Sorbus americana (American Mountain-ash)
|Also known as:|
|Life cycle:||perennial woody|
|Habitat:||part shade, sun; moist; cool forest, swamps, stream banks, forest edges, rocky shores, bluffs|
|Bloom season:||June - July|
|Plant height:||10 to 30 feet|
|Wetland Indicator Status:||GP: none MW: FAC NCNE: FAC|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Flat or rounded cluster to 5 inches across of short-stalked flowers at the tips of 1-year-old branches, blooming when leaves are mature. Flowers are ¼ to 1/3 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 19 leaflets. Leaflets are lance-oblong, 1½ to 3 inches long, 3/8 to ¾ inch wide (3.3 to 4+ times as long as wide), tapering 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 on the underside. The compound leaf stalk is hairless and often reddish. Leaves turn red in fall. Buds are shiny, sticky, green to purple, and hairless or with short, yellowish to light brown hairs at the tip and/or around the edge of scales. New twigs are green to brown and hairless, becoming reddish brown with scattered, pale lenticels (pores) the second year. Older bark is gray to bronzy, smooth but developing plates that eventually peel off. Trunks are single or multiple from the base and can reach up to 10 inches diameter at breast height (dbh).
American Mountain-ash reaches the western edge of its range in Minnesota and typically grows as a small tree in swamps and cool, moist, forested edges, but in drier or harsher conditions, such as the rocky north shore of Lake Superior, it can take on a smaller, more shrub-like growth. It is easily mistaken for the related, and much more common, Showy Mountain-ash (Sorbus decora), and it is highly likely that the 2 herbarium records on the county distribution map that are outside of the Arrowhead region are actually S. decora. It has leaflets that are less than 3.3 times as long as wide (divide length by width to determine), typically hairy along veins on the underside, and has somewhat larger flowers and fruits (both to ½ inch). European Mountain-ash (Sorbus aucuparia), a landscape tree than occasionally escapes cultivation, also has smaller leaflets, about half the size of American Mountain-ash, and is hairy on leaf stalks, sepals, buds, and leaflets, 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.
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- American Mountain-ash on the shore of Lake Superior
- American Mountain-ash tree
- American Mountain-ash tree
- scan of compound leaf
- more leaves
- leaves emerging in spring
Photos courtesy Peter M. Dziuk taken in Cook County.
Have you seen this plant in Minnesota, or have any other comments about it?