A shrub or tree up to 30 ft high, with a trunk 11⁄2 to 2 ft in girth; young shoots red, and covered at first with a fine down, glabrous by autumn; buds stalked, downy. Leaves oval or ovate, 2 to 4 in. long, two-thirds as wide, rounded or slightly heart-shaped at the base, pointed; veins in about ten pairs, each vein ending at the point of a toothed lobe; dark green above, with down on the midrib and nerves; paler green and more or less downy beneath; stalk 1⁄2 to 1 in. long, downy. Male catkins expanding in March in clusters of three or four, each 11⁄2 to 21⁄2 in. long. Fruits narrowly egg-shaped, 1⁄2 to 5⁄8 in. long, three to five in a cluster.
Native of western N. America, from British Columbia to California. It is, perhaps, most nearly allied to A. rubra, but the leaves are not greyish beneath, the male catkins are shorter, and the fruits smaller. According to Sargent, the wing of the seed in A. tenuifolia is reduced to a narrow border, whilst it is broad in A. rubra.
var. occidentalis (Dipp.) Callier A. occidentalis Dipp. – This distinct variety was introduced to Europe by A. Purpus, and first cultivated at Zoeschen in Germany by Dr Dieck, from whom it came to Kew in 1889. The leaves are larger than in the type, and on young trees very large; at Kew they have been 7 in. long by 5 in. wide. Veins in ten to twelve pairs, the entire under-surface at first downy. Fruits 5⁄8 to 7⁄8 in. long. Native of British Columbia and Oregon.
From the Supplement (Vol. V)
The relationship of this alder is with A. rugosa rather than with A. rubra, and it has, like that species, been given subspecific rank under A. incana as A. incana subsp. tenuifolia (Nutt.) Breitung (which also includes A. tenuifolia var. occidentalis). It ranges in the north as far as southern Alaska, and in the south to northern Mexico.