Flower:

Male and female flowers are borne separately on the same plant (monoecious), in clusters called catkins, blooming in spring when leaves emerge. Two to 4 male catkins are clustered at the tip of 1 year old twigs, pendent in flower, 1¼ to 3½ inches long. At anthesis, female catkins are erect, red, narrowly cylindric, up to about 1/3 inch long, in one or more separate clusters near the male catkins on the same branch, with 2 to 10 catkins in a cluster. Catkins develop in late summer, the males as slender spikes of tightly appressed scales and females more bud-like, persist through winter and open up the following spring.

Leaves and stems:

Leaves are alternate and simple, 2 to 3½ inches long, 1 to 2½ inches wide, egg-shaped to elliptic, pointed to rounded at the tip, the base rounded to somewhat heart-shaped, on a hairy stalk up to about ¾ inch long. Edges are finely toothed and not double-toothed. The upper surface is dark green, hairless to sparsely hairy, the lower surface shiny green and hairless to sparsely hairy, especially along the veins, and typically sticky when young. Twigs are brown to reddish brown to grayish, with scattered white lenticels (pores), new growth hairless to sparsely hairy and becoming hairless and smooth the second year. Buds are erect, elliptic, have 4 to 6 scales and are stalkless or nearly so.

Older bark is grayish to reddish brown, smooth with scattered pale horizontal lenticels. Stems are multiple from the base, the larger trunks up to 1¾ inches diameter.

Fruit:

Female catkins become stout, oval to egg-shaped, cone-like clusters up to about ¾ inch long, containing broadly-winged nutlets each 1/8 inch long, green drying to brown.

Notes:

Green Alder is a shrub of our northern forests and the rocky north shore of Lake Superior. It is very similar to the related Speckled Alder (Alnus incana), which has buds that are more distinctly stalked, narrowly-winged nutlets, leaves that are not shiny on the underside, typically more coarsely double-toothed with shallowly-lobed edges, female catkins are shorter, stalkless and not erect, and flowers open before leaves emerge in spring. While Green Alder can spread from root suckers, it spreads slowly and rarely forms dense thickets like Speckled Alder does. There are at least 4 recognized subspecies of Aluns viridis: subsp. viridis limited to Europe, subsp. sinuata and fruticosa present in the northwest US into Canada and Alaska with double-toothed leaves, and subsp. crispa with finely serrated leaves present throughout most of Canada and into New England and the northern Great Lakes region, including Minnesota.