Fruit and sporadic fall leaf color of Korean mountainash. Photo by Michelle Sutton

Underutilized Trees for Urban Use: Korean mountainash (Sorbus alnifolia)

By Michelle Sutton, Council Editor

I can think of few trees with the year-round beauty of Korean mountainash (Sorbus alnifolia). It is notable for its delicate and showy white spring flowers, handsome deeply veined summer foliage, yellow to orange fall color, reddish-pink berries in fall and early winter, and elephantine bark in winter (beech-like, but with distinct white markings). Its potential height is 20 to 40 feet and potential width 15 to 25 feet. The cultivar ‘Redbird’ has rosy-red fruits and a more upright form.

Sorbus alnifolia in fall, on Vassar College campus in Poughkeepsie. Photo by Michelle Sutton

Despite its common name, Korean mountainash is a member of the Rose family, and like so many other members of that family tends to bear its flowers and fruit more heavily in alternate years. Birds are attracted to the fruits, which are pomes, botanically speaking. The tree is native to Japan and China as well as Korea. It prefers full sun but can tolerate part shade.

Delicate Korean mountainash flowers, five-petaled as per the Rose family. Photo Courtesy Missouri Botanical Garden Plant Finder

Korean mountainash has stronger wood and more borer resistance than its European mountain ash counterparts. To reduce the chance for fireblight infection, it should be pruned in the winter. Its early structural pruning should take into account the tree’s tendency toward narrow crotch angles.

Vertically striated trunk of mature Korean mountainash. Photo by Michelle Sutton

The Cornell Urban Horticulture Institute (UHI) reports that in Ithaca, NY, Korean mountainash is successfully transplanted bare root at two-inch caliper or less, spring or fall. UHI has had success growing it in CU-Structural Soil installations. It is recommended as a specimen tree for parks, large parking lot islands, and generous tree lawns where it can get its water needs met, as it doesn’t tolerate prolonged periods of drought. It can thrive in a wide range of soil pH (5.0 to 8.0). Korean mountainash performs best in USDA Hardiness Zones 4b to 7. As an underutilized tree, this beauty carries extra charm.