Trident maple

Acer buergerianum comes from the warm, humid lowlands of eastern China and was taken to Japan centuries ago, where it became one of the three or four species the whole art of deciduous bonsai was worked out on. It is a street tree in its own country, a tough, fast, thirsty thing that shrugs off pollution, heat and hard pruning. It is less often seen in gardens than it deserves, which is a pity, because in a pot it is the most forgiving deciduous tree a beginner can buy and the one that will teach them most.

What makes it a bonsai tree is its vigour. A trident thickens its trunk faster than any temperate species in common use, so a seedling grown hard in the ground or a big box for five or six years, chopped and regrown, becomes a trunk with real taper and a flared base in a decade rather than a lifetime. The leaves are small, three-lobed and glossy, and they reduce to the size of a fingernail under pot culture. The bark starts smooth and grey and on an old tree flakes into orange and grey plates. The roots are fleshy and fuse readily, which is why the species is the standard choice for root-over-rock and for the great flat spreading nebari that make a trunk look as if it has stood for a century.

The same vigour is what beginners get wrong. A trident left to grow will put out shoots a metre long, with internodes you could fit a finger between, and all of that energy goes to the top of the tree and the ends of the branches while the inner shoots fade. Keeping one is a matter of cutting back constantly in the growing season and, on a tree that is being refined rather than thickened, pinching the shoots as they open. The second mistake is the roots. Tridents build so much fleshy root that a tree in a bonsai pot can lift itself out in two seasons and become impossible to water; they want repotting far more often than a Japanese maple, every year or two while young. The third is winter: those thick roots are full of water and sugar and can rot or freeze, so the pot needs protection in a hard frost even though the tree above it is perfectly hardy.

Through the year a trident lives outside in full sun. Unlike its Japanese cousin it does not scorch in an ordinary temperate summer, and the sun keeps the internodes short and the leaves small; a trident in shade grows long and coarse. It wants a great deal of water in summer. In winter it stays outside, bare, with the pot on the ground or in a cold frame when the forecast is for several days of hard frost, and back out when it softens. A greenhouse that gets warm on a sunny February day will break its dormancy early and set it up for frost damage, so unheated and shaded is better than warm.

The growth habit is simple. Buds swell in late March, the leaves open in April, and the shoots run until the tree is stopped, either by you or by August. It is strongly apically dominant and back-buds readily on old wood, even from a bare trunk after a hard chop, which is the quality that makes trunk-building possible. Cuts heal fast and large wounds close in a few seasons if they are sealed and the tree is fed. In refinement, pinching the opening shoots to one pair of leaves, cutting back in early summer to one or two pairs, and defoliating a strong tree in June all work well and the tree takes them without complaint. In development, the opposite: let the sacrifice branches run, feed hard, cut back once in late winter.

Wire works on young shoots in late winter or just after leaf drop, but the bark marks and branches thicken so fast that wire must be checked weekly in May and June and is usually off within six to eight weeks. Most of the shaping is done by clip-and-grow: let a shoot extend, cut it back to a bud pointing the way you want, and let the next shoot take the line. It is slower than wire and the result looks like a tree.

The seasons give you glossy green through summer, then an autumn colour that depends on the year and the clone: orange and red in a good year after cool nights, yellow in a wet, mild one. Winter shows the twig structure, and on an older tree the bark. A trident is not as refined as a Japanese maple and does not pretend to be. It is a strong, honest tree that rewards work, and the best first deciduous bonsai there is.

• Informal upright (moyogi): the trunk thickens and tapers quickly, which is what the style needs most
• Broom (hokidachi): fine ramification from a cut trunk builds a dome of twigs in a few seasons
• Root over rock (sekijoju): the thick, grasping roots clasp a stone and fuse to it better than any other species commonly grown
• Forest group (yose-ue): cheap, fast-growing seedlings make a convincing grove quickly
• Slanting (shakan): a leaning trunk with a strong flare at the base is easy to develop from field-grown stock
• Formal upright (chokkan): possible from seed with patience, though the species' energy makes a perfectly straight, tapered trunk harder to hold than it looks

2 parts akadama, 1 part pumice, 1 part lava (by volume); for a tree being thickened in a large training pot, a coarser mix with more lava

Tridents eat and drink hard and build fleshy roots fast; the akadama holds the water they want and breaks down as the roots fill it, which is why they are repotted often.

Glazed, like all deciduous trees, in a shallow oval or rectangle about two thirds the tree's height in length and about as deep as the trunk is wide at the base, which for a developed trident with a heavy base means a surprisingly shallow, wide pot. A tree still being thickened belongs in a large plastic training pot or a wooden box, not a bonsai pot; the pot comes last.

The glossy, slightly coarse foliage takes a stronger glaze than a Japanese maple: a deep blue, a green, an oatmeal or a brown glaze all suit it, and a sharper-cornered rectangle suits a thick masculine trunk. Root-over-rock trees usually go in a shallow tray or on a slab, so the roots on the stone are the picture.

Aphids on the spring shoots, dealt with by hand or with a soft-soap spray, and vine weevil grubs in the roots, dealt with by a nematode drench in late summer and spring. The thick roots are also attractive to woodlice and slugs in a damp pot; keep the surface clean and the pot off the ground on feet.

The tree is otherwise robust. Leaf scorch on a trident is unusual and usually means the pot dried out. A branch that dies back suddenly in summer, with the leaves still attached, should be removed to clean wood and the cut sealed; keep the tree strong and do not reuse its soil elsewhere.

What every month's line rests on: RHS H6: hardy in the ground; the fleshy roots in a pot are more frost-tender than the top, so protect the pot in a hard frost and never let it freeze solid for days.