American Mountain Ash is a native deciduous small tree growing to 40 feet high and 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, and with lenticels when young but becomes grayish-brown and scaly with cracks and splits when older.
Twigs are moderately stout, hairy when young, somewhat shiny, with a 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) of 125 to 400+ flowers, 3 to 6 inches in width, the cluster on separate shoots from the twig. 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 small apples. 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 in 1785 - Arboreteum Americanum: The American Grove, an Alphabetical Catalogue of Forest Trees and Shrubs, Natives of the American United States. An older 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 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.
Above: Photo - Leaflets of American Mountain Ash have the widest ratio of length to width (i.e. the narrowest leaf) of the three species shown on this page - always more than 3.4x or more longer than wide. Tips gradually taper to a point. Photo courtesy USDA-NRCS Plants database. Line drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: Leaflet Comparison: Leaflets of the two other species noted on this page. 1st photo - Showy Mountain Ash, S. decora, leaflet l/w ratio less than 3.2 (usually less than 3), tips more abruptly tapering with a distinct longer tooth at tip. 2nd photo - European Mountain Ash, S. aucuparia, leaflet l/w ratio less than 3.7 (usually less than 3), leaflet tips acute to rounded, no projecting point, hairy underside.
Below: Flowers: All three species of Mountain Ash referenced on this page have similar looking inflorescences and flowers. 1st photo - the inflorescence is a wide, dense corymb - somewhat flattened branched clusters. The flowers have 5 white petals, very small sepals, numerous stamens and 3 to 4 styles from a central receptacle. Those of Showy Mt. Ash shown here.
Below: Twigs of American Mountain Ash are stout, shiny gray to reddish brown with pale lenticels. Buds are a darker reddish-purple and sticky, and lacking the hair seen on those of other species.
Below - the red drupe clusters.
Notes: American Mountain Ash is not indigenous to the Garden, but was presumably brought in by Eloise Butler in 1909, presumably because she just listed 'Mountain Ash'. She obtained it from the Park Board Nursery but what species were they growing? The question could be answered in 1917 when she again bought in a tree from the Park Board Nursery and used the name Pyrus americana which was the name in vogue at the time. Thus, it is probable that the 1909 tree was the same species. Martha Crone planted 2 in 1935 using the current scientific name.
In Minnesota American Mountain Ash grows only in 4 counties in the NE corner section of the state as that is about the limit of tree's natural western range in the U.S. It is principally found in Canada from Ontario eastward to the coast. There is some range south into the states bordering the Great Lakes and into New England. It can be found in other areas of Minnesota outside of its natural range as a landscape planting. Three species of Sorbus grow in Minnesota. See 'comparisons' above.
Uses: American Mountain Ash is a decorative species prized for showy flower clusters and bright berries of autumn, however perhaps less showy than S. decora as that species does grow better in central and southern Minnesota.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"