A deciduous, semi-shrubby plant often less than 11⁄2 ft high, but much higher in milder climates; young shoots, leaves, flower-stalks, and ovary covered with bristly, pale down. Leaves ovate-oblong, rounded or tapered at the base, slender-pointed, margined with numerous fine, bristle-like teeth, 3 to 6 in. long, 1 to 21⁄2 in. wide, roughened, especially above; stalk 1⁄4 to 1 in. long. Corymb 3 to 5 in. across, enclosed in the bud state by about six large broadly ovate bracts, the largest about 1 in. long, covered with a felt of appressed whitish down. Sterile flowers at the margin of the corymb, 3⁄4 to 1 in. across, the three to five sepals white or blue-white, slightly downy. Small fertile flowers blue. Blossoms from August to October.
Native of Japan and Formosa. The distinguishing feature of this species is the whorl of bracts (involucre) at the base of the inflorescence, which persists through the flowering. It is very pretty when in bloom, the blue (sometimes rosy-lilac) fertile flowers making an effective contrast with the large sterile whitish ones. Unfortunately it is not very hardy, and is often killed back more or less in winter, the flowers being borne on the new shoots which spring from the base. It thrives well in the west country.
In floral characters and foliage H. involucrata is near to H. aspera but it differs from all the other species of the section Hydrangea (the ‘true’ hydrangeas) in having the inflorescence enclosed in the bud-stage by an involucre, as in the section Cornidia.
cv. ‘Hortensis’. – Sterile flowers double and also more numerous than in the wild form, coloured an attractive shade of buff-pink (but almost white on plants grown in shade). Of Japanese garden origin, described in 1867 but not introduced to Britain until 1906. It is not completely hardy but has lived for many years in the gardens of the Royal Horticultural Society at Wisley, in a sheltered border.