Sessile oak

Sessile oak: how do you recognise it?

Habit

Large tree of 20–40 m with a straight trunk that often continues up into the crown, and a more regular crown than the English oak.

Leaf

Alternate, 7–14 cm, regularly lobed with 5–8 pairs of shallow lobes. Leaf stalk 1–2.5 cm; leaf base wedge-shaped, without ears.

Bark

Grey, more finely and regularly fissured than in English oak.

Flower

Hanging yellow-green catkins (male) and inconspicuous female flowers, in April–May with the young leaves.

Fruit

Acorns in clusters, (almost) without a stalk, sitting directly on the twig, ripe in September–October.

Difference from English oak

English oak has a very short leaf stalk (under 1 cm) with two small ears at the leaf base, and acorns on a long stalk (3–8 cm).

Flowering: May · Harvest and fruit: May–June and September–October · Pruning: July–August and November–February

Identify by feature: Lobed leaves (like an oak) · Nuts, acorns and chestnuts

Looks like it, but isn’t

• English oak Quercus robur: very short leaf stalk with ear lobes, and acorns on long stalks. Difference from English oak

Why grow it?

• Just as rich for wildlife as the English oak: hundreds of species of insects, fungi, birds and bats depend on it.
• A native oak for drier, acid sandy and loamy soils, where it does better than the English oak.
• In autumn its acorns feed jays, squirrels, woodpeckers and mice.
• A straight, stately tree for generations.

Sessile oak: where does it grow best?

• Full sun.
• Dry to moist, acid, well-drained soil: sand, loam or stony ground. No waterlogging or high water table; English oak does better there.
• Deep roots: plant it at least 15 m from buildings, drains and pipes.
• Allow for a crown 15–20 m wide: only for a large garden, a park, a meadow or a line of trees.

Light

Full sun

Legal basis

Acid soil

Moisture

Dry

Size

35 m tall, 18 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Holly: Likes to grow in the shade of oaks: winter green in the understorey.
• Silver birch: Like the sessile oak, it loves dry, acid soil; together a light woodland.
• Bilberry: A low shrub for the acid soil under oaks, as in the Ardennes.
• Common honeysuckle: Climbs into the lower branches and is fragrant in the evening for moths.

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

Sessile oak: when and how to plant it?

1. Plant from November to March when the ground isn’t frozen; young, small bare-root trees of 60–120 cm establish best.
2. Dig a hole deep enough for the taproot without bending it.
3. Put a stake on the windward side and tie the trunk loosely; remove stake and tie after 2–3 years.
4. Protect the trunk from rabbits and deer with a guard or tree shelter.
5. Water generously after planting and during dry spells in the first year.

Care

Watering

In dry spells during the first and second year; after that its deep roots cope, even on dry soil.

Feeding

Not needed. Leave the fallen leaves under the tree: they turn into humus.

Under the tree

Don’t mow or dig right up to the trunk; leave a strip of long grass or leaf litter for the roots and soil life.

Old trees

Leave dead wood where it is safe: it is valuable for beetles, woodpeckers and fungi.

Oak processionary moth

From May to July, check for nests on the trunk or thick branches, especially near a play area or patio.

Tip: Torn between English oak and sessile oak? Look at your soil: on dry, acid sandy or loamy soil choose the sessile oak, on damp clay the English oak.

Sessile oak: when to prune it?

Pruning months: July–August and November–February.

When: Dead, diseased and broken branches can be removed at any time of year. Formative pruning in winter or summer.

How:

• As for English oak: prune as little as possible.
• Train a young tree on a single leader; gradually remove competing leaders and branches that hang too low.
• Cut large branches in three steps, just outside the branch collar.

Caution: Do you see nests of the oak processionary moth? Don’t prune yourself but have them removed by a professional.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

In October, gather fresh, heavy acorns without holes and sow them straight away, 3–5 cm deep, in a pot or where they are to grow. Don’t let them dry out, or they won’t germinate.

Moving a seedling

Move a self-sown seedling in its first or second year; later the taproot is too long.

Sessile oak: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy, even in severe winters.
• In winter, protect young trees from rabbits and deer with a guard or shelter.

Diseases and pests

• Oak processionary moth: Caterpillars in groups with irritating hairs, in nests on the trunk (May–July). Don’t touch them; have the nests removed by a professional. Tits eat the caterpillars: put up nest boxes.
• Oak powdery mildew: White fungal coating on young leaves, especially on the second flush in summer. Harmless to an old tree; it slightly slows young oaks.
• Gall wasps: Balls or growths on leaves and twigs. Harmless: leave them.
• Wet soil: On wet, poorly drained soil the sessile oak declines. Plant an English oak there instead.

Through the year

• January: Remove dead and broken branches. Plant bare-root trees in mild weather.
• February: Last month for planting and for winter formative pruning.
• March: Planting season ends; water young trees in dry spells.
• April: Leaves and flowers (catkins) open: don’t prune.
• May: Watch for oak processionary moth nests.
• July: Summer pruning of young trees; keep watching for processionary caterpillars.
• August: Summer pruning still possible; water young trees in dry spells.
• October: Gather acorns for sowing; autumn colour.
• November: Planting season starts; winter formative pruning possible.
• December: Remove dead branches; protect young trees from browsing.

What can you use it for?

Wood

Top-quality oak timber; sought after for wine and whisky barrels.

For wildlife

Just as rich in wildlife as the English oak.

Caution: The oak processionary caterpillar can occur here too.

Edible and useful

Nuts (September–October)

Gather acorns in September and October. Raw, they are bitter and hard to digest because of their tannins; once thoroughly leached in water they become a nourishing ingredient for flour, porridge or a coffee substitute. Keep only sound, heavy acorns without holes; any that float in a bucket of water go on the compost.

Leaves (May–June)

Young, soft leaves in May and June, for oak-leaf wine, an old aperitif from the south of France.

Storing

Dry acorns for a few weeks in an open crate somewhere airy; in their shells they then keep for months. Leached, thoroughly dried meal keeps for a few months in a closed jar in a cool, dark place, or longer in the freezer.

Caution: Never eat acorns raw or insufficiently leached: the tannins cause stomach upset and an astringent taste. Do not confuse them with conkers (horse chestnuts), which are poisonous. From May to August the oak processionary caterpillar may be in the tree: keep away from the nests and do not pick or gather anything under a tree that has them, as the stinging hairs stay active for a long time. Pick well away from busy roads and sprayed fields, and never eat anything you have not identified with certainty.

Eating and using is at your own risk: see the terms of use.

Recipes

• Leached acorn flour (Dried · ± 300 g flour · 1 hr + 3–7 days leaching + drying)
• Acorn coffee (Other · ± 150 g · 1 hr 15 + leaching)
• Oak-leaf wine (Liqueur · ± 1.2 litres · 15 min + 6 weeks steeping + 1 month resting)
• Chocolate acorn pudding (Dish · 4 portions · 20 min + 2 hrs cooling)
• Trout baked in oak leaves (Dish · 2 portions · 40 min)

Good for wildlife

Caterpillars

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

Birds

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

Bats

Attracts insects in the evening that bats hunt.

Hedgehogs

Shelter and a layer of leaf litter full of insects.

Did you know?

• Prefers drier, acid soil; common in the Ardennes. Hybrids with English oak are not unusual.
• Sessile oak timber is of top quality and sought after for wine and whisky barrels.
• Its Latin name, petraea, means ‘of the rocks’: it also grows on stony slopes.
• The species name petraea comes from the Greek petra, ‘rock’: sessile oak likes to grow on stony slopes.
• It copes with drought better than English oak and is therefore seen as an oak with a future in a warmer climate.