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Overview
Alnus rubra is a large deciduous tree reaching 40–80 feet (12–24 m) tall and 20–30 feet (6–9 m) wide, with a narrow, upright to broadly columnar crown and smooth, pale gray bark with white patches from lichens. Leaves are ovate to elliptical with doubly serrated margins and a slightly rolled-under edge, 3–5 inches (8–13 cm) long, dark green above with a faintly rusty or greenish underside — the reddish tint of young buds and inner bark gives the species its common name. Bark is smooth and gray-white, becoming slightly rougher with lenticels on older trees. In late winter (February through March), pendulous male catkins 3–5 inches (8–13 cm) long appear before leaf emergence; female catkins are smaller and become hard, woody, cone-like structures 0.5–1 inch (1.5–2.5 cm) long that persist through winter. Growth rate is fast, 2–4 feet (60–120 cm) per year in favorable sites. Red alder fixes atmospheric nitrogen through a symbiotic relationship with the actinobacterium Frankia, significantly enriching soil nitrogen levels and facilitating the establishment of subsequent plant communities. It is a primary successional species in the Pacific Northwest, colonizing disturbed sites, logged areas, and stream banks rapidly. The tree is short-lived for its size — 50–100 years — and does not transplant well as a large specimen. Widely used in riparian restoration, erosion control, and large-scale habitat planting throughout the PNW.
Native Range
Native to western North America from southeastern Alaska south through British Columbia, Washington, Oregon, and California to Baja California. Found in moist soils along streams and rivers, in bottomlands, disturbed ground, and lower coastal forest slopes from sea level to approximately 2,500 feet (760 m) elevation.Suggested Uses
Used primarily in riparian restoration, stream bank stabilization, erosion control, wetland buffer plantings, and large-scale habitat restoration throughout the Pacific Northwest. Nitrogen fixation makes it valuable for establishing vegetation on nutrient-poor, disturbed, or recently logged soils. Not suited to small residential landscapes due to size, shallow roots, and litter. Provides nesting habitat and winter food for birds through the persistent woody cones.How to Identify
Appearance
Size & Dimensions
Height40' - 80'
Width/Spread20' - 30'
Reaches mature size in approximately 15 years
Colors
Bloom Information
Blooms February through March, with pendulous male catkins 3–5 inches (8–13 cm) long appearing before leaf emergence — among the earliest native trees to bloom in the Pacific Northwest. Female catkins are smaller and green, maturing by late spring into hard, woody, cone-like structures 0.5–1 inch (1.5–2.5 cm) long. Woody female cones persist through winter, providing winter identification and bird foraging habitat.Detailed Descriptions
Flower Description
yellow-brown (male catkins)Foliage Description
dark green above, pale greenish-rusty beneath; yellow-green in fallGrowing Conditions
Sun Requirements
Requires 4-8 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Plant in moist to wet soil — red alder is not suitable for dry, well-drained upland sites without regular irrigation. Performs best in full sun with consistent soil moisture; established trees in riparian zones are largely self-sufficient. The tree fixes nitrogen and does not require fertilizing. Plant from seed or small container stock — alder does not transplant reliably as a large specimen due to its extensive, shallow lateral root system. In garden settings, the tree drops catkins, leaves, and woody cones; site away from paving and water features if litter is a concern. Provides rapid cover and ecological function on disturbed or degraded sites but will eventually be overtopped by conifers in forest succession.Pruning
Prune during dormancy (November through January). Remove dead, damaged, or crossing branches. The tree develops a straight central leader naturally; minimal corrective pruning is needed. Large wounds from heavy pruning compartmentalize slowly and can introduce decay. Avoid removing more than 20% of the canopy in a single season. In restoration plantings, pruning is rarely necessary.Pruning Schedule
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winter