A deciduous shrub of somewhat loose habit 4 to 10 ft high; young shoots rather downy at first, becoming glabrous. Leaves broadly ovate, oval or roundish, 3 to 7 in. long, 2 to 6 in. wide, pointed at the apex, rounded or heart-shaped at the base, coarsely toothed, upper surface bright dark green, lower one paler, both glabrous, or with down only on the veins or in the vein-axils beneath; stalk 1 to 3 in. long. Corymbs flattish, much branched, usually 4 to 6 in. across, with few or no large sterile flowers. Fertile flowers dull white, very small and crowded; flower-stalks downy. Seed-vessels eight-ribbed, with calyx adhering at the top.
Native of the eastern United States, from the State of New York southwards; introduced by Peter Collinson in 1736. A vigorous and hardy species, which flowers freely in July and August, but is not particularly attractive.
cv. ‘Grandiflora’. – A very beautiful form, in which all the flowers are of the large sterile type and pure white. It is a clone descended from a plant found growing wild in Ohio late in the 19th century and was introduced to Britain in 1907. It is quite hardy, and showy enough to be regarded as an admirable substitute for the hortensias in the colder parts of the country. It blooms from July to September, and is probably the best hydrangea to cultivate out-of-doors near London and in places with a similar climate. Its one defect is that its flower-heads are often so heavy that the stalk is not stout enough to hold them upright.
var. oblonga Torr. & Gr. – Leaves rounded or tapered at the base, narrow-ovate to elliptic-oblong. There is a form of this – f. sterilis (Torr. & Gr.) St John – in which all the flowers are sterile. It was originally described by Torrey and Gray in 1840 from a plant found wild in Pennsylvania, but so far as is known the original plant was never propagated.
In previous editions, two allies of H. arborescens were described under the names H. cinerea and H. radiata, but these are really only part of the variation of H. arborescens and cannot reasonably be maintained as distinct species. Following Miss McClintock, they are treated here as subspecies:
subsp. discolor (Ser.) McClintock H. arborescens var. discolor Ser.; H. cinerea Small – Undersides of leaves sparsely downy, or with a continuous but thin greyish indumentum; hairs covered with numerous microscopic ‘warts’ (tubercles). This subspecies is mainly represented in cultivation by the cultivar ‘Sterilis’, in which all or most of the flowers are of the sterile type. The hydrangea named H. canescens by Kirchner (Arb. Muscav. (1864), p. 198) is near to subsp. discolor but according to Rehder the hairs lack the characteristic tubercles.
subsp. radiata (Walt.) McClintock H. radiata Walt.; H. nivea Michx. – Upper surface of leaves dark green, the lower covered with a close snow-white felt. Native of the southern Appalachians; introduced in 1786. The vividly white undersurface of the leaf distinguishes this hydrangea from all others in cultivation and gives it a peculiar interest.