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Alnus Mill.

KingdomPlantaePlants, but not fungi, lichens, or algae
SubkingdomTracheobiontaVascular plants—plants with a “circulatory system” for delivering water and nutrients
DivisionMagnoliophytaFlowering plants, also known as angiosperms
ClassMagnoliopsidaDicotyledons—plants with two initial seed leaves
SubclassRosidaeRoses, legumes, proteas, dogwoods, hydrangeas, mistletoes, euphorbias, grapes, many more
OrderFagalesBirch, she-oak, beech, walnut, bayberry, others
FamilyBetulaceaeBirch family: birches, alders, hazels, hornbeams and hop-hornbeams
GenusAlnusThe classical Latin name for this genus

About plant names...

The Alnus genus consists of alder trees, of which there are about 30 species. Alders are members of the birch family. Most are small or medium-sized trees with multiple trunks and branches, often appearing as tall woody shrubs. Their small cone-like fruits, which remain on the trees throughout the winter, are their most recognizable feature.

This table compares some of the common species.


Alnus Mill.

Alder (Alnus) · 9/11/2010 · Nashua River Rail Trail, Groton Center, Groton, Mass­a­chu­setts · ≈ 6 × 4″ (15 × 10 cm) Species not yet identified

Alnus incana ssp. rugosa

Alnus serrulata

Alnus viridis
Common Name

speckled alder

smooth alder

green alder
Flowers Male flowers 2½-5″ (6.3-12 cm) long, hanging downward; female flowers about ½″ (1.3 cm) around Bright yellow in spring, make catkins green-brown, female flowers about ½″ (1.3 cm) Male flowers about 2″ (5 cm), female flowers point up, about ½″ (1.3 cm)
Leaves Oval-shaped, 2½-5″ (6.3-12 cm) long, 2-4″ (5-10 cm) wide Measure(2-4"), rounded edges or small serrations, dark green above, paler and with fine hairs below Relatively large serrations
Stem Speckled with lenticles—horizontal lens-shaped spots Bark is smooth, does not have lenticles Has diamond-shaped patches
Seeds Small cones, about ⅝″ (1.6 cm), on short stalks Small cones, about ⅝″ (1.6 cm), on long thin stalks On short stalks
Range/ Zones

Habitats Moist areas, bordering streams, lakes and rivers; bogs and nutrient-rich swamps.    
Type Wild Wild Wild


Online References:

Alnus on Flora of North America (technical description)

Alnus on Wikipedia


Sibley, David Allen, The Sibley Guide to Trees, Alfred A. Knopf, 2009, pp. 159-165

Little, Elbert L., National Audubon Society Field Guide to North American Trees, Eastern Region, Alfred A. Knopf, 1980


Alnus description by Thomas H. Kent, last updated 16 Sep 2020.

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Alnus Mill.

Green alder (Alnus viridis) · 9/20/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 4½ × 3″ (11 × 7.9 cm) ID is uncertain


About this map...