Acer japonicum (Downy Japanese Maple or Fullmoon Maple; Japanese: ハウチワカエデ hauchiwakaede) is a species of maple native to Japan, on Honshū, Hokkaidō, Kyūshū (Nagasaki Prefecture), and also southern Korea.
It is a small deciduous tree growing to 5–10 m (rarely 15 m) tall, with a trunk up to 40 cm diameter. The bark is smooth on young trees, becoming rough and scaly on old trees. The shoots are slender, and thinly downy with whitish hairs. The leaves are rounded, 7-15 cm diameter with 9–13 (rarely 7) serrate lobes incised to half or less of the diameter of the leaf; they are downy at first with white hairs, the hairs mostly lost by late summer except on the veins and the underside of the leaf; the petiole is 2–4 cm long and hairy. In autumn, the leaves turn bright orange to dark red. The flowers are 1 cm diameter, dark purplish-red with five sepals and petals; they are produced 10–15 together in drooping corymbs in early spring as the leaves start to open. The fruit is a paired samara with the nutlets 7 mm diameter with a 20–25 mm wing, hanging under the leaves.
The closely related species Acer shirasawanum (Japanese, オオイタヤメイゲツ ooitayameigetsu) from southern Japan is sometimes included as a subspecies of A. japonicum. It is distinct in its hairless shoots, and usually smaller leaves. Another related species, Acer sieboldianum (Japanese: コハウチワカエデ kohauchiwakaede), is best distinguished by its yellow (not red) flowers, and smooth bark even on old trees. It is more easily distinguished from Acer palmatum, as that species rarely has leaves with more than seven lobes.
Its preferred growing conditions are similar to those of A. palmatum, but it is sometimes considered more tolerant of cold, especially compared to the more delicate cultivars of the latter.
Numerous cultivars have been selected, some of which have their own common names (e.g. "Grape-leaf maple" for A. japonicum 'Vitifolium'). Other popular cultivars are 'Aconitifolium' which has deeply incised leaves, and 'Green Cascade', with drooping to pendulous branches. Because of their variation from the wild species, some cultivars are difficult to assign to species, and they have often been mis-labeled as cultivars of the other related species; notably 'Aureum' has commonly been cited as a cultivar of A. japonicum, but is actually derived from A. shirasawanum.
Cultivars of this maple are found in almost every maple collection including Esveld Aceretum (Boskoop, Netherlands) and the large Acer section of Arnold Arboretum (Boston, Massachusetts). They are also common in more general collections of horticulture, such as Valley Gardens (Surrey, England).