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Trees & Shrubs of the Eloise Butler Wildflower Garden

The oldest public wildflower garden in the United States

Speckled Alder

Common Name
Speckled Alder (Swamp Alder, Tag Alder)

 

Scientific Name
Alnus incana subsp. rugosa (Du Roi) R. T. Clausen

 

Plant Family
Birch (Betulaceae)

Garden Location
Woodland - Marsh

 

Prime Season
Spring flowering

 

 

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.

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 in 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: As 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 generally remain 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: The common name, Speckled Alder, refers to the bark that appears 'speckled' by the many lenticels. For many years the scientific name of this plant was A. rugosa, but that has now be classified as a subspecies of A. incana, the Gray Alder. 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.

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.

See bottom of page for notes on the Garden's planting history, distribution in Minnesota and North America, lore and other references.

tree drawing

Above: A multi-stem shurb in fall color. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.

Below: Leaves are broadest near or below the middle, with an irregular double saw-toothed edge, with a network of 9 to 12 parallel veins that appear sunken into the leaf producing a wrinkled appearance. The underside has soft hair and a hairy leaf stalk. 2nd photo - Twigs are reddish-brown to gray-brown with stalked overwintering buds that grow mostly parallel to the twig.

Leaf Speckled Alder twig

Below: 1st photo - The male flowers are on catkins that form at the end of twigs in the fall and then flower in the spring. Spring catkins before elongation shown. 2nd & 3rd photo - Bark is smooth and can be reddish-brown to gray in color as these two examples show. There are prominent whitish horizontal lenticels that sometimes shade to light orange in color.

Speckled Alder Spring Catkins Speckled Alder Bark Speckled Alder Bark

Below: 1st photo - Male catkins elongating in the spring. 2nd photo - Detail of the male flowers. Stamens are clustered beneath a brown bract.

Speckled Alder male catkins Speckled Alder male flowers

Below: The female catkins. These are small cone shapes just back from the much larger male catkins. The individual reddish female are quite small.

Speckled Alder male and female catkins Speckled Alder female flowers

Below: Fertilized female flowers mature to a green cone that becomes brown to blackish and hard and contains small rounded brown nutlets that then drop from the cone. Old cones remain on the shrub over winter.

Speckled Alder cone Speckled Alder green cones

Below: 1st photo - the seeds of Speckled Alder with their leathery wings, photo ©Steven Hurst. 2nd photo - a comparison leaf of A. viridis, the Mountain Alder, the only other Alnus species in Minnesota. Photo ©Robert H. Mohlenbrock. Both photos ©USDA-NRCS Plants Database.

seeds mountain alder leaf

Notes:

Notes: Speckled Alder is not indigenous to the Garden. Eloise Butler planted it in Oct. 1912 with plants obtained from the Fort Snelling area (Minneapolis). She planted again in 1913. Cary George reported planting it in 1998. Garden Curator Susan Wilkins added additional plants in 2008. In North America, Speckled Alder is found in the NE Section. From Saskatchewan and Manitoba eastward in Canada and in the US from North Dakota eastward to the coast, south as far as Illinois and Virginia. Within Minnesota it is found east and north of a diagonal line drawn from Houston county in the SE to Norman and Polk in the NW. This includes Hennepin, Wright and Dakota counties in the metro and those further east.

Uses: The wood of Speckled Alder has no commercial use, nor is it a preferred browse plant for animals. It is a good colonizer of shorelines and thus good for erosin control.

References and site links

References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.

graphicIdentification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.



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