A spreading shrub; twigs glabrous, terete; pith chambered. Leaves ovate to broad-ovate, 7–12 × 3–6 cm, short-acuminate, base rounded; margin with irregular tiny teeth or more often entire; underside pubescent at least along the main veins; petiole pubescent. Flowers in mid spring; calyx about half as long as the corolla tube; corolla bright yellow, small (c. 25 mm wide), with narrowly oblong lobes. (Bean 1981).
Distribution Japan SW Honshu
Habitat Mountain forests
USDA Hardiness Zone 5
RHS Hardiness Rating H6
Conservation status Not evaluated (NE)
Like many Forsythia species F. japonica appears to be a rare plant in the wild, surviving in a few mountain refuges, though detailed information seems lacking. While the Chinese F. suspensa had long been cultivated in Japanese gardens, this native species was named by Tomitaro Makino as recently as 1914. A separate population on Shōdoshima Island, off the south coast of Honshu, was described in 1964 as a variety (var. subintegra H. Hara), but Hara elevated this to species level nine years later as F. togashii, which does not seem to have been introduced to the west.
Forsythia japonica itself was introduced to the Arnold Arboretum in Massachusetts in 1950 from the Acclimatisation Garden in Kyoku, but is no longer grown here (Arnold Arboretum 2018). Plants, possibly from the Arnold stock, were available in Poland later in the 1950s for Bolesław Suszka’s breeding programme (Suszka 1959), but it is not confirmed that any of the contemporary Forsythia cultivars of Polish origin have genes from F. japonica in their make-up. Donald Wyman (Wyman 1961) states that F. ‘Arnold Dwarf’ was bred at the Arnold Arboretum as early as 1941 by crossing F. × intermedia with F. japonica, but it seems that this parent was really the Korean F. saxatilis, a species known to be in America at this time.
The date of introduction to the UK is not known, and the rather vague comment in Trees and Shrubs Hardy in the British Isles that Forsythia japonica was ‘not common in gardens’ (Bean 1981) might be suspected to cover an absence, either of knowledge or more probably of the plant itself. The only specimen traced during research for this article is one at Lord Heseltine’s Thenford Arboretum in Northamptonshire, which was received as F. japonica from Arboretum Waasland in the Netherlands in 1999 (M. Heseltine pers. comm.)