4. Alnus glutinosa (Linnaeus) Gaertner, Fruct. Sem. Pl. 2: 54. 1790.
Black alder, European alder
Betula alnus Linnaeus var. (a) glutinosa Linnaeus, Sp. Pl. 2: 983. 1753
Trees , to 20 m; trunks often several, crowns narrow. Bark dark brown, smooth, becoming darker and breaking into shallow fissures in age; lenticels pale, horizontal. Winter buds stipitate, ellipsoid to obovoid, 6--10 mm, apex obtuse; stalks 2--5 mm; scales 2--3, outer 2 equal, valvate, usually heavily resin-coated. Leaf blade obovate to nearly orbiculate, 3--9 × 3--8 cm, leathery, base obtuse to broadly cuneate, margins flat, coarsely and often irregularly doubly serrate to nearly dentate, major teeth acute to obtuse or rounded, apex often retuse or obcordate, or occasionally rounded; surfaces abaxially glabrous to sparsely pubescent, often more heavily on veins, both surfaces heavily resin-coated. Inflorescences formed season before flowering and exposed during winter; staminate catkins in 1 or more clusters of 2--5, 4--13 cm; pistillate catkins in 1 or more clusters of 2--5. Flowering before new growth in spring. Infructescences ovoid to nearly globose, 1.2--2.5 × 1--1.5 cm; peduncles 1--10(--20) mm. Samaras obovate, wings reduced to narrow, thickened ridges. 2 n = 28.
Flowering early spring. Stream banks, moist flood plains, damp depressions, borders of wetlands; 0--200 m; Ont.; Conn., Ill., Ind., Iowa, Mass., Mich., Minn., N.J., N.Y., Ohio, Pa., R.I., Wis.; Europe.
Alnus glutinosa is cultivated as an ornamental tree throughout eastern North America and is available in a variety of cultivars, including cut-leafed and compact-branching forms. This species has also been used extensively to control erosion and improve the soil on recently cleared or unstable substrates, such as sand dunes and mine spoils. It has escaped and become widely naturalized throughout the temperate Northeast, occasionally becoming a weedy pest. In Europe the black alder has served for many centuries as an important source of hardwood for timbers and carved items, including wooden shoes.
Alnus glutinosa has been called A . vulgaris Hill in some older literature; that name was not validly published.