Alnus incana 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 doubly-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 have only 2 outer bud scales.

Flowers of Alnus incana 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 (Symplocarpus foetidus) and the earliest silver maples (Acer saccharinum). The mature female ament is a tough, persistent structure that disperses the mature fruit in summer and often remains on the plant through the next winter (providing a handy character for identification). The single-seeded dry fruit is narrowly winged or nearly wingless in some cases.

Speckled alder grows throughout Wisconsin in wet soils and full sun to very light shade. It is the namesake of a type of wetland known as "alder thicket". It is also found in or adjacent to sedge meadows, 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. Wisconsin plants of this species belong to the subspecies rugosa and were long known as Alnus rugosa (DuRoi (Spreng.) in the Midwest and that name is likely to be found in many of the botanical books for this area. The largest individual of Alnus incana known in the state of Wisconsin is located in Brown County.