Grey alder

Grey alder: how do you recognise it?

Habit

Tree of 10–20 m with an oval crown about 8 m across, often multi-stemmed; forms root suckers.

Leaves

Alternate, 5–10 cm, oval with a pointed tip, sharply double-toothed; underside grey-green and softly hairy. Not sticky.

Bark

Smooth and pale grey, even on older trees; twigs hairy.

Flowers

Hanging male catkins in February–March, before the leaves.

Fruit

Small woody cones 1–1.5 cm long, stalkless or short-stalked, in small clusters.

Difference from alder

Common alder has a round leaf with a rounded or notched tip, sticky young leaves and dark, cracked bark.

Difference from hornbeam

Hornbeam has a pleated leaf with many more parallel veins, and nutlets on winged bracts instead of cones.

Flowering: February–March · Pruning: November–March

Identify by feature: Winter flowers

Looks like it, but isn’t

• Alder Alnus glutinosa: round leaf with a rounded or notched tip, sticky young leaves and dark, cracked bark. Difference from alder
• Hornbeam Carpinus betulus: leaf with many more parallel veins, pleated, and nutlets on winged bracts instead of cones. Difference from hornbeam

Why grow it?

• A fast pioneer on poor, wet or dry soil, where other trees struggle to get going.
• Fixes nitrogen from the air and so improves the soil for the trees that follow.
• A quick windbreak or shelter for young trees.
• Early catkins in February–March, and little cones full of seed for siskins and goldfinches in winter.
• Regrows readily after cutting back: suitable as coppice and for a hedgerow.

Grey alder: where does it grow best?

• Sun to partial shade.
• Not fussy: wet or dry soil, sand, loam or clay, poor soil included.
• Allow for 10–20 m in height and a crown of about 8 m: for a large garden, a hedgerow or a windbreak. As coppice it stays smaller.
• It forms root suckers: don’t plant it right next to a lawn or border you want to keep tidy.

Light

Sun or partial shade

Legal basis

Any garden soil

Moisture

Dry to wet

Size

15 m tall, 8 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Alder: Our native alder for the wet part of the garden; grey alder takes the drier part.
• Silver birch: Another fast pioneer for poor soil; together they quickly form a light woodland edge.
• Goat willow: Flowers in March–April and gives bees early food; also a fast grower for a windbreak.
• Hawthorn: A thorny native shrub that fills in the base of a hedgerow or windbreak.

Good and bad companions

Goes well with

• Tolerate the shade under this tree: oregon grape, rhododendron, oleaster, japanese skimmia, garden privet, camellia, laurustinus, box-leaved honeysuckle

Grey alder: when and how to plant it?

1. Plant from November to March when the ground isn’t frozen.
2. Young forestry transplants of 60–100 cm establish best and are cheap.
3. For a windbreak or hedgerow, plant about 1–1.5 m apart, if you like in two staggered rows.
4. No fertiliser needed: it makes its own nitrogen. Water after planting if the soil is dry.

Care

Watering

Only in the first year during prolonged drought.

Feeding

Never needed.

Root suckers

Mow or cut suckers in the grass; in a hedgerow you can leave them to fill gaps.

Coppicing

To keep it low, cut it back every few years (see pruning).

Tip: Use grey alder as a ‘nurse tree’: plant it between young oaks or beeches on poor soil. It gives them shelter and nitrogen, and once the other trees are big enough, you cut the alder back or remove it.

Grey alder: when to prune it?

Pruning months: November–March.

When: During winter dormancy (November–March).

How:

• Regrows readily after cutting back: manage it as coppice or in a hedgerow and cut it down low every few years.
• A free-standing tree needs little pruning: just remove dead, broken and crossing branches.
• Mow or cut off root suckers.

Caution: Don’t coppice in hard frost, and cut at a slant so that rainwater runs off the stool.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

In autumn pick ripe cones and let them dry indoors: the seeds fall out. Sow them thinly on moist soil in spring and barely cover them.

Root suckers

In winter, dig up a sucker with a piece of root and replant it in its new spot.

Grey alder: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy: a tree of the mountains and the north.
• In winter you can recognise it by its smooth, pale grey bark, its hairy twigs and the little cones that hang on the bare branches.

Diseases and pests

• Alder leaf beetle: A small blue-black beetle and its larvae chew holes in the leaves in summer. Unsightly, but harmless.
• Alder dieback: A Phytophthora that mainly attacks alders along streams: small yellow leaves, a thin crown and dark, tarry patches at the base of the trunk. There is no treatment.
• Suckers in the lawn: Normal for this alder. Mow them regularly; pulling them out rarely works.

Through the year

• January: Pruning; cut coppice; siskins in the cones.
• February: Catkins in flower; pruning and planting.
• March: Last month for pruning and planting.
• April: Leaves emerge.
• June: Mow off suckers in the grass.
• October: Cones ripe: collect seed.
• November: Planting season; pruning can start again.
• December: Pruning; cut coppice.

What can you use it for?

Garden and landscape

Fast pioneer for poor, wet or dry soil, as a windbreak or to shelter other trees.

For wildlife

The seeds in the little cones are winter food for siskins and goldfinches.

Caution: The pollen in February–March can cause hay fever. Forms root suckers.

Did you know?

• Naturally a tree of mountains and riverbanks in central and northern Europe. Here it was mostly planted as a pioneer on poor soil: like common alder, it fixes nitrogen.
• Bacteria (Frankia) living in nodules on the roots take nitrogen from the air and pass it on to the tree.
• The scientific name incana means ‘greyish’, after the grey, hairy underside of the leaf.