Italian alder

Italian alder: how do you recognise it?

Habit

Fast-growing tree 15 m tall and 8 m wide, conical when young and broader later.

Leaves

Heart-shaped to broadly oval, 5–10 cm, glossy dark green and finely toothed; falls late in autumn, often still green.

Flower

Hanging, yellow-brown male catkins of 5–10 cm in February–March, before the leaves; the female catkins are small.

Fruit

Egg-shaped, woody cones of 2–3 cm, larger than those of common alder, that stay on all winter.

Roots

With nodules housing bacteria that fix nitrogen; they improve the soil.

Flowering: February–March · Plants: March–April and October–December · Pruning: January–February

Why grow it?

• Grows fast and forms a windbreak or green screen within a few years.
• Tolerates dry, poor and chalky soil, where most other alders fail.
• Improves the soil: its roots live with bacteria that fix nitrogen from the air.
• Glossy green foliage until late autumn and long catkins as early as February–March.
• The small cones provide seed for small birds in winter.

Italian alder: where does it grow best?

• Sun or partial shade; it grows fastest in sun.
• Almost any soil: sand, loam or clay, acid to chalky, even poor or urban soil.
• Dry to wet: it is very tolerant, but grows best on moisture-retentive soil.
• Allow 8 m or more all round, far from houses, drains and paving. In a windbreak, plant in a row 2–4 m apart.
• Tolerates wind and city air well.

Light

Sun and partial shade

Soil type

Sand, loam or clay

Acidity

Not fussy

Moisture

Dry to wet

Size

15 m tall, 8 m wide

Hardiness

Hardy (to -20 °C)

Goes well with

• Hazel: Also flowers early and fits well in a mixed windbreak.
• Blackthorn: A tough shrub for a dry, windy spot.
• Dogwood: Brings red stems in winter beside the green alder.
• Hawthorn: A thorny shrub that gives food and shelter to birds.

Italian alder: when and how to plant it?

Plant in March–April and October–December.

1. Plant in October–December or March–April, when the soil is neither frozen nor too wet. Bare-root trees in November–March.
2. Dig a hole twice as wide as the root ball and loosen the sides and bottom.
3. Set the tree at the same depth as in the pot or nursery and backfill with the soil you dug out; fertiliser is not needed.
4. Drive in a sturdy stake and tie the trunk loosely; check the tie every year.
5. Water thoroughly when planting and, for the first 2 summers, in dry weather.
6. Lay a mulch ring 5–8 cm deep of compost or wood chips, kept away from the trunk.

Care

Water

For the first 2 years, water thoroughly once a week during long dry spells. After that it looks after itself.

Feeding

Not needed: the tree fixes nitrogen itself. On very poor soil a layer of compost helps it establish.

Mulching

A mulch ring keeps the soil moist and weed-free; keep the trunk itself clear.

Support

Young trees get a stake for the first 2–3 years, which you then remove.

Fallen leaves

The leaves fall late; raking is not compulsory: they rot down quickly and are nitrogen-rich for the compost heap.

Tip: Unlike other alders it also tolerates dry and chalky soil. It grows fast and fixes nitrogen: a good tree for windbreaks and urban soil.

Italian alder: when to prune it?

Pruning months: January–February.

When: As little as possible. If needed between January and February, in the dormant season.

How:

• Remove dead, damaged and crossing branches.
• Give a young tree a single leading stem and remove competing tops.
• Saw large branches off in three steps, just outside the branch collar, so that the wound can heal.
• Leave reducing or felling large trees to a tree surgeon.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

Pick the cones in October–November and let them dry so that the tiny seeds are released. Sow in spring (March–April) on moist, light soil and barely cover them. Prick out the seedlings and plant them out after a year.

Italian alder: how does it get through the winter?

Hardy to −20 °C.

• Fully hardy, down to about −20 °C: no protection is needed, even in harsh Belgian winters.
• Young trees: leave the stake in place and add an extra layer of mulch around the base in the first year.
• The woody cones and bare branches make a pleasing silhouette in winter.

Diseases and pests

• Alder leaf beetle: Metallic blue beetles and dark larvae eat the foliage in summer. A healthy, large tree recovers by itself; on young trees, pick off the larvae by hand.
• Alder tongue gall: Red, tongue-shaped outgrowths on the female catkins caused by a fungus (Taphrina alni). Harmless and purely cosmetic; nothing needs doing.
• Poor establishment: A young tree struggles in dry summers or on compacted soil. Water for the first 2 years, mulch and loosen the soil when planting.

Through the year

• January: Prune if needed. Catkins begin to swell.
• February: In flower: long catkins. Prune if needed.
• March: In flower. Plant (container trees). Sow.
• April: Plant. Leaves unfold. Water the young tree in dry weather.
• May: Water young trees in dry weather.
• June: Fast growth. Water in dry weather.
• July: Watch for alder leaf beetle on the leaves.
• August: Watch for alder leaf beetle. Water in dry weather.
• September: Cones ripen. The tree stays green.
• October: Collect cones for sowing. Plant.
• November: Plant (bare-root too). Leaves fall, still green.
• December: Plant if the soil is open. Rest.

Did you know?

• The root nodules house bacteria (Frankia) that fix nitrogen from the air. That way the alder enriches the soil itself.
• The species name cordata means ‘heart-shaped’ and refers to the shape of the leaves.
• The leaves stay green until late autumn and often fall while still green; the cones are larger than those of common alder.