Willow oak

Willow oak: how do you recognise it?

Habit

Conical when young, later an airy, rounded crown; 15–20 m tall and 10–12 m wide.

Leaves

Narrow and lance-shaped, 5–12 cm long and 1–2 cm wide, unlobed with a smooth margin; light green, yellow to russet in autumn.

Flowers

Yellow, hanging catkins in May, along with the leaves. Wind-pollinated.

Fruit

Small, round acorns about 1 cm across in a shallow cup; they ripen in the second year.

Bark

Smooth and grey on young trees, later shallowly furrowed.

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

Why grow it?

• Fine, willow-like foliage: a light, graceful oak without the heavy, dark look of many other oaks.
• Grows fairly fast for an oak and gives shade quickly.
• Tolerates wet soil and temporary flooding better than many other oaks.
• The small acorns feed birds and small mammals; the tree is home to insects.
• Lovely autumn colour, yellow to russet.

Willow oak: where does it grow best?

• Full sun: it grows best there and the crown stays well filled.
• Acid to neutral sandy or loamy soil that stays moist. On chalky soil the leaves turn yellow (chlorosis).
• Moderately moist to wet; it tolerates temporarily waterlogged soil, but not permanently standing water.
• Allow 10–12 m of space, away from buildings and pipes.

Light

Sun

Soil type

Sand or loam

Acidity

Acid or neutral

Moisture

Normal or wet

Size

20 m tall, 12 m wide

Hardiness

Hardy (to -25 °C)

Goes well with

• Sweet gum: Comes from the same region, likes the same acid, moist soil and has fine autumn colour.
• Swamp cypress: A deciduous conifer for wet, acid soil.
• Rhododendron: Evergreen shrub for the light shade under the crown, on acid soil.
• Royal fern: A large fern for moist, acid soil.
• Holly: Evergreen understorey that gives a green backdrop in winter.

Willow oak: when and how to plant it?

Plant in March–April and October–December.

1. Plant in March–April or from October to December, when the soil is not frozen.
2. Choose a young tree of 2–3 m: oaks have a long taproot and establish best when young.
3. Dig a hole two to three times as wide as the root ball, but no deeper. Loosen the sides.
4. Plant at the same depth as in the nursery and backfill with the soil you dug out, without lime or fresh manure. For a standard tree, add a stake and tie loosely.
5. Water in at once with 20–30 litres.
6. Spread a mulch ring about 1 m wide of leaf mould or bark, keeping it clear of the trunk.

Care

Water

For the first 2–3 summers give 20–30 litres a week in dry spells, deeply and not too often. It likes moist soil: do not leave it too long without water in a dry summer.

Feeding

Not needed. Do not add lime or chalky fertilisers: they raise the pH.

Mulching

A mulch of leaf mould or bark keeps the soil moist and acid. Keep the circle free of grass.

Support

For a standard tree, a stake for 2–3 years; remove it afterwards so the trunk grows strong.

Autumn leaves

The narrow leaves rot down faster than those of many other oaks. Make leaf mould or leave them under shrubs.

Tip: Plant it in acid, moisture-retentive soil: that is where it grows best. Mulch young trees with leaf mould, which keeps the soil moist and acid.

Willow oak: when to prune it?

Pruning months: December–February.

When: Only when needed, during dormancy from December to February, in dry, frost-free weather.

How:

• In the early years, build up a single leader and a balanced crown: remove competing tops and crossing branches.
• Take out dead or broken branches; cut just outside the branch collar.
• For thick branches and large trees, call in a tree surgeon.

Caution: Do not prune oaks during the growing season, from April to September: fresh wounds are more prone to infection then.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Seed

Sow fresh acorns in October–November where they are to grow or in deep pots; they germinate in spring. Acorns dry out quickly: do not store them. Seedlings can differ from the parent tree.

Grafting

Special selections are grafted by nurseries. This is not a job for the gardener.

Willow oak: how does it get through the winter?

Hardy to −25 °C.

• Hardy to about −25 °C: no problem in Belgian winters.
• On young trees, a layer of mulch protects the roots during hard frosts.
• Protect the trunk of young trees from game, rabbits and the mower with a tree guard.

Diseases and pests

• Yellow leaves (chlorosis): Pale yellow leaves with green veins on chalky soil. Mulch with leaf mould and do not add lime. On strongly chalky soil, choose a different oak.
• Oak processionary moth: Nests of hairy caterpillars on the trunk and branches from May to July. The stinging hairs can cause severe skin irritation. Do not touch the nests and have them removed by an approved contractor or the council.
• Caterpillar defoliation: In May, caterpillars can strip the young leaves. The tree usually leafs out again. A healthy tree needs no treatment.
• Drought stress: Limp or yellowing leaves and early leaf fall on young trees in a dry summer. Water deeply for the first few years and keep the mulch in place.

Through the year

• January: Remove dead branches in dry, frost-free weather.
• February: Last chance to prune. Check stakes and ties.
• March: Plant once the soil has thawed and is not too wet. Water in well.
• April: Plant. Mulch. The tree comes into leaf.
• May: Yellow catkins. Watch for oak processionary moth nests.
• June: Water young trees in dry weather. Have processionary nests removed.
• July: Water young trees deeply in dry weather.
• August: Water young trees deeply in dry weather.
• October: Yellow to russet autumn colour. Acorns fall; planting can start again.
• November: Plant. Collect fallen leaves for leaf mould.
• December: Plant. Pruning of dead branches possible.

Good for wildlife

Birds

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

Did you know?

• Unlike most oaks it has no lobed leaves: they are narrow and smooth, just like a willow’s.
• In its native land it grows in moist river-bottom forests and tolerates temporary flooding.
• In the southern United States it is widely planted as a street and shade tree.