Culture
Best grown in moist, acidic, humusy, well-drained soils in full sun. As the common name suggests, this is a tree of cool mountain climates that dislikes dry soils and hot and humid summers. It will not grow well in the deep South below USDA Zone 6. It is somewhat intolerant of urban pollution. It generally requires little pruning. Prune if needed from late fall to early spring.
Noteworthy Characteristics
Sorbus americana, called American mountain ash, is a small, deciduous, understory tree (sometimes a shrub) that is native to northeastern North America from Newfoundland to Manitoba south to northern Illinois, northern Michigan and New Jersey plus further south in the Appalachians to Georgia. It typically grows to 15-30’ tall with an open rounded crown. It is noted for its attractive form, white spring flowers, serrate compound-pinnate leaves and bright orange-red fall fruit. Smooth, gray bark becomes scaly with age. Dense flattened clusters (corymbs to 3-6” across) of very small 5-petaled white flowers (each to 1/4” wide) appear in May. Flowers are followed by bright orange-red berries (each to 5/16” diameter) that ripen in late summer and remain on the tree after leaf-drop. Berries are attractive to birds and animals, but too acidic to be eaten fresh off the tree by humans. Berries may be made into jellies. Each odd pinnate leaf (6-10” long) typically has 9-17 sharply serrated, lance shaped, dark green leaflets (2-4” long) with gray-green undersides. Foliage turns yellow in fall. Mountain ashes usually 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).
Specific epithet means of North or South America.
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 borers, aphids, sawfly and scale. Stressed trees are generally more susceptible to attack from canker and borers.
Uses
Lawn specimen or small shade tree for cool northern climates. It is not recommended for the St. Louis climate.