Imagine a shrub that produces large, rounded clusters of white flowers and you'd understand why it may be called a snowball bush. Although two species of hydrangeas may be called snowball bushes, this common name typically is assigned to three species of viburnum. Hydrangeas and viburnums are not closely related plants taxonomically.
Hydrangea shrubs are members of their own family, Hydrangaceae. The two species of white-flowering hydrangeas that may have snowball-like flowers are the paniculate hydrangea (Hydrangea paniculata) and the tree hydrangea (Hydrangea arborescens). Varieties of these two hydrangeas include 'PeeGee, Grandiflora and Annabelle.
Viburnums that are commonly called snowball bushes are members of the honeysuckle family, Caprifoliaceae. They include the European snowball (Viburnum opulus Roseum or Sterile), Japanese snowball (Viburnum plicatum var. plicatum), and the Chinese snowball (Viburnum macrocephalum).
Generally speaking, the white-flowering hydrangeas flower in the warmth of midsummer and later. Snowball viburnums flower in early spring to late spring, usually before their leaves have fully emerged or matured.
If mature leaves are present, texture and shape aids in differentiating between hydrangea and snowball viburnum species. Both the paniculate hydrangea and the tree hydrangea have smooth, broadly oval leaves that taper to a point, sometimes with small teeth on the leaf edges. The snowball viburnums have varying foliage, but the common characteristic is the depressed leaf veins, which makes all leaves look crepe paper-like, rippled or crinkled. The European snowball's leaves are lobed, looking like those of a maple. Japanese snowball leaves are tapering and heart-shaped, while Chinese snowball leaves are oval and taper to a sharp point.
Through close examination of both hydrangea and viburnum flowers, these two plant types can be distinguished by the number of stamens. Stamens are the male organs of the flowers, from which pollen is shed. In fertile hydrangea blossoms, each floret will have seven or more stamens surrounding the female pistil organ. Viburnum plants have five stamens in each blossom.
Each of the species of hydrangea and viburnum hail from differing locations across the world. The hydrangeas are native either to extreme eastern Asia (Hydrangea paniculata) or the eastern United States (Hydrangea arborescens). Snowball viburnums reflect their nativity in naming: from Japan, the Japanese snowball (Viburnum plicatum var. plicatum); from China the Chinese snowball (Viburnum macrocephalum) and from Europe to western Asia is the European snowball (Viburnum opulus Roseum).