Full-moon maple

Acer japonicum is the second Japanese maple species and the one most growers don't know exists. The species sits alongside Acer palmatum (entry #1) in the broader "Japanese maple" category but is genuinely distinct — leaves are more rounded with shallower lobes (typically 7–11 lobes versus palmatum's 5–7), the species grows more vigorously, and certain refined cultivars produce some of the most striking maple bonsai available.

Care is essentially identical to Acer palmatum — same watering, feeding, repotting, pruning techniques apply. The substantive differences are in habit and cultivar selection. Where palmatum has hundreds of selected cultivars covering every possible leaf shape, colour, and habit, japonicum has a much smaller cultivar pool but includes two standout selections: 'Aconitifolium' (Maiku-jaku — peacock's tail), with deeply cut filigree-like leaves and exceptional gold-orange-red autumn colour, and 'Vitifolium', with grape-like rounded leaves and brilliant red autumn colour. Both are widely planted in UK gardens and both produce striking bonsai.

For UK growers, the species offers an alternative to palmatum with slightly less competition and slightly different character. The wider leaves of typical A. japonicum are larger than the most refined palmatum cultivars but produce a more "tree-like" appearance — A. japonicum bonsai often read as full trees in miniature rather than refined miniaturisations.

Botanical confusion is worth flagging: Acer japonicum is sometimes confused with Acer pseudosieboldianum (Korean maple) and Acer shirasawanum (Shirasawa's maple, golden full-moon), both of which are related species with similar characteristics. A. shirasawanum 'Aureum' (golden full-moon) is particularly fine as bonsai. All three species can be cared for with the same techniques.

Daily through the growing season. Identical needs to Acer palmatum — twice daily in summer heat, reduced in winter.

In hot summer weather, twice daily for trees in shallow pots. Position with morning sun and afternoon shade is preferred — the species' broader leaves are slightly more sensitive to scorch than palmatum's finer foliage.

Use rainwater where possible.

Weekly half-strength liquid feed from late April through to mid-September. Slow-release organic pellets in spring and early summer.

The species responds to feeding similarly to palmatum. Restrained feeding produces better proportioned trees than heavy feeding.

Stop feeding by mid-September.

Free-draining and slightly acidic. Identical requirements to Acer palmatum.

Repot every 2–3 years on young trees, every 3–4 on mature specimens. The window is mid-February through to early April. Repot as buds swell.

A. japonicum tolerates moderate root work — up to a third of the root mass on healthy trees. The species recovers slightly faster than refined palmatum cultivars but slower than vigorous deciduous like elms or hornbeams.

Standard Japanese maple pruning technique applies. The species back-buds reliably and tolerates moderate cutbacks.

Structural pruning in late winter (February). Through the growing season, allow new shoots to extend to four or five leaves then cut back to two. Pinch every two to three weeks.

The larger leaves of typical A. japonicum mean defoliation produces dramatic leaf-size reduction on the second flush. Use defoliation on healthy refined trees every 2–3 years rather than annually.

For 'Aconitifolium' specifically, the deeply cut leaves don't reduce dramatically with defoliation — the leaf shape rather than size is the cultivar's aesthetic asset. Don't expect defoliation to produce dramatically smaller leaves on this cultivar.

Wire after leaves harden in early summer or on bare branches in winter. Bark is thin — apply loosely and check fortnightly. Aluminium for almost all work.

The species' more vigorous habit compared to refined palmatum cultivars means wire bites in slightly faster — check fortnightly during active growth rather than monthly.

Informal upright is the natural fit. The broader leaf habit suits naturalistic compositions rather than the most refined miniaturised styles. Twin-trunk and clump styles work well. Forest plantings of A. japonicum (particularly 'Aconitifolium' for autumn colour displays) are exceptional.

Cascade and semi-cascade work on the weeping cultivars ('Green Cascade', 'Filigree'). Formal upright and pure broom styles don't suit the species' habit.

Hardy across most of the UK with no protection needed in normal winters. Slightly more cold-tolerant than typical palmatum cultivars due to the species' more northern origin.

In exposed northern positions, light shelter in severe winters is sensible while trees are developing. Mature trees tolerate UK winters without complaint.

Never bring indoors.

The species is grafted onto Acer palmatum rootstock in most commercial production. From seed possible — produces variable seedlings, not true to cultivar. Air layering works on established branches. UK garden centres widely stock 'Aconitifolium' and 'Vitifolium' as ornamental trees.

Generally healthy. Inherits the typical Japanese maple vulnerabilities (verticillium wilt, chlorosis from alkaline conditions) but with slightly better resistance than palmatum on average.