What hardiness zones is Alnus suitable for?

Alnus is suitable for USDA hardiness zones 3, 4, 5, 6, 7, 8.

Where is Alnus native to?

The genus Alnus is distributed across the Northern Hemisphere, with species native to North America, Europe, and Asia. North American species include A. rubra (Pacific Northwest), A. incana subsp. {rugosa} (eastern wetlands), and A. rhombifolia (western stream banks). Grows along streams, riverbanks, bogs, and wet lowlands from sea level to 8,000 feet (2,400 m) elevation.

Broadleaf Deciduous Trees

Alnus spp.

Alder

Betulaceae

Northern Hemisphere (North America, Europe, Asia)

Learn more

At a Glance

TypeTree

HabitUpright

FoliageDeciduous

Height30-60 feet (9-18 m)

Width20-40 feet (6-12 m)

Maturity20 years

Growing Zones

USDA Hardiness Zones

3 - 8

Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8

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

What's my zone? →

Frost Tolerancehardy

Key Features

Attracts Pollinators

Native to North America

Maintenancelow

Overview

Alnus spp. is a genus of approximately 35 species of deciduous trees and large shrubs in the family Betulaceae, reaching 30–60 feet (9–18 m) tall and 20–40 feet (6–12 m) wide depending on species. Bark is smooth and gray on young trees, becoming dark and fissured with age in most species. Leaves are alternate, ovate to elliptic, 2–5 inches (5–13 cm) long with toothed margins and prominent parallel veins. Male and female flowers are borne as separate catkins on the same tree (monoecious). Male catkins are pendulous, 2–4 inches (5–10 cm) long, yellowish-brown, appearing in late winter before leaf emergence. Female catkins develop into small, woody, cone-like strobili 0.5–1 inch (1.3–2.5 cm) long that persist on branches through winter. Root nodules containing nitrogen-fixing bacteria (Frankia spp. species) enable growth in nutrient-poor soils. Growth rate is rapid at 24–36 inches (60–90 cm) annually; many species reach 30 feet (9 m) within 10 years. Alders are typically short-lived, with lifespans of 40–60 years depending on species. Foliage drops green to brown in fall without significant color change. Wood is weak and brittle; branch breakage occurs in ice storms and high winds.

Native Range

The genus Alnus spp. is distributed across the Northern Hemisphere, with species native to North America, Europe, and Asia. North American species include A. rubra (Pacific Northwest), A. incana subsp. rugosa spp. (eastern wetlands), and A. rhombifolia (western stream banks). Grows along streams, riverbanks, bogs, and wet lowlands from sea level to 8,000 feet (2,400 m) elevation.

Suggested Uses

Commonly planted for streambank stabilization, wetland restoration, and riparian buffers at 15–25 foot (4.5–7.5 m) spacing. Nitrogen-fixing ability improves soil fertility in reclamation sites. Not suited to dry, well-drained garden soils or sites where a long-lived specimen tree is needed.

How to Identify

Identified by alternate, ovate leaves with toothed margins and prominent parallel venation. Male catkins are pendulous, appearing before leaves in late winter. Persistent small, woody, cone-like strobili 0.5–1 inch (1.3–2.5 cm) long are present year-round on mature trees and are a reliable field mark. Bark is smooth and gray on young trees. Distinguished from Betula spp. (birch) by persistent woody strobili (birch strobili disintegrate) and non-peeling bark in most species.

Appearance

Size & Dimensions

Height30' - 60'

Width/Spread20' - 40'

Reaches mature size in approximately 20 years

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~3 weeks

Winter Spring

Male catkins expand and release pollen February through April depending on species and zone. Female catkins are pollinated by wind at the same time. Catkin emergence occurs before leaf-out, 2–4 weeks ahead of foliage. In zone 8, catkins may expand as early as January.

Detailed Descriptions

Flower Description

Male catkins yellowish-brown; female catkins green maturing to brown

Foliage Description

Dark green, ovate to elliptic, toothed margins

Growing Conditions

Sun Requirements

Full Sun Partial Shade

Requires 6-10 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 - 7.5(Neutral)

357912

Soil Types

loam clay sand silt

Drainagewet

Water & Climate

Water Needs

High

Frost Tolerance

hardy

Time to Maturity

10-15 years

Care & Maintenance

Care Guide

Alders tolerate saturated soils and seasonal flooding that would damage most trees. Established trees are intolerant of prolonged summer drought and decline rapidly on dry, well-drained sites. Nitrogen-fixing root nodules reduce or eliminate the need for supplemental fertilization. Tent caterpillars, alder flea beetles, and woolly alder aphids are common insect pests. Phytophthora root rot affects trees in waterlogged soils with poor oxygen exchange. Wood is weak; prune to develop a strong central leader. Lifespan is short compared to other hardwood trees, typically 40–60 years.

Pruning

Prune during winter dormancy (November–January) to establish a central leader and remove dead, crossing, or structurally weak branches. Alders bleed sap heavily when pruned in late winter or spring; early dormancy pruning minimizes sap flow. Mature trees require minimal pruning beyond dead wood removal. Suckers at the base can be removed annually if a single-trunk form is desired.

Pruning Schedule

winter