ninebark, any plant of the genus Physocarpus of the family Rosaceae (rose family). Ninebarks are North American (one is Asian) deciduous, hardy, spring-blooming shrubs, with thin bark which peels off in many layers. The most common American species is P. opulifolius, found along rocky streams E of the Rockies. Ninebarks are similar to the closely related spiraeas and are also cultivated as ornamentals. Ninebarks are classified in the division Magnoliophyta, class Magnoliopsida, order Rosales, family Rosaceae.