As the heat of midsummer sets in, it’s a delight to see the cooling white flowers of many hydrangea species now blooming in the Arboretum. Our collections hold close to 30 taxa of hydrangea (their genus name is, conveniently, Hydrangea), all native to Asia or North America. The inflorescences of most hydrangeas are notable for bearing both small, fertile florets and a spangling of large, sterile florets.
One showy North American species is oakleaf hydrangea [pdf] (H. quercifolia). This Southeastern native features long, conical flower panicles with snowy white sterile florets that turn rose-pink as they age, as well as bold foliage that often develops striking purple to red fall color. Smooth hydrangea (H. arborescens), also an American native, bears domed clusters of fertile florets with just a sprinkling of sterile florets, though gardeners are more likely to grow cultivars like ‘Annabelle’ that bear big, globe-shaped inflorescences composed of mostly sterile florets.
Panicle hydrangea (H. paniculata), an adaptable Asian species, has become increasingly popular as many new cultivars, including compact forms, have been introduced in recent years. This large shrub produces loads of conical flower panicles featuring a mix of sterile and fertile florets, the latter of which are eagerly visited by honeybees and many other pollinators (mostly-sterile-flowered cultivars like ‘Grandiflora’ are less appealing). Be sure to check out the impressive centenarian specimens (14714*A and 14714-1*A) of early-flowering cultivar ‘Praecox’ [pdf] growing in the Bradley Rosaceous Collection, as well as the many other hydrangeas in the Arboretum’s collections.
Note: This hasn’t been a good year for bigleaf hydrangeas (H. macrophylla), ever popular for the showy blue, purple, pink, or white flowers of this species’ many cultivars. The extreme cold temperatures in February plus an early April freeze killed many bigleaf hydrangea stems to the ground. Damaged specimens of cultivars that bloom on old wood will have no flowers this year, but cultivars that bloom on both old and new wood should still produce a late summer floral show on new stems.