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Wild hydrangea canes produce a flat terminal panicle 3-6" across with 100-500 flowers of two kinds — small, fertile flowers filling most of the area and a few, sterile pseudo-flowers around the margin that are 3-4 times larger than the flowers in the center and seem to function only as an advertisement for pollinators. The central, fertile flowers are about 1/4" across with a green calyx made of five separate sepals; five greenish-white or cream petals about 1/8" long; 8-10 stamens with long filaments and oblong, white anthers; and a pistil bearing two styles with knob- or club-like stigmas. The sterile, outer pseudo-flowers have 3-4 fused petal-like, white bracts (or sepals; sources differ) with a central structures that visually is reminiscent of stamens or styles; the pseudo-flowers persist long after the inner flowers have been fertilized and have produced a two compartment seed pod that is ribbed and bears a pair of tiny horns (the styles) on its upper end. Wild hydrangea is relatively rare in northern Illinois; in Jackson Park it prefers wooded area by walking paths.

Hydrangea

Hydrangea arborescens

wild hydrangea [Blooms: Jun-Aug]

Panicled hydrangea (Hydrangea paniculata) is an exotic perennial native to China and Japan that may grow as either a shrub or a tree. Neither Natureserve Explorer nor the USDA Plants database acknowledge the presence of panicled hydrangea in Illinois, but the Morton Arboretum and the University of Illinois Extension both explicitly do. Panicled hydrangea is widely sold in nurseries; commercial cultivars reach 10-15 feet high and wide. It definitely occurs in Jackson Park, near our native species — wild hydrangea (Hydrangea arborescens) — but I have not seen it in direct competition with wild hydrangea. The inflorescence is a conical panicle typically covered with two types of flowers — large, nominally-sterile flowers and smaller, bisexual flowers — scattered across the surface of the panicle. The "sterile" flowers are white or greenish white with typically 4-5 (3 in Jackson Park) elliptical to round lobes (actually sepals) 1/2-1" long and 1/3-2/3" across (8-15 mm). In Jackson Park, at least half the nominally sterile flowers bear a small, budlike structure in their center that exhibits three lobes (petals?), have 3-6 distinct stamens with small anthers, and a two-lobed central structure that is probably a reduced pistil. It is not clear whether either the pistils or stamens are functional. The bisexual flowers usually have five tiny (less than 1 mm long and wide) sepals, five petals 3-4 mm long and 0.1-0.3 mm wide (which often fall off the flower), 2-5 mm long stamens, and an ovary with 2-4 spreading styles (two in Jackson Park specimens) that are 2-3.5 mm long and are fused at their bases. After fertilization, the petals and stamens wither and fall off; the remainder of the flower forms an oblong, 3-5 mm long, seed capsule. The easiest ways to tell panicled hydrangea (H. paniculata) from wild hydrangea (H. arborescens) is: (1) the inflorescence is conical or pyramid-shaped in panicled hydrangea but a flat disc or shallow dome in wild hydrangea, and (2) the showy, "sterile" flowers in panicled hydrangea are scattered evenly across the surface of the inflorescence in panicled hydrangea, but are restricted to the margins of the inflorescence of wild hydrangea (rather like the inflorescence of the much more common but unrelated American cranberry, Viburnum opulus var. americanum). Wild hydrangea is much more common than panicled hydrangea in Jackson Park.

Hydrangea

Hydrangea paniculata

panicled hydrangea [Blooms: Jul]