Ninebark (Physocarpus opulifolius)* of the Rose (Rosaceae) family is a large shrub which bears tight clusters of small white flowers. The genus name, from Greek words for “bladder” and “fruit,” refers to the inflated carpels of the fruit. The specific epithet, from Latin, compares the leaves to those of Viburnum opulus. Ninebark is widespread in the eastern U.S., occurring principally from Minnesota to Maine, south to Arkansas, Alabama, and the Carolinas. (A disjunct population occurs in the Rocky Mountains of Colorado.) The only species of the genus in Arkansas, it grows in our Interior Highlands. Habitat preference is dry to moist, rocky to clayey soils of stream banks, bases of bluffs, and bog margins, in full to partial sunlight. Other common names include Atlantic ninebark and Eastern ninebark.
Ninebarks are rounded to upright, deciduous shrubs, reaching heights of 8+ feet and often equally as broad. The shrubs have a multitude of closely spaced, long, slender stems from their base which arch and recurve over the top of the shrub, typically bearing short, fertile branches from the leaf axils. Shrubs are typically glabrous, except for minute pubescence in the inflorescence. Stems begin to exhibit exfoliation in their second growth year, and older stems have several layers of thin, light brown bark loosened from underlying darker brown to reddish brown bark, especially nearer the base of the shrub.
The inflorescence, in May, consists of hemispherical corymbs (closely spaced flowers on a short axis). Small white flowers terminate newly developed, short (3-6 inches long), fertile branches, growing from axillary buds of the previous year. Corymbs are aligned along the upper side of the stems, often, in sunny sites, in an impressively long, continuous series. Corymbs measure 1½ inches wide and 1 inch tall, with 20-40 flowers on straight pedicels that decrease in length toward the top of the corymb. Individual flowers, about 3/8 inch across, bloom from the outside in, i.e., from the corymb’s perimeter to its center. Clusters bloom for up to two weeks.
Flowers are white overall, however, buds may be highlighted with pink and petals may be faintly pink. Flower parts arise, like those of a rose, from a basal, bowl-like structure called a hypanthium. Five sepals, five rounded petals, and 22 to 30+ stamens are attached at the hypanthium rim; 3-5 tightly appressed, erect ovaries somewhat fused at the base are attached at the bottom of the bowl. When first appearing, anthers are burgundy, but become dark brown as they split to disperse light yellow pollen. At anthesis, anthers are positioned well above the corolla and stigmas tend to be slightly higher. Sepals are persistent through fruiting.
With fertilization, the 3-5 tightly appressed ovaries of the flower quickly enlarge and inflate to form pale-green to reddish follicles. Follicles have a papery, minutely pubescent outer surface. Dry follicles become dark brown and split along the inner seam. They produce one to several smooth, light tan, pear-shaped seeds, each with a hilum (umbilical scar) toward the smaller end. Follicles are to ½ inch long and ⅛ inch wide. Seeds are slightly less than ⅛ inch long. With fruiting in August to October, the previously ascending corymbs become dangling. As well as dispersal of seeds by small mammals and birds, seeds remaining in follicles that fall into water may be dispersed by water flow.
For a larger garden or natural area, ninebark is a leafy, dense shrub which may produce cascades of flowers in sunnier sites. It may also be a nice shrub for a more shaded site, but may not bear flowers. Ninebark breaks dormancy in early spring, with flower clusters quickly developing. It does not produce suckers and self-seeding does not seem to be a problem. “Extra” young plants can be easily pulled or transplanted. New plants can be established from soft cuttings or by layering. Along with the spring flower clusters, the compound follicles provide visual interest in late summer into fall. Fall color is not showy.
* Some authorities recognize as either a variety of Physocarpus opulifolius or as a separate species the entity Physocarpus intermedius. It has a more western distribution, occupying the central U.S., and is reportedly the primary, if not only, ninebark in Arkansas. It differs in having carpels/follicles densely stellate-hairy, sometimes only on the sutures. Variety opulifolius has mostly glabrous or glabrescent carpels/follicles and is more eastern and northern in distribution.
Article and photographs by ANPS member Sid Vogelpohl