Cork oak

Cork oak: how do you recognise it?

Habit

Evergreen tree of 8–15 m with a broad, irregular crown; in Belgium usually smaller and slower-growing.

Leaf

Alternate, evergreen, 3–7 cm, oval, with small bristle-tipped teeth; dark green above, grey-felted below.

Bark

Very thick, pale grey, deeply furrowed, corky and springy: you can press your thumb into it. After the cork harvest the trunk is reddish-brown.

Flower

Yellow, hanging catkins in April–May.

Fruit

Acorns 2–3 cm long in a cup with somewhat spreading scales.

Difference from holm oak

Holm oak has hard bark cracked into small blocks, without cork.

Difference from Lucombe oak

Lucombe oak (Quercus × hispanica) is a cross with Turkey oak, semi-evergreen, with a thinner layer of cork; sometimes in parks.

Flowering: May · Pruning: May–July

Identify by feature: Nuts, acorns and chestnuts

Looks like it, but isn’t

• Holm oak Quercus ilex: hard bark cracked into small blocks, without cork. Difference from holm oak
• Lucombe oak (Quercus × hispanica): a cross with Turkey oak, semi-evergreen, with a thinner layer of cork; sometimes in parks.

Why grow it?

• Unique bark: thick, soft and spongy, you can press your thumb into it.
• Evergreen, with small, grey-green leaves: a touch of the Mediterranean in the garden.
• An eye-catcher: few gardens have a living cork tree.
• Little pruning needed.

Cork oak: where does it grow best?

• Full sun, in a warm spot sheltered from cold east winds: for example in front of a south-facing wall or in a sheltered town garden.
• Acid to neutral, dry and free-draining soil, such as sand; on chalky soil the leaves turn yellow.
• No wet soil: wet feet in winter make it even more vulnerable to frost.
• It is mildest on the coast and in towns; in the Campine and the Ardennes the risk of frost damage is high.
• About 8 m across: suits a medium-sized to large garden.

Light

Full sun

Legal basis

Acid soil

Moisture

Dry

Size

10 m tall, 8 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Heather: Also likes acid, dry, sandy soil in the sun.
• Broom: Yellow flowers in May on the same poor, acid soil.
• Rosemary: A fragrant Mediterranean shrub for the sunny foot of the tree.

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

Cork oak: when and how to plant it?

1. Plant in spring (April–May), once the hardest frosts are over, so that it can root before winter.
2. Buy a young plant in a pot; evergreen oaks don’t transplant well.
3. Plant it at the same depth as it was in the pot and, on loamy soil, mix coarse sand into the planting soil so that water drains away well.
4. Water after planting and during dry spells in the first summers; after that it is drought-tolerant.
5. In a cold area you can also grow a young cork oak in a large pot for a few years and move it to a cool, light, frost-free place in severe winters.

Care

Watering

In dry spells during the first summers; rarely needed after that.

Feeding

Not needed; no lime and no fresh manure.

Winter protection

Protect a young tree in severe winters with fleece over the crown and a layer of mulch around the base.

Bark

Don’t peel off any cork: you don’t harvest cork in a garden, and damaging the living layer beneath the cork weakens the tree.

Tip: To show off the cork, gradually raise the crown and don’t plant tall shrubs in front of it: the thick, gnarled corky trunk is its finest feature.

Cork oak: when to prune it?

Pruning months: May–July.

When: In late spring or early summer (May–July), after the last frost.

How:

• Little pruning needed: remove dead, diseased and crossing branches.
• If branches have been killed by frost in winter, wait until May–June to cut them out. Protect a young tree in severe winters with fleece and a layer of mulch around the base.
• Gradually raise the crown of a young tree so that the corky trunk is on show.

Caution: Don’t prune in autumn or winter: fresh wounds and the new growth that follows are then vulnerable to frost.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

Sow fresh acorns in a deep pot in autumn and keep them frost-free; don’t let them dry out. Plant the young trees out only after a few years, in spring.

Cork oak: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Less hardy than holm oak: an older tree withstands about −10 °C for a short time, but in harder or prolonged frost the leaves turn brown and branches die back.
• Young trees are the most vulnerable: protect them from hard frost with fleece over the crown and a thick layer of mulch around the base, or grow them in a pot that you can move somewhere frost-free.

Diseases and pests

• Frost damage: Brown leaves and dead twigs after a severe winter. Wait until May–June, then cut out the dead wood.
• Yellow leaves: On chalky soil the leaves turn yellow between the veins. Plant it in acid to neutral soil.
• Wet soil: In wet, heavy soil the roots rot and the tree is more vulnerable to frost.
• Leaf drop in spring: After the new flush, some of the old leaves fall: that is normal for an evergreen oak.

Through the year

• March: Remove winter protection as the weather turns milder.
• April: Planting season for young potted plants; flowering (catkins) begins.
• May: Pruning possible: remove frost-killed branches after the last frost.
• June: Pruning still possible; water young trees in dry spells.
• July: Last month for pruning.
• October: Acorns ripe: sow them in a pot.
• November: Mulch around the base of young trees.
• December: Protect young trees from hard frost with fleece.

What can you use it for?

Also used for

Cork for bottle stoppers, insulation, flooring and shoe soles; the tree stays alive when the cork is harvested.

Garden and landscape

A special tree for a sheltered, sunny spot; here only in mild areas, and frost-tender especially when young.

For wildlife

The cork oak woodlands (montados) of Portugal and Spain are rich in birds, such as wintering cranes.

Did you know?

• Every 9 to 12 years the cork is stripped from the trunk by hand, without harming the tree. Portugal produces about half of the world’s cork.
• The first cork, the ‘virgin cork’, is too rough for bottle stoppers; only from the third harvest onwards is the cork good enough. By then the tree is already about 40 years old.
• In Portugal, after the harvest, the last digit of the year is painted on the trunk: that way people know when the tree may be stripped again.