Black alder (also common alder or European alder) is a small to medium-sized deciduous tree that is distinguished by its gummy young twigs and leaves (glutinosa means sticky) and its obovate to rounded, glossy dark green leaves (to 4” long) with doubly-toothed margins and blunt to sometimes notched apices.
As a single trunk tree, it typically grows to 40-60’ tall in cultivation (to 100’ or more in its native habitat) with a narrow pyramidal form. As a multi-trunked tree, it grows much smaller and spreads. It has naturalized in many parts of eastern North America, particularly along streams and in low or swampy sites.
Flowers are monoecious. Drooping male catkins (to 4” long) form in fall and overwinter on the tree. Small, plump, rounded female catkins form in late winter to early spring. The catkins flower in March before the foliage emerges. After flowering, the female catkins develop into 3/4-inch long woody cones (strobiles), with winged seeds forming therein. The seeds disperse in fall at maturity, but the woody cones typically remain on the tree over winter and persist into the following growing season.
Alders are easily identified in winter by the presence of the drooping male catkins and the woody cones, both of which are ornamentally attractive. Winter branches are often added to floral arrangements. Dark brown bark with warty striping gives rise to the black alder common name. It shows no appreciable fall color.
Nitrogen fixing microorganisms develop in nodules on the tree roots in somewhat the same manner as with the legumes. Black alder prefers full sun to part shade and medium to wet soil. This tree has low maintenance, showy flowers and is suitable for rain gardens. Black alder tolerates drought, wet and clay soils as well as air pollution.