tree

Black Alder

Alnus glutinosa

The Story

Plant Bio

Alnus glutinosa, the black alder, is a deciduous tree reaching forty to sixty feet in cultivation, distinguished by its dark green, rounded leaves with notched tips and sticky young twigs. The species is highly adapted to saturated soils, producing wind-pollinated, reddish-brown male catkins in early spring before the foliage emerges.

This species forms a symbiotic relationship with nitrogen-fixing bacteria, allowing it to thrive in nutrient-poor, waterlogged soils where other trees fail. The female flowers mature into woody, cone-like structures that persist on the branches through winter, providing food for small birds.

Uses & Benefits:

Features glossy dark green leaves and persistent, cone-like woody strobiles that provide winter interest.

Excellent for stabilizing wet banks, controlling erosion, and reclaiming waterlogged or infertile soils.

Fixes atmospheric nitrogen through root nodules and provides winter seeds and cover for small birds.

Essentials

At a Glance

Plant Type

Tree

Height

708"

Width

472"

Light

Full Sun, Partial Sun

Zones

4, 5, 6, 7

Water

High

Humidity

High

Lifespan

50 to 100 years

Native To

Europe, western Asia, northern Africa

Toxicity

Non-toxic but can cause minor skin irritation from resin

Bloom period

Early spring

In flower about 1 month of the year

Planting

How to Grow

Plant in a full sun to partially shaded location with moist, wet, or poorly drained soil.
Water regularly during the first two growing seasons to establish a deep root system.
Prune in mid-winter to remove root suckers if a single-trunked tree form is desired.
Avoid applying nitrogen fertilizers, as the root nodules fix sufficient nitrogen naturally.

Pro Tip

Prune root suckers in mid-winter to maintain a single-trunked tree form; cutting aggressively into old, leafless wood without visible buds results in poor recovery.

Keep It Thriving

Care Guide

Do

Prune during winter dormancy to shape the canopy and control suckers.
Plant in wet areas, rain gardens, or boggy sites where other trees struggle.
Monitor for Japanese beetles and apply targeted insecticidal soaps if damage is severe.

Don't

Do not plant in dry, upland landscape beds where the tree will suffer from drought stress.
Do not prune aggressively into old, leafless wood lacking visible growth buds.
Do not apply nitrogen-heavy fertilizers, which can disrupt the natural root nodule symbiosis.

Seasonal Care Calendar

Task	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Pest Check
Plant
Prune
Spring Cleanup

Watch Out For

Pests & Diseases

Common Pests

Common Diseases

In the Garden

Landscape Uses & Companions

Landscape Uses

Rain GardenErosion ControlWetland RestorationScreening

Companion Plants

Betula nigra Cornus sericea Carex stricta

Multiply

Propagation Methods

Good to Know

Common Questions

Black Alder questions

What zones can Black Alder grow in?

Black Alder is hardy in USDA Zones 4–7. Inside that range it survives winter in the ground; outside it, grow it as an annual or a container plant you protect.

Is Black Alder deer resistant?

Not reliably. Deer may browse Black Alder, especially tender spring growth — plan on repellents or fencing where pressure is high.

When does Black Alder bloom?

Black Alder typically blooms in early spring through early spring. Exact timing shifts a week or two with your zone — Sow's bloom calendar maps it to your garden.

Does Black Alder need full sun?

Black Alder does best in full sun to part shade. In hot climates, afternoon shade keeps blooms fresher longer.