Flower:

Flat or rounded cluster to 6 inches across of short-stalked flowers at the tips of 1-year-old branches, blooming when leaves are mature. Flowers are ¼ to ½ inch across, white with 5 rounded petals. In the center are 2 to 4 short, yellowish styles surrounded by about 20 white, ascending to spreading stamens. The calyx around the base of the flower has 5 short, triangular lobes, is usually hairless though may have a few glands. Flower stalks are hairless or with sparse, long, soft hairs.

Leaves and bark:

Leaves are alternate, 6 to 10 inches long, compound with 11 to 17 leaflets. Leaflets are lance-oblong, 1½ to 2½ inches long, ½ to 1 inch wide (2.5 to 3.3 times as long as wide), often abruptly tapered to a pointed tip, asymmetrical and rounded to wedge-shaped at the base, and stalkless. Edges are sharply toothed, sometimes only on the tip half, and the tooth at the very tip is typically slender and elongated. Surfaces are initially downy hairy but quickly become smooth, though a few hairs may persist along the midvein and vein axils on the underside. The compound leaf stalk is hairless and often reddish. Leaves turn red in fall.

Buds are shiny, sticky, dark purple, and with short, yellowish to light brown hairs at the tip and/or around the edge of scales. New twigs are green to brown, hairless to hairy, becoming reddish brown and smooth with scattered, pale lenticels (pores) the second year.

Older bark is gray to gray-brown, smooth but developing plates and scaly patches that eventually peel off. Trunks are single or multiple from the base and can reach up to 12 inches diameter at breast height (dbh).

Fruit:

Fruit is a cluster of bright red, ¼ to ½-inch, round, berry-like pomes (fleshy with a chamber of seeds in the center, like an apple). Fruit matures in late summer and persists through winter.

Notes:

Showy Mountain-ash reaches the western edge of its US range in Minnesota and typically grows as a small tree on bluffs, in upland and swamp forests, and is a common sight on the rocky north shore of Lake Superior. It is easily mistaken for the related, and less common, American Mountain-ash (Sorbus americana), which has leaflets that are 3.3 times or more as long as wide (divide length by width to determine), typically hairless all over, and has somewhat smaller flowers and fruits (both to 1/3 inch). European Mountain-ash (Sorbus aucuparia), a landscape tree than occasionally escapes cultivation, has smaller leaflets up to 1½ inch long, and is hairy on leaf stalks, sepals, leaflets, has more densely hairy buds, and the terminal tooth on leaflets is much the same as the lateral teeth, not slender and elongated. Note that Mountain-ash is unrelated to Ash trees (Fraxinus species) and not at risk from emerald ash borer.