Holm oak

Holm oak: how do you recognise it?

Habit

Evergreen tree of 10–20 m (in Belgium usually 5–15 m) with a dense, rounded crown about 10 m across; also grown as a shrub or hedge.

Leaf

Alternate, evergreen, 3–7 cm, oval to lance-shaped, untoothed or (especially on young trees) with spiny teeth; dark green above, greyish-white felted below. They stay on the tree for 2–3 years.

Bark

Dark grey to blackish, cracked into small square blocks.

Flower

Yellow, hanging catkins in May–June.

Fruit

Small, pointed acorns 2–3 cm long in a grey-felted cup; they ripen in their first year.

Difference from holly

Holly has a hairless, glossy leaf without a felted underside, and red berries instead of acorns.

Difference from cork oak

Cork oak has thick, soft and spongy cork bark.

Flowering: May · Pruning: May–August

Identify by feature: Spiny leaves · Evergreen leaves · Nuts, acorns and chestnuts

Looks like it, but isn’t

• Holly Ilex aquifolium: hairless, glossy leaf without a felted underside, and red berries instead of acorns. Difference from holly
• Cork oak Quercus suber: thick, soft and spongy cork bark. Difference from cork oak

Why grow it?

• Evergreen: a dark green crown or hedge all year round.
• Tolerates sea wind and salt: one of the best trees for coastal gardens.
• Takes pruning well: as a clipped tree, a ball, a tall hedge or a windbreak.
• Copes well with drought and heat, in towns too.

Holm oak: where does it grow best?

• Sun to partial shade, sheltered from cold east winds.
• Any free-draining soil, including poor sand and chalk; not wet ground where water lies in winter.
• Ideal on the coast and in towns, where it is milder; in colder areas, such as the Ardennes, the risk of frost damage is high.
• Allow for a dense crown about 10 m across: as a free-standing tree for a large garden or a park; as a clipped tree or hedge also for a smaller garden.

Light

Sun or partial shade

Legal basis

Any garden soil

Moisture

Dry

Size

15 m tall, 10 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Bay laurel: Another evergreen Mediterranean plant that can be clipped into shape.
• Rosemary: A fragrant Mediterranean shrub for the sunny edge in front of the holm oak.
• English lavender: Summer flowers for bees and butterflies in full sun, on the same dry soil.
• Laurustinus: An evergreen shrub that flowers in winter and spring and also tolerates partial shade.

Good and bad companions

Goes well with

• Like the acid soil under this tree: heather, rhododendron, japanese skimmia, raspberry, highbush blueberry, witch hazel, broom, gorse
• Tolerate the shade under this tree: oregon grape, rhododendron, oleaster, japanese skimmia, garden privet, camellia, laurustinus, box-leaved honeysuckle

Best kept away from

• Sun-loving plants get too little light under the dense crown: english lavender, heather, butterfly bush, blackthorn, highbush blueberry, sea buckthorn, japanese rose, broom

Holm oak: when and how to plant it?

1. Plant in spring (April–May), once the hardest frosts are over: that way it can root well before its first winter.
2. Buy a young plant in a pot: evergreen oaks don’t transplant well bare-root.
3. Plant it at the same depth as it was in the pot, in a generous hole; on heavy soil, mix coarse sand or gravel into the planting soil.
4. For a tall hedge or windbreak: about 2 young plants per metre.
5. Water after planting and during dry spells in the first two summers.

Care

Watering

In dry spells during the first two years; after that it is very drought-tolerant.

Feeding

Not needed; a little compost in the planting hole is enough.

Clipped trees and hedges

Clip a shaped tree or hedge once a year, in summer.

Young trees

In the first few winters, protect young trees from hard frost with fleece over the crown and a layer of mulch around the base.

Tip: On the coast, holm oak is one of the best windbreaks: plant a row of young holm oaks in spring and clip them every summer so they stay dense right down to the ground.

Holm oak: when to prune it?

Pruning months: May–August.

When: In late spring or summer (May–August), not in winter: frost on fresh wounds causes damage.

How:

• Tolerates pruning well and resprouts from old wood: an overgrown tree or hedge can be cut back hard.
• Clip a shaped tree or hedge once a year in summer.
• Prune a free-standing tree lightly: remove dead, crossing and low branches.

Caution: Severe winters can turn the leaves brown; wait until spring before removing dead branches.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

In October–November, gather ripe acorns and sow them straight away in a deep pot; don’t let them dry out. Protect the seedlings from hard frost in their first winters and plant them out only after 2–3 years, in spring.

Holm oak: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• An established tree withstands about −15 °C. In harder or prolonged frost, especially with cold east winds, the leaves turn brown and young shoots can die; the tree usually sprouts again in summer.
• Young trees are more vulnerable: plant them in a sheltered spot and protect them from hard frost with fleece and a layer of mulch around the base.

Diseases and pests

• Frost damage: Brown, dried-out leaves and dead shoot tips after a severe winter. Wait until late spring, then cut out the dead wood; the tree usually sprouts again.
• Leaf drop in summer: After the new flush, in May–July, it sheds many old leaves. That is normal for an evergreen oak; the tough leaves are slow to rot down.
• Wet soil: On wet, heavy soil it grows poorly and is more vulnerable to frost. Choose a free-draining spot.

Through the year

• March: Remove winter protection as the weather turns milder.
• April: Planting season for young potted plants.
• May: New growth and flowering (catkins); pruning possible, and remove dead branches after a severe winter.
• June: Clip shaped trees and hedges; old leaves fall.
• July: Pruning still possible; water young trees in dry spells.
• August: Last month for pruning, so the wounds have closed before winter.
• October: Acorns ripe: gather them for sowing.
• December: Protect young trees from hard frost with fleece.

What can you use it for?

Garden and landscape

Evergreen tree for coastal gardens: tolerates sea wind and salt. Also as a clipped specimen or tall hedge.

Wood

Very hard, heavy wood; in the south mainly firewood and charcoal.

For wildlife

In southern Europe jays, wild boar and pigs feed on its acorns.

Did you know?

• Around the Mediterranean it forms entire forests, and it is one of the favourite host trees of the black truffle.
• Ilex was the Latin name for the holm oak. The name later passed to holly, whose leaf resembles that of young holm oaks.
• Each leaf stays on the tree for 2–3 years; the tree renews them gradually and so is never bare.