Culture
Best grown in moist, acidic, humusy, well-drained soils in full sun. As the common name of mountain ash 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 decora, commonly known as showy mountain ash or northern mountain ash, is a small, deciduous, understory tree (sometimes a tall shrub) that typically grows to 20-30’ (rarely to 60’) tall and features showy white spring flowers, pinnate-compound leaves and red fall fruit. It is native from Labrador and Newfoundland to Quebec south to Iowa, New York and Maine with scattered populations further south in the Appalachian mountains to North Carolina. It is very similar to and often grows in areas populated by American mountain ash (Sobus americana).
Each odd pinnate leaf (to 10” long) has 11-15 lanceolate bluish-green to gray-green leaflets (each 2-4” long) which have pointed tips, serrate margins and are broadest in the middle. Foliage turns yellow to red in fall. Five-petaled white flowers (to 1/ 4” across) bloom in dense clusters (flattened corymbs to 5” across) in May. Flowers are followed by clusters of shiny red fruits (each to 3/8” diameter) that ripen in late summer but remain on the tree after leaf drop. Branches typically bend downward from the weight of the ripe fruit. Each fruit (botanically a pome) contains 1-2 seeds. Fruits are attractive to birds and animals, but too bitter/acidic to be eaten fresh off the tree by humans. Fruits may be made into jellies. Smooth, gray bark becomes scaly with age.
Genus name comes from the Latin name sorbum for the fruit of the service tree (Sorbus domestica).
Specific epithet comes from the Latin word decoratus meaning decorative or showy in probable reference to the appearance of the fruit.
Common name of mountain ash is in reference to the ash-like leaves and preference for cool mountain climates. Notwithstanding ash-like leaf appearance, this mountain ash is a member of the rose family and not the family of the true ashes (Fraxinus), which are in the olive family.
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.