Ninebarks tolerate more cold than most ornamental plants and thrive in U.S. Department of Agriculture plant hardiness zones 2 through 8. Even in the coldest climates, ninebark flowers reliably in late spring to early summer. The bloom period lasts for three to four weeks. Thereafter, the plants form red capsule-like fruits.
Ninebark flowers are actually masses of tiny flowers that form in 1- to 2-inch clusters. The flat-topped clusters are typically white or pink and resemble the blooms of bridalwreath spirea (Spiraea x vanhouttei), a close relative found in USDA zones 2 to 8. Ninebark blooms have an "unpleasant" odor, notes the University of Wisconsin-Extension.
As a native plant, ninebark tolerates most growing conditions. It blooms best in full sun, although it tolerates partial shade. Ninebark tolerates most soil types, including sand and clay. Newly planted shrubs need regular watering. Established plants are somewhat drought tolerant, although they'll grow best with moderately moist soil.
Ninebark is generally a low-maintenance shrub, but it can grow to 12 feet tall and 8 feet wide. Similar to spirea, it sometimes becomes mangy as it ages. Prune it back to the ground after blooming in the spring to control its growth and create a neater form. The shrub is prone to powdery mildew, fireblight and witch's broom. To keep it healthy, prune it annually to remove disease spores and open the shrub to light and air. Avoid overcrowding it, and use soaker hoses instead of overhead sprinklers.