Alnus Mill.

Alder

Some alders compared

	Alnus incana	Alnus incana ssp. rugosa	Alnus serrulata
Common Name	grey alder	speckled alder	smooth alder
Plant	Small to medium in size, often with multiple trunks and branches. Speckled alders grow to 12–36′ (3.7–10 m), rarely reaching 60′ (18 m), usually less than 20′ (6.1 m) tall. “Speckled” refers to the presence of many lenticels—white, or light gray or orange oval patches, wider than they are flat, on the trunk.	Trunk is speckled with lenticles—horizontal lens-shaped spots.	8–13′ (2.5–4 m) tall, somewhat disheveled-look­ing trees, with many trunks and branches that form a nearly impenetrable thicket. Bark is smooth, lacking lenticels.
Leaves	Dark green, alternate, roughly oval, 2–4″ (5–10 cm) long, and doubly serrated.	Oval-shaped, 2½–5″ (6.3–12 cm) long, 2–4″ (5–10 cm) wide	2–5 ⨉ 1–2½″ (5–12 ⨉ 3–7.1 cm), obovate, with rounded edges or small serrations, dark green above, paler and with fine hairs below.
Flowers	Male flowers are catkins 1½–3½″ (3.8–8.9 cm) long. Female flowers are cylindrical catkins attached directly to the branch, ⅛″ (4.8 mm) long.	Male flowers 2½–5″ (6.3–12 cm) long, hanging downward; female flowers about ½″ (1.3 cm) around	Bright yellow in spring, becoming brown-green, male catkins hang in groups of 2-5, and are 2–4″ (5–10 cm) long. Female flowers, also catkins, are upright, cylindrical, up to ¼″ (6.3 mm) long, and bright red or reddish-green. Flowers appear from March to April.
Fruit	Egg-shaped cones about ⅝ ⨉ ¼″ (1.6 cm ⨉ 7.9 mm), green at first, becoming dark brown and often persisting through the winter.	Small cones, about ⅝″ (1.6 cm), on short stalks	Small cone-like fruits called strobiles, which remain on the tree through the winter. They are up to ¾″ (1.9 cm) long, containing winged nutlets.
Range/ Zones			USDA Zones: 4-9
Habitats	Swamps and wetlands.	Moist areas, bordering streams, lakes and rivers; bogs and nutrient-rich swamps.	Moist soil near streams, pond margins, and riversides.

	Alnus viridis
Common Name	green alder
Plant	Small trees 9½–39′ (3–12 m) in height, with mulitple trunks and smooth, gray bark. Green alders have lenticels—white or light gray patches, that are roughly diamond-shaped.
Leaves	Alternate, egg-shaped to elliptic, with pointed or rounded tips. They are dark green on top, shiny green below, with finely toothed edges, and 1–3 ⨉ ¾–2″ (3–8 ⨉ 2–6 cm) in size.
Flowers	Male and female flowers appear on the same tree. Male catkins are 1¼–3½″ (3.2–8.9 cm) long, hanging in groups of two to four. Female catkins are erect (upward-pointing), red, roughly cylindrical, and up to ¼″ (8.5 mm) tall. They appear on the same branches as male catkins, in groups of 2-10. Flowers appear in late summer.
Fruit	Oval to egg-shaped small “cones" about ¾″ (1.9 cm) long, green, turning brown, with winged nutlets ⅛″ (3.2 mm) long.
Range/ Zones
Habitats	Alpine or subalpine zones, forest edges, meadows and fields, shores of rivers or lakes, swamps, talus and rocky slopes, wetland margins.

Alnus description by Thomas H. Kent, not updated.

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