5a. Alnus incana subsp. rugosa (Du Roi) R. T. Clausen, Cornell Univ. Agric. Exp. Sta. Mem. 291: 8. 1949.
Speckled alder, tag alder, swamp alder, aulne blanchâtre
Betula alnus Linnaeus var. rugosa Du Roi, Diss. Observ. Bot., 31. 1771; Alnus glauca Michaux; A. incana var. americana Regel; A. rugosa (Du Roi) Sprengel var. americana (Regel) Fernald; A. rugosa var. rugosa; A. rugosa var. tomophylla (Fernald) Fernald
Shrubs , open, spreading, to 9 m. Bark dark grayish to reddish brown, smooth; lenticels whitish, prominent, horizontal. Winter buds parallel to twig, ellipsoid, 3--7 mm, apex acute or obtuse; stalks 2--4 mm. Leaf blade ovate to elliptic, 4--11 × 3--8 cm, firm, base cuneate to narrowly rounded, margins usually coarsely doubly serrate, toothed to base, apex acute or short-acuminate to slightly obtuse; surfaces abaxially often glaucous, glabrous to sparsely pubescent, often more densely so on veins, slightly to not noticeably resin-coated. Inflorescences: staminate catkins in 1 or more clusters of 2--4, 2--7 cm; pistillate catkins in 1 or more clusters of 2--6. Infructescences ovoid, 1--1.7 × 0.8--1.2 cm; peduncles 1--5 mm. 2 n = 28.
Flowering early spring. Stream banks, lake shores, bogs, swamps, margins of wet fields, swales, and roadsides, often forming dense thickets; 0--800 m; St. Pierre and Miquelon; Man., N.B., Nfld., N.S., Ont., P.E.I., Que., Sask.; Conn., Ill., Ind., Iowa, Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.Dak., Ohio, Pa., R.I., Vt., Va., W.Va., Wis.
Alnus incana subsp. rugosa is an important shoreline and meadow colonizer in boreal and north temperate areas of the Canadian Shield, and a weedy successional species in damp areas along roadsides throughout its range. It overlaps in range and intergrades with A . incana subsp. tenuifolia to the west (in Saskatchewan and Manitoba) and with A . serrulata to the south. It is only slightly differentiated from the more treelike European A . incana subsp. incana .