Alnus viridis is most often a shrub (perhaps occasionally a small tree) with simple, alternate, finely toothed leaves. The female aments (catkins) of Alnus viridis are covered by exterior bud scales and the winter leaf buds are covered by 3 or more imbricate outer scales and they are short-stalked. The female aments of our other native alder species, Alnus rugosa, are lacking the outer scales and the leaf buds have only 2 outer bud scales and are conspicuously stalked. Flowers of Alnus viridis open as the leaves unfold, much later than A. rugosa in any given area. The single-seeded dry fruits are winged (samaras).

Alnus viridis is much less common in Wisconsin than Alnus incana, being relatively common mostly in the counties near Lake Superior. It appears to be more closely associated with banks and sandy shorelines rather than the low wet habitats of Alnus incana. Wisconsin plants of Alnus viridis belong to subspecies crispa and were long known as Alnus crispa (Aiton) Pursh. in the Midwest and may be referenced under that name in many botanical books for the area.