Trees & Shrubs of the Eloise Butler Wildflower Garden

American Mountain Ash is a native deciduous small tree growing to 40 feet high as a tree, but usually to 30 feet, and up to 8 inches in diameter or as a large multi-stemmed shrub. As a tree it grows upright with a narrow crown that spreads and opens to a rounded structure as the tree ages.

The bark is a light gray, smooth, with lenticels when young but becomes grayish-brown and scaly with cracks and splits when older.

Twigs are moderately stout, hairy when young, with a somewhat shiny gray to reddish-brown color. Winter buds are a darker purplish-red color and sticky, up to 1/2 inch long.

The leaves are pinnately compound, 6 to 8 inches long with 11 to 17 stalkless lance shaped leaflets. Leaflets are each up to 1-1/2 to 4 inches inches long and 1/2 to 1 inch wide [important - leaflets are more than 3x as long as wide, ranging from 3.4 to 5 to 1]. Leaflets taper gradually to a pointed tip, and have saw-toothed edges on at least the upper half, but more often almost to the leaflet base. They are dull green to yellowish-green above and smooth, paler in color under with some hair remaining on the veins. Stalk bases may have some short tawny or whitish hair. The leaves have reddish stalks and are alternate on the twigs. Leaf stipules fall away early. Fall color is yellow.

The inflorescence is a dense showy upright, somewhat flattened branched cluster (a corymb), 3 to 6 inches in width, of 125 to 400+ flowers, the cluster on a separate shoot from the twig, usually near the end. Cluster stalks may have sparse or no hair.

The flowers: Each flower is stalked, about 1/4 inch wide with 5 rounded white petals that have narrowed bases. The sepals are very short, often with glands. The hypanthium, flower stalk and cluster stalk are usually without hair. Stamens number 14 to 20; there are 3 to 4 styles.

Fruit: Each fertile flower produces a small shiny bright red to orange-red pome, about 1/4 inch in diameter, looking like a small apple. The sepals, being small, are inconspicuous on the pome. Pomes contain a few seeds and have a bitter taste. These mature in early autumn and can persist on the tree into the winter.

 

Habitat: American Mountain Ash grows in moist soils of valleys and slopes in the more northern temperate climate areas. It requires sunny locations to flower and fruit and is not very tolerant of drought.

Names: The genus Sorbus is taken from the Latin sorbum referring to the fruit of a tree similar to the Mountain ashes. The species americana means 'of America', referring to its North American native status. The author name for the plant classification - ‘Marshall’ is for Humphry Marshall (1722-1801), American Botanist, who published the description in 1785 in - Arboreteum Americanum: The American Grove, an Alphabetical Catalogue of Forest Trees and Shrubs, Natives of the American United States. An unaccepted name scientific name for the tree is Pyrus americana.

Comparisons: S. americana is distinguished from the two other species of Sorbus that grow in Minnesota as follows: S. aucuparia, European Mountain Ash, has buds that are red-brown but more densely hairy with gray hair, and are not sticky. Leaflets are on average less than 3x long as wide, leaflet tips acute to rounded, no projecting point, and the underside is hairy. Flowers and fruit are larger. S. decora, the Showy Mountain Ash, also has leaflets that average less than 3x as long as wide but do not have fine hair on the underside. It also has sticky buds. It flowers about a week later than S. americana, the leaflet tip is abruptly tapered to a projecting tooth; the flowers and the fruit are larger.