Showy Mountain Ash is a native deciduous small tree or large shrub growing to 30 feet high and as a tree, up to 6 inches in diameter. 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 purplish to brownish-gray, smooth, and with lenticels when young but becomes a lighter gray and scaly with cracks and splits when older.
Twigs are stout, hairy when young, somewhat shiny, with a reddish-brown color. Buds are a darker color and sticky.
The leaves are pinnately compound, 4 to 10 inches long with 15 (usually, but sometimes 11 to 17) stalkless leaflets. Leaflets are each up to 1-1/4 to 2-3/4 inches long and up to 1 inch wide [important - leaflets are less than 3x as long as wide], dull green to bluish-green above, the underside a paler color but without hair. Leaflets are elliptical to oblong in shape, abruptly tapering to a short-pointed tip, the tip tooth longer than the others; the leaflet margins serrate, at least in the upper 1/2, but with teeth more fine that European Mt. Ash, S. aucuparia. The leaves have reddish stalks and are alternate on the twigs. Fall color is a deep yellow.
The inflorescence is a dense showy upright, somewhat flattened branched cluster (a corymb) of 75 to 400+ flowers, 3 to 6 inches in width, the cluster on separate shoots from the twig. Cluster stalks may have sparse to dense fine hair.
The flowers: Each flower is stalked, about 3/8 inch wide with 5 rounded white petals that have narrowed bases. The flowers are perfect, stamens number 15 to 20, have white filaments and reddish-brown anthers; there are 3 or 4 styles. The central receptacle is yellow-green in color. The outer sepals are small, are also yellow green with 5 pointed lobes. The hypanthium of the flower is without dense hair.
Fruit: Each fertile flower produces a small orange-red pome, about 3/8 to 1/2 inch in diameter, looking like small apples. These contain a few seeds and have a bitter taste. These mature in early autumn and can persist on the tree into the winter.
Habitat: Showy 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 decora means decorative - the showy corymb of flowers. The author names for the plant classification are as follows: The first to publish was ‘Sarg.’ which refers to Charles Sprague Sargent (1841-1927), American botanist, first director of the Arnold Arboretum and specialist on trees of the American forest. His work was extended by ‘C.K.Schneid.’ - Camillo Karl Schneider (1876-1951), Austrian botanist who published several important papers on trees.
Comparisons: S. decora 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 densely hairy with gray hair, and are not sticky. Leaflets are on average also less than 3x long as wide, but leaflet tips acute to rounded, no projecting point, and the underside is hairy. The flower hypanthium has dense fine hair. Flowers and fruit are the same size. S. americana, American Mountain Ash, has lateral leaflets averaging more than 3x as long as wide and are finely hairy beneath on the veins. Flowers and fruit are smaller. Twig buds are a dark reddish-purple and sticky.
Above: The flowers from a dense upright showy cluster. Flowers have 5 rounded petals, stamens with reddish-brown anthers and a yellow-green central receptacle.
Below: The leaves have 11 to 17 stalkless leaflets with serrate edges. Leaflets are less than 3x as long as wide and have a projecting tip tooth.
Below: 1st photo - Fruit is an orange-red pome with a bitter taste. 2nd photo -Older bark is a gray color and scaly with cracks and splits
Below: Twigs are hairy when young, somewhat shiny, with a reddish-brown color. Winter buds are a darker color with fine hair on the scales and sticky. Younger bark (2nd photo) is smoother with lighter colored lenticels showing. Older bark (3rd photo) is a gray color and scaly with cracks and splits
Below: 1st photo - This young tree is more pyramidal in shape. With age, the crown will open and spread. 2nd photo - Maturing fruit is held in clusters along the outside edge of the branches.
Below: The Fall leaf color, contrasting with the reddish stalks.
Notes: Showy Mountain Ash is not indigenous to the Garden, but has been present for some time as the tree is quite large. There is no clear record of when it was planted, although Eloise Butler's log shows a mountain ash planted in 1931 without a species identified. The large old specimen at the top of the Fern Grove shows leaves to be Showy Mountain Ash, but that area was not part of the Garden until 1944.
In Minnesota Showy Mountain Ash is found in a group of counties in the NE 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 Saskatchewan 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, such as in the metro area where it does well. Three species of Sorbus grow in Minnesota. See 'comparisons' above.
Uses: Showy Mountain Ash is a decorative species prized for showy flower clusters and bright berries of autumn. It and European Mountain Ash, S. aucuparia would be better choices for Central and Southern Minnesota than the American Mountain Ash, S. americana.
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"