mountain ash, name for any species of the genus Sorbus of the family Rosaceae (rose family), hardy ornamental trees and shrubs native to the Northern Hemisphere, not related to the true ashes. They are deciduous and bear flat-topped clusters of white flowers followed by orange or brilliant red berrylike fruits, for which they are widely cultivated as ornamentals. The astringent pome fruits are often used in domestic remedies. Of native kinds, the most common is the American mountain ash ( S. americana ), ranging from Newfoundland to North Carolina. Introduced species are often cultivated, especially the common European mountain ash or rowan tree ( S. aucuparia ). This tree is one of the most revered plants in the folklore of the Old World. It warded off evil influences and was "Thor's helper"; bits of the wood were thought to avert almost any disaster. Mountain ash is classified in the division Magnoliophyta, class Magnoliopsida, order Rosales, family Rosaceae.