Alder

Alder: how do you recognise it?

Habit

Tree of 15–25 m with a narrow, oval to conical crown about 10 m across; multi-stemmed when coppiced.

Leaves

Alternate, round to obovate, 4–10 cm, double-toothed, tip rounded or notched. Young leaves sticky.

Bark

Dark grey to blackish-brown, cracked into small plates. Buds purple and stalked.

Flowers

Long, hanging male catkins as early as February.

Fruit

Small woody cones 1–2 cm long.

Difference from hazel

Hazel has a leaf with a pointed tip that is softly hairy.

Flowering: February–March · Pruning: November–March

Identify by feature: Winter flowers

Looks like it, but isn’t

• Hazel Corylus avellana: leaf with a pointed tip, softly hairy. Difference from hazel

Why grow it?

• Grows with its feet in the water and stabilises the banks of streams and ponds.
• The seeds in the alder cones are winter food for siskins and goldfinches.
• Early catkins in February, and cones that hang on all winter.
• As coppice it provides firewood every 5–10 years.
• Improves the soil: it takes nitrogen from the air.

Alder: where does it grow best?

• Sun to partial shade.
• Moist to wet soil, along streams, ditches and ponds; it tolerates flooding and any soil type, including poor soil.
• It is out of place on dry ground.
• Left to grow freely, allow for 15–25 m and a crown of about 10 m: for a large garden, a bank or a hedgerow. Coppiced or pollarded, it stays much smaller.

Light

Sun or partial shade

Legal basis

Any garden soil

Moisture

Moist to wet

Size

20 m tall, 10 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• White willow: Together the classic trees of the stream bank: alder and pollard willow.
• Guelder rose: A native shrub of wet ground with white flower heads and red berries.
• Alder buckthorn: A shrub of wet woodland and damp woodland edges, food plant of the brimstone butterfly.

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

Alder: when and how to plant it?

1. Plant from November to March when the ground isn’t frozen.
2. Young whips (forestry transplants of 60–100 cm) establish best.
3. Plant on the bank just above the normal water line; for a hedgerow, about 1 m apart.
4. No fertiliser needed; water after planting if the soil is dry.

Care

Watering

Never needed on wet ground; elsewhere, in dry spells during the first year.

Feeding

Never needed: it makes its own nitrogen.

Coppicing

Cut it back every 5–10 years (see pruning).

Leaves

The leaves fall while still green in autumn and rot down quickly: leave them, they enrich the soil.

Tip: Along a ditch or pond in a large garden, plant a row of alders and manage them as coppice. Every 5–10 years you will have firewood, the bank stays firm and the stools sprout again each time.

Alder: when to prune it?

Pruning months: November–March.

When: During winter dormancy (November–March).

How:

• Traditionally managed by coppicing or pollarding, cut back every 5–10 years. Regrows readily.
• Cut coppice low with a sloping cut, so that rainwater runs off the stool.
• A free-standing tree needs little pruning: just remove dead and broken branches.

Caution: Freshly cut alder wood turns orange-red within minutes: that is normal and not a sign of disease.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

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

Self-sown seedlings

On bare, wet ground it often seeds itself; such a seedling can be moved in winter.

Alder: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy.
• In winter the cones and purple buds decorate the bare branches; siskins and goldfinches come to pick out the seeds.

Diseases and pests

• Phytophthora dieback: Mainly along streams: small, yellow leaves, a thin crown and dark, tarry spots at the base of the trunk, caused by a Phytophthora. There is no treatment; fell badly affected trees.
• Alder leaf beetle: A small blue-black beetle and its larvae eat holes in the leaves in summer. Unsightly, but harmless.
• Struggling on dry ground: In a dry spot an alder stays small and sickly. Only plant it on moist to wet soil.

Through the year

• January: Pruning; cutting coppice; siskins in the cones.
• February: The long catkins flower; pruning.
• March: Last month for pruning and planting.
• April: The sticky young leaves emerge.
• June: Alder leaf beetles on the leaves: leave them be.
• October: Cones ripe: collect seed.
• November: Planting season; pruning can start again.
• December: Pruning; cutting coppice.

What can you use it for?

Wood

Wood that hardly rots under water: once used for foundation piles and sluices.

Garden and landscape

Stabilises the banks of streams and ponds.

For wildlife

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

Good for wildlife

Birds

Berries, seeds or insects as food, or a place to nest.

Caterpillars

Host plant: caterpillars of butterflies or moths eat the leaves.

Bats

Attracts insects in the evening that bats hunt.

Did you know?

• Grows along streams and in marshes. Root nodules containing bacteria take nitrogen from the air.
• Alder wood hardly rots under water: it was once used for foundation piles and sluices.
• The name glutinosa means ‘sticky’, after the sticky young leaves.
• Freshly felled alder wood turns a striking orange-red shortly after cutting, as if the tree were bleeding.
• The ‘Erlkönig’ of Goethe’s famous poem literally means alder king, but it is probably a mistranslation of a Danish word for the king of the elves.