What hardiness zones is Alnus rubra suitable for?

Alnus rubra is suitable for USDA hardiness zones 4, 5, 6, 7, 8, 9.

How much water does Alnus rubra need?

Alnus rubra has High water needs.

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Broadleaf Deciduous Trees

Alnus rubra

red alder

Betulaceae

Western North America, southeastern Alaska to Baja California

At a Glance

TypeTree

HabitUpright

FoliageDeciduous

Height40-80 feet (12-24 m)

Width20-30 feet (6-9 m)

Maturity15 years

Growing Zones

USDA Hardiness Zones

4 - 9

Zone 4

Zone 5

Zone 6

Zone 7

Zone 8

Zone 9

These zones indicate the coldest temperatures this plant can typically survive.

Frost Tolerancehardy

Key Features

Native to North America

Maintenancevery low

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

Identified by ovate leaves with doubly serrated margins and a distinctive rolled-under edge, 3–5 inches (8–13 cm) long. Pale gray-white smooth bark with horizontal lenticels and lichen patches is characteristic. Hard, woody, cone-like female catkins 0.5–1 inch (1.5–2.5 cm) long persist on the branches through fall and winter — a reliable year-round identification feature. Long pendulous male catkins 3–5 inches (8–13 cm) appear in late February through March before leaf-out. Inner bark, when scraped, is reddish-orange.

Appearance

Size & Dimensions

Height40' - 80'

Width/Spread20' - 30'

Reaches mature size in approximately 15 years

Colors

Flower Colors

yellow

brown

Foliage Colors

dark green

Fall Foliage Colors

yellow

green

Bloom Information

Bloom Period

~4 weeks

Spring

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 fall

Growing Conditions

Sun Requirements

Full Sun

Partial Shade

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

Soil Requirements

pH Range5.0 - 7.0(Neutral)

357912

Soil Types

loamclaysiltsand

Drainage

moist

Water & Climate

Water Needs

High

Frost Tolerance

hardy

Time to Maturity

10-20 years

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

winter

Maintenance Level

very low

⚠️ Toxicity Warning

Non-toxic