves open in a week or two of astonishing speed, and the shoots extend until early summer, after which the tree slows and may push a smaller second flush in August if it is strong. It is strongly apically dominant: left alone, the top and the outer tips race away and the inner shoots weaken and die, so the work of keeping a maple is the work of holding that energy back. Pinching the new shoots to the first pair of leaves as they open keeps the internodes short; cutting back the extension in June to one or two pairs of leaves builds the twigging; a partial or full defoliation in June, on a strong tree only, doubles the ramification in a season and gives a smaller second set of leaves. None of that is for a tree that is weak, newly repotted, or in its first year with you.

It responds to wire, but not kindly. The bark marks easily, the branches thicken fast in early summer, and wire left a month too long scars for years. Most growers shape a maple by pruning and by guy wires, and use wire on young shoots in the autumn or early spring when the sap is low, checking it every week. Large cuts heal slowly and should be sealed, and they are best made in late winter, when the tree will bleed least and callus through the spring. Wounds low on the trunk can take several seasons to close.

The seasonal interest is the reason most people keep one. The spring leaves open red or bronze and turn green; the summer canopy is a cool, layered thing that moves in the wind; and in late October, with cool nights and a bit of sun, the leaves go scarlet and orange for a fortnight before they drop. Then the winter silhouette shows what the year's work achieved. A well-ramified maple with its leaves off, on a stand, is as good as bonsai gets in this country. A tree that has spent the year being ignored shows that too, in long bare internodes and a crown of three fat shoots. The difference is not skill so much as attention, little and often, in the right months.

• Informal upright (moyogi): the natural habit of the species: a trunk that moves gently with branches held in layered, open pads
• Broom (hokidachi): fine, even twigging from a short straight trunk shows the winter silhouette at its best
• Slanting (shakan): a trunk that leans with the branches balancing it reads naturally for a tree of woodland edges
• Forest group (yose-ue): several thin trunks of different heights make a grove whose autumn colour is the point
• Cascade (kengai): possible with a young, flexible trunk, though the species would rather grow up than down; a style for an experienced hand

2 parts akadama, 1 part pumice, 1 part lava (by volume), sieved to remove dust; a little more akadama for a tree that dries too fast

Maples want a mix that holds moisture and still drains freely; fine roots build quickly in akadama, and the pumice and lava keep air in the pot through a wet winter.

A deciduous tree takes a glazed pot. For a Japanese maple the usual choice is a shallow oval or a soft-cornered rectangle, about two thirds as long as the tree is tall, and about as deep as the trunk is wide at the base. Those two figures are rules of thumb, not laws; a tree with a wide, flat canopy can take a longer pot, and a young tree in training wants something deeper and plainer so the roots can develop.

Glaze colour follows the leaves. Cream, pale blue, soft green and a muted grey-blue all sit well under green summer foliage and let the autumn reds do the work; a red or brown glaze fights the autumn colour. Avoid high-gloss, loud glazes on a tree this refined. A feminine, rounded form suits the species; a heavy, sharp-cornered unglazed pot does not.

Aphids on the soft spring shoots are the commonest problem, and a pinch between finger and thumb or a jet of water deals with most of them; a soft-soap spray for the rest. Scale insects sometimes appear on the undersides of branches and are picked off. Vine weevil grubs eat the roots of pot plants in many places, and a maple that wilts for no reason, or lifts out of its pot with no roots, has probably been eaten; a nematode drench in late summer and again in spring is the safe treatment.

The leaf problems are mostly not pests. Brown, crisp leaf margins are scorch from sun or wind, not disease. Coral spot, orange pustules on dead wood, means dead branches have been left on the tree; cut back to live wood and seal. Verticillium wilt, where one branch suddenly dies with the leaves still on, is the one serious disease and has no cure; keep the tree strong and do not reuse soil from a tree that has had it.

What every month's line rests on: RHS H6: hardy in the ground (to about -15 to -20 °C); the roots in a shallow pot are less hardy than the tree in the ground, so the pot needs shelter in a hard frost.