Maples used as bonsai are cool-weather trees in a small volume of soil. Everything that goes wrong with one traces back to heat, drought or a room with no movement in it.
Where to keep it
Maple likes a cool position and flinches from hard, direct light, so a bonsai of it is best maintained somewhere shaded and airy rather than in full sun. An indoor shelf is possible but poor: rooms are still, dim and dry, and a maple in one deteriorates quietly. The practical limit for keeping a specimen inside is about a week at a time before it goes back outside. In winter, a cold frame, an unheated porch or a sheltered corner of the garden does better than a warm room. In the milder southern areas the pot can simply be buried in the ground for the cold months and lifted out again in spring, which insulates the roots far better than any tray of bark.
Water and fertiliser
Through the growing season a maple in a shallow pot needs watering often, because the volume dries out fast and a drought-stressed maple scorches its leaf edges within days. When temperatures run high, wet the ground and the bench around the pot as well, so the immediate air stays reasonably moist. In winter, watering drops back sharply; barely moist is the target, and a waterlogged cold tray is the way root loss happens.
Feeding a maple bonsai should not be enthusiastic. Skip it entirely through winter, and one or two applications across the rest of the year is generally plenty. The reason is the foliage: rich feeding produces big coarse leaves and long internodes on a tree whose whole point is refinement. Weak, infrequent food keeps the leaf small and the branchlets close.
Buds, and the summer leaf cut
Maple pushes new buds with real vigour, which is both the pleasure and the chore of the plant. Unwanted shoots coming from the trunk or the base of a branch should be rubbed off while they are small, because a bud left for a month becomes a thickening that has to be cut and then disguised. Prune sprouting back to one or two pairs of leaves where you want the ramification to stay tight.
The best colour on a maple is the first flush of spring leaf and the fresh growth after a summer cut. Shearing the canopy in summer, taking the leaves off by their stems, followed by a light feed, brings on a second set of smaller, cleaner foliage, which is why display material is leaf-pruned before a show. Do it on a strong tree only, and not in the year you have repotted.
Pests, and the two habits that matter
Insects appear on maples often enough that inspecting the underside of the leaves should be part of the watering routine rather than an emergency response. As soon as damage shows, treat it, and treat the plant as a whole; spraying the visible leaves and leaving the rest is how an infestation comes back. Mites and boring pests each need a product aimed at them, so identify what is on the tree before choosing, and repeat at weekly intervals rather than hoping a single application finishes the job.
Open granular mixes give the root system the drainage and the oxygen that a fine compost in a shallow tray cannot. Feeding discipline matters just as much: reading what a plant actually needs before feeding it prevents the coarse, over-fed growth that ruins the shape of the tree.
For a garden specimen rather than a bonsai, the small purple-leaved tree written up under purple-leaf breeding answers the same appetite for colour in a border.