Douglas Justice took today’s Botany Photo of the Day and wrote the accompanying entry as well.
Hydrangeas are popular garden subjects, and the genus Hydrangea has been written about extensively. At least three popular volumes have appeared in the last few years, and the quality of writing and images in those books is impressive compared with what I had to put up with early in my horticultural career. Thankfully, modern hydrangea books take pains to discuss both the wild species and the cultivars (of which there are hundreds). Our collection of hydrangeas at UBC runs to about 20 wild taxa (species, subspecies, varieties, etc.)—most of them derived from Japan, China, and Taiwan—and includes deciduous small and large shrubs (some nearly tree-like) along with deciduous and evergreen climbers.
Michael Dirr, the respected woody plant expert and author of numerous horticultural books, writes of Hydrangea aspera in his excellent Hydrangeas for American Gardens (Timber Press, 2004): "All taxa umbrellaed by this species have large fuzzy leaves and stems, lacecap inflorescences, typically blue or purple in the central fertile flowers, with an outer ring of white to lilac-pink showy sepals. Large, knobbily rounded bud clusters foreshadow the flowers. Depending on one’s aesthetic bent, the bud clusters are somewhat attractive to rather scary". Indeed.
Hydrangea aspera is native to an enormously wide geographical range, from the western Himalayas through Indo-China to Malesia and east to central and western China and Taiwan. Plants vary considerably both morphologically and with respect to cold-hardiness and heat tolerance, which shouldn’t be surprising given that the species is found in both temperate and tropical habitats. The ability of plants to absorb aluminium—and to thereby colour their flowers blue—is also evidently variable. I’ve read that the flowers of H. aspera are unaffected by soil pH, but my own experience with the species (the cultivar in today’s Photo of the Day, in particular) suggests that regular springtime applications of aluminium sulphate effectively cause the fertile flowers to appear bright blue when fully open. They are normally pink, mauve, or lilac-purple in unamended acid soil.