Described by Bean (1981) as Platycrater arguta. Text transferred to Hydrangea in August 2022 (JMG).
[Platycrater was formerly segregated from Hydrangea by having the sepals of the ray-flowers united and numerous stamens in the fertile flowers.]
A low, deciduous, sometimes creeping shrub, with slender, glabrous stems. Leaves opposite, narrowly oval-lanceolate, the largest 5 to 8 in. long and 1 to 2 in. wide, tapering at both ends, the margins set with slender teeth, bristly hairy beneath; stalk 1⁄4 in. long. Flowers of two kinds, viz. perfect and sterile, as in Hydrangea, produced in a lax terminal corymb. Perfect flowers 1 in. across, with four white, broadly ovate petals, two styles, very numerous yellow stamens, and a four-lobed calyx; the lobes 1⁄2 in. long, pointed, narrowly triangular. Fruits top-shaped, with the calyx-lobes persisting. Sterile flowers consist only of the united calyx-lobes, and form a white, flat, three- or four-sided disk, 3⁄4 in. across, all the other parts of the flower being absent.
Native of Japan; introduced by way of St Petersburg about 1868. The plant is rather tender and apt to be cut to the ground in winter, or killed outright in severe frosts. I have never seen the sterile flowers above described on cultivated plants, usually there have been three perfect flowers produced in a corymb, the middle one opening first, each on a slender stalk 1 in. or less long. Both Siebold and Regel include sterile flowers in their figures (see Flora Japonica, t. 27, and Gartenflora, t. 516). The cultivated form without sterile flowers has been distinguished as var. hortensis Maxim. [it is not certain that this name would be valid in Hydrangea – JMG, August 2022]. Siebold says he found the plant growing on humid rocks with its branches flat on the ground. He mentions a curious use the Japanese made of the plant; this was to make an infusion of the leaves with which the images of Buddha were washed or baptised. But that was in 1835. The plant is easily increased by rather soft cuttings.