A small deciduous shrub with slender branches; young stems glabrous or appressed-downy. Leaves rather thin, lanceolate, ovate elliptic, acute, acuminate or caudate at the apex, 2 to 6 in. long, 1 to 21⁄2 in. wide, finely to coarsely serrate, usually glabrous above, veins beneath often covered with short appressed or curled hairs. Flowers in flat-topped corymbs; ray-flowers present, with variously shaped pink or bluish toothed or entire sepals. Capsules as in H. macrophylla but smaller, c. 1⁄8 in. long (including styles).
Native of the mountains of Japan and of S. Korea (Quelpaert Island); introduced in 1843. It is allied to H. macrophylla, in which it is included as a subspecies or variety by some botanists. It is a less robust shrub, with more slender stems, thinner, relatively narrower leaves and smaller flowers and seed-capsules. It is also mainly a plant of montane woodlands, whereas H. macrophylla occurs at low altitudes and never far from the coast. This no doubt serves to explain its greater hardiness (see further below).
H. serrata is represented in cultivation mainly by cultivars, discussed below. First the following variety must be described:
var. thunbergii (Sieb.) H. macrophylla var. thunbergii (Sieb.) Mak. – As represented in gardens it is a small, neat, deciduous shrub, usually 2 to 3 ft in height, the young stems smooth and very dark, ultimately almost black. Leaves oval or ovate-oblong, tapered about equally towards both ends, toothed at the terminal part only, 11⁄2 to 31⁄2 in. long, 5⁄8 to 11⁄2 in. wide, dull green and with appressed hairs above, paler and glabrous beneath except for occasional tufts of down in the vein-axils. Corymbs 2 to 4 in. across, the sterile flowers 1⁄2 to 3⁄4 in. across, the sepals overlapping, broader than long, blue or pink, often with a shallow notch at the apex; flower-stalks downy. Hardier than the hortensias, but liable to be injured in hard winters. A dainty shrub, flowering in July and August. Similar plants occur wild in S. Japan. A tea is made from the dried leaves of this variety.
The following cultivars are usually placed under H. serrata and appear to belong to it:
‘Bluebird’. – Ray-flowers pale pink or pale blue, entire. Early flowering. A vigorous and hardy variety unusually resistant to drought. 5 ft. A.M. 1960. Journ. R.H.S., Vol. 74 (1949), fig. 191; The Hydrangeas, plate V.
‘Diadem’. – Ray-flowers serrated, clear blue or pink; inflorescences on side-shoots all the way up the stems, opening in June. The foliage is purple in an open position. Compact habit. 21⁄2 ft. A.M. 1963.
‘Grayswood’. – Ray-flowers about nine in each corymb, white at first, ageing to pink and eventually, in sun, to bright red; sepals coarsely toothed towards the apex, the outermost longer than the other three. 5 ft. A.M. 1948. Journ. R.H.S., Vol. 65 (1940), fig. 105; The Hydrangeas, plate XIX.
‘Preziosa’. – This variety is unusual in having an inflorescence of the hortensia type, though smaller. Ray-flowers at first pink, shaded deeper at the edge, later clear crimson, blotched with deeper shades of the same colour. Young stems reddish brown. Very hardy and flowering well in sun or half-shade. About 4 ft. F.C.C. 1964.
‘Rosalba’. – Leaves yellowish dull green. Ray-flowers six or seven in each corymb, white at first, becoming heavily blotched with pure crimson; sepals serrated. Of upright habit, to about 4 ft, usually somewhat less. This variety has long been grown in British gardens, indeed the first introduction of H. serrata, figured in Bot. Reg. (1844), t. 61, as H. japonica Sieb., closely resembles the present-day clone. A very similar plant is grown in Japanese gardens under the name Benikaku and is figured in Siebold and Zuccarini, Fl. Japonica (1840), t. 53 (as H. japonica). ‘Rosalba’ is the ‘type’ of a group of garden varieties of H. serrata with ray-flowers that never ‘blue’ even in acid soils. Here belong ‘Grayswood’ and ‘Preziosa’, described above, and also the hydrangeas known as ‘Intermedia’ and ‘Macrosepala’ (not described here).
A number of Japanese garden forms of H. serrata with multisepalous ray-flowers and often with double or proliferous fertile flowers have been grouped under the name H. serrata f. prolifera (Reg.) Wils. Such plants have been introduced to this country but none has been traced by Mr Haworth-Booth. The plant figured in Fl. Japonica, t. 59 (I), as H. stellata, belongs here.