Trees & Shrubs of the Eloise Butler Wildflower Garden

Speckled Alder is a fast-growing low clump forming shrub, making dense thickets, or that can develop a single trunk for a small tree, up to 20 feet high with open spreading branches. There are two recognized subspecies.

The bark is a smooth reddish-brown to gray, with prominent whitish horizontal lenticels that sometimes shade to light orange in color.

Twigs are reddish-brown to gray-brown, slender, slightly hairy when young and with a 3-angled pith. The over-wintering buds are plump, stalked, parallel to the twig, and reddish-brown with two outer scales.

Leaves are alternate, simple, elliptical to ovate, 2 to 4 inches long and 1-1/4 to 3 inches wide, broadest near or below the middle, with an irregular double saw-toothed edge (pointed teeth). Some leaves may have shallow lobes. The base is usually rounded and the tip roundly tapers to a soft point. The upper side is a dull dark green with a network of 9 to 12 parallel veins that appear sunken into the leaf producing a wrinkled appearance. The underside is paler, often with soft hair and a hairy leaf stalk. Leaves do not have a noticeable resin-coat.

Flowers: The shrub is monoecious, that is there are separate male and female flowers on the same plant. The male flowers are on drooping clusters of 2 to 6 slender greenish catkins the formed the previous fall at the end of twigs and then, like many birches, elongate in the spring. The small male flowers on the catkins are yellowish-green contained behind a brown scale. There are 3 flowers per scale, each flower has 4 stamens. The female flowers are also formed late in the growing season but remain un-exposed during the winter. These are very small and in the Spring are formed into a 1/2 inch long short-stalked reddish cone shape at the ends of twigs. Pollination is by wind. The female cone also has scales with usually two flowers per scale. Scales are 5 lobed.

Seed: The fertilized female flowers mature to a green cone that becomes brown to blackish and hard, with the 5-lobed scales each harboring two small rounded brown nutlets that may have two small leathery wings. These cones release their seed in the summer but the cone structure generally remains intact on the tree over the following winter. The plant needs to be 5 to 10 years old before flowering begins.

Habitat: Speckled Alder grows in moist to wet areas such as marshes, stream banks and roadside ditches as long as the site is open and not understory. The root system is rhizomatous, shallow and spreading and has nodules that support nitrogen fixing bacteria. Thickets are usually formed from the underground rhizomes, not from seed. Re-sprouting from crowns is common.

Names: In prior years there were a number of subspecies grouped under the name Alnus incana, the Gray Alder. These have been reduced to two. For many years the scientific name of our plant was A. rugosa, but that has now been classified as a subspecies of A. incana. The common name, Speckled Alder, refers to the bark that appears 'speckled' by the many lenticels. The genus Alnus is Latin for alder, the species, incana, means gray and rugosa, refers to the wrinkled appearance of the leaves. As to the authors for the plant classification, first must be mentioned ‘Du Roi’ which refers to Johann Philipp Du Roi (1741-1785), German physician and botanist who published work on North American trees. His major work was Dissertatio inauguralis observationes botanica. His work was updated by ‘R.T.Clausen’ which refers to Robert Theodore Clausen (1911-1981), American botanist, Professor of Botany at the L. H. Baily Hortorium and for 23 years curator of Cornell’s Wiegand Herbarium. His major research contributions were in the genus Sedum.

Comparisons: Only one other alder is native to Minnesota and it could be confusing with our species - the Green (or Mountain) Alder, Alnus viridis (Vill.) DC. subsp. crispa, but its range is restricted to the NE part of the state. In that species the leaf edge is finely saw-toothed, the leaf appears to have a resinous coating (see comparison leaf image below.) and the bark does not have the lenticels. Also, the winter buds are not stalked.

Subspecies: The 2nd subspecies of A. incana is subsp. tenuifolia, which is a plant of the western North America. Its leaves are thin and papery and the secondary teeth are blunt or rounded. It also tends to be more tree-like in shape.