Culture
Best grown in moist, acidic, well-drained soils in full sun. As the common name suggests, this is a tree of cool mountain climates that dislikes hot and humid summers, and will not grow well south of USDA Zones 5-6. It does poorly in the St. Louis area.
Noteworthy Characteristics
Sorbus aucuparia, commonly called European mountain ash, is native to Europe and Asia. It has been widely planted in northern North America as an ornamental and has naturalized in some parts of Canada and the northern U.S. It is primarily cultivated for its compound green leaves and attractive clusters of fall fruit. It is a small, deciduous tree that grows to 20-40’ tall with a narrow, upright-oval crown, usually rounding and opening with age. Compound, odd-pinnate, flat medium green leaves have 9-15 serrate, oblong-lanceolate leaflets (each leaflet to 2.5” long). Leaves turn yellow to reddish-purple in fall. Small, white, 5-petaled flowers (1/3” across) bloom in flattened corymbs in May. Flowers give way to pendant clusters of orange-red berry-like drupes in late summer. Cultivars are available in commerce featuring pink, yellow and bright red fruits. Mountain ashes have ash-like leaves, but are members of the rose family, and are not related to true ashes (Fraxinus), which are in the olive family.
Genus name comes from the Latin name sorbum for the fruit of the service tree (Sorbus domestica).
Problems
Bacterial fireblight can be a severe problem, causing scorched leaves at the branch ends. Scab can cause significant defoliation. Cankers, crown gall, powdery mildew and rust may also occur. Insect visitors include aphids, sawfly, scale and borers. Stressed trees are particularly vulnerable to borers and cankers.
Uses
Lawn specimen or small shade tree for cool northern climates. May also be an effective street tree in areas where road salt is infrequently applied in winter. Not recommended for the St. Louis area.