Physocarpus, commonly known as ninebark, is a genus of deciduous flowering shrubs in the rose family (Rosaceae), placed in the order Rosales. The genus comprises six to around twenty species, nearly all native to North America, with two outlier species — P. amurensis and P. ribesifolia — native to northeastern Asia.
Plants are deciduous shrubs with a distinctive characteristic: the bark of mature branches peels away in thin, papery strips — a trait that inspired the common name "ninebark." Leaves are arranged alternately and are palmately lobed, typically with three to seven lobes and often toothed margins. Flowers are borne in rounded corymb-like clusters; each flower is bell-shaped with five rounded white or pink petals and numerous stamens, giving the inflorescences a fluffy appearance. The fruit is a flat or inflated dehiscent follicle that splits open at maturity to release seeds.
The most widely cultivated member is Physocarpus opulifolius (common ninebark), native to eastern North America, which has given rise to numerous garden cultivars prized for foliage color — from yellow-leaved forms like 'Dart's Gold' and 'Luteus' to deep reddish-purple selections such as 'Monlo' and 'Seward'. Other notable species include P. capitatus (Pacific ninebark) of the western coast, P. malvaceus (mallow ninebark) of the Intermountain West, P. monogynus (mountain ninebark), and the compact P. alternans (dwarf ninebark). The Asian representative P. amurensis extends the genus range into the Russian Far East and northeastern China.
Etymology
The genus name Physocarpus derives from the Greek words for "bladder" and "fruit," a reference to the inflated, bladder-like follicle fruits characteristic of several species. The common name "ninebark" alludes to the peeling bark of mature stems, which separates into multiple thin papery layers — though the name is colloquial rather than a precise count.
Distribution
Almost all species of Physocarpus are native to North America, ranging from eastern woodlands (P. opulifolius) to western mountains and the Pacific coast (P. capitatus, P. malvaceus, P. monogynus, P. alternans). Two species — P. amurensis and P. ribesifolia — are native to northeastern Asia (Russian Far East and adjacent regions).
Cultivation
Physocarpus species can be propagated from seed sown as soon as ripe, or stored dry in airtight containers in a cool place for up to a year before sowing. Established plants can be divided in early spring by splitting the rootball with a sharp spade or axe. Both softwood and hardwood cuttings root readily. Garden writers note that ninebark species are generally trouble-free, though aphid infestations and powdery mildew (particularly in coastal or humid conditions) can occasionally occur.
