Speckled alder can occasionally grow to tree size, but is most often considered a large shrub. It commonly grows in wet soils and develops the multiple stemmed growth form as shown above. Leaves are simple and alternate, with toothed margins and often a few very shallow lobes as shown in the leaf photo above. The bark is often marked with conspicuous light-colored, horizontal lenticels, and the pith of twigs is angled or star-shaped. The relatively large, blunt leaf buds are stalked and there are only 2 outer bud scales.

Flowers of speckled alder are in the form of conspicuous, elongate aments (catkins) of male flowers and much smaller reddish aments of female flowers, lacking outer scales ("naked" female ament buds). They are among the first flowers to open in the spring in northeastern Wisconsin in late March or early April, preceded only by skunk cabbage and the earliest silver maples. The mature female ament is a tough, woody structure that disperses its unwinged achenes (single-seeded dry fruits) in summer and often remains on the plant through the next winter (providing a handy character for identification).

Speckled alder grows throughout Wisconsin in wet soils and full sun to very light shade. Common habitats are adjacent to sedge meadow, shrub carr and swamps, along streams and in roadside ditches. It sometimes aggressively colonizes cut-over northern conifer swamps, as appears to be the case in many dense alder stands along small streams in northern Wisconsin. The largest individual of speckled alder known in the state of Wisconsin is in Brown County. This species was long known as Alnus rugosa in the Midwest and that name is likely to be found in many of the botanical books for this area.