Shrub 1 - 3 m tall Leaves: alternate, stalked, medium to dark green, 2 - 7 cm long, egg-shaped to circular with a wedge-shaped to squared or slightly heart-shaped base, usually three- to five-lobed, irregularly toothed. Inflorescence: borne terminally, somewhat flat-topped, 3 - 5 cm across, many-flowered. Flowers: white or pinkish, 7 - 10 mm across, with five triangular sepals, five nearly round petals, 20 to 40 purplish stamens, and three to five pistils. Fruit: capsule-like, borne in drooping clusters, shiny reddish to purplish, 5 - 10 mm across, egg-shaped, inflated, papery, splitting on two sides to reveal two to five seeds. Seeds are shiny yellowish to pale brown, rounded, and hard. Twigs: shiny reddish brown when young, peeling in multiple layers of papery strips with age, exposing reddish to pale brown inner park. Buds: brown, small, usually having five visible scales. Form: upright to spreading with recurved branches, leading to an overall rounded appearance.
Similar species: Physocarpus opulifolius is easy to distinguish in the Chicago Region by its multiple layers of peeling bark, and clusters of reddish to purplish inflated fruit.
Flowering: late May to late June
Habitat and ecology: This species is not very common in the Chicago Region. It grows on dry rocky slopes, in moist swale borders, and along streambanks and lakeshores.
Occurence in the Chicago region: native
Notes: A few cultivars of ninebark are available for landscape use, including a dwarf form and purple-leaved form. Bees and butterflies collect nectar from the flowers, while beetles feed on the pollen and birds eat the seeds.
Etymology: Physocarpus comes from the Greek words physa, meaning bladder, and karpos meaning fruit, referring to the inflated fruit. Opulifolius refers to the resemblance of the leaves to those of Viburnum opulus, the Guelder rose.
Shrub to 3 m, the bark exfoliating in narrow strips; petioles 1-3 cm; lvs ovate to obovate, the principal ones ±3-lobed, irregularly serrate, broadly cuneate to truncate at base; pedicels 1-2 cm; sep 1.5-2.5 mm; fls 7-10 mm wide; fr 5-10 mm, glabrous or stellate-pubescent; 2n=18. Moist, sandy or rocky soil, especially along streambanks and shores; Que. to N.D. and Colo., s. to N.C., Tenn., and Ark., also escaped from cult. May-July. The form with stellate fr, commoner toward the west, has been called var. intermedius (Rydb.) B. L. Rob. (Opulaster opulifolius; O. australis)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.