Oak fern

Oak fern: how do you recognise it?

Habit

Low, creeping fern 15–25 cm tall that slowly forms a carpet 50 cm wide or more.

Fronds

Triangular, two to three times divided, soft and thin; the lowest pair of pinnae is large, so the frond seems to consist of three triangles.

Colour

Fresh green to yellow-green. The fronds die back in autumn.

Stalk

Thin, straight and often longer than the frond itself, which therefore stands like a small parasol above the ground.

Spores

Small, round spore clusters without a covering membrane under the frond; ferns do not flower.

Roots

Thin, creeping, dark rhizomes just below the surface.

Plants: March–May and September–October

Why grow it?

• A fine, fresh green ground cover for acid, humus-rich soil in the shade.
• Native: it fits perfectly into a natural woodland garden or a shady wildlife garden.
• The light, almost floating fronds bring brightness and calm to dark corners.
• Small and unobtrusive: ideal between stones, along a path or under shrubs.
• Almost no work: the old fronds rot down by themselves, so no pruning is needed.

Oak fern: where does it grow best?

• Partial shade to shade, for example under deciduous trees or on the north side of a wall.
• Loose, humus-rich, acid to neutral soil: sand, loam or peat, ideally with plenty of leaf mould. No lime.
• Normally moist: cool and moisture-retentive, but not waterlogged and certainly not drying out.
• A sheltered spot without strong wind and without competition from vigorous spreaders.
• Plant 25–30 cm apart; the carpet then closes in two to three years.

Light

Partial shade and shade

Soil type

Sand, loam or peat

Acidity

Acid or neutral

Moisture

Normal

Size

0.25 m tall, 0.5 m wide

Hardiness

Hardy (to -35 °C)

Goes well with

• Wood anemone: Flowers in March–April, before the fern fronds unfurl, in the same woodland soil.
• Wood sorrel: A low, native ground cover for acid, shady woodland soil.
• May lily: A small native woodland plant with white spikes in May, on the same acid soil.
• Beech fern: Another small, creeping native fern that often grows in the same places in the wild.
• Hosta: Large, broad leaves as a calm contrast to the fine fern fronds.

Oak fern: when and how to plant it?

Plant in March–May and September–October.

1. Plant from March to May or in September–October, when the soil is neither frozen nor too dry.
2. Loosen the soil and work in plenty of leaf mould or crumbled, rotted tree leaves.
3. Dig a shallow hole slightly wider than the pot and lay the rhizomes just below the surface, no deeper than before.
4. Leave 25–30 cm between plants and firm the soil gently.
5. Water thoroughly straight away, ideally with rainwater, and keep the soil moist for the first few weeks.
6. Spread a thin layer of leaf mould between the plants as a mulch.

Care

Water

Water in dry spells, ideally with rainwater: tap water is often limy. The soil must not dry out.

Feeding

Not needed: a layer of leaf mould in spring is enough. No chemical fertiliser and no lime.

Mulching

Leave fallen tree leaves where they lie: they rot down into the loose woodland soil this fern likes best.

Weeding

Weed carefully by hand between young plants until the carpet has closed; hoeing damages the rhizomes.

Old fronds

The fronds rot away by themselves in autumn and winter. Leave them.

Tip: Give it loose, acid soil with plenty of leaf mould and simply leave fallen leaves where they lie: that recreates the woodland soil it grows in in the wild. Water with rainwater.

Oak fern: when to prune it?

When: No pruning is needed: the fronds die back in autumn and rot down by themselves into a natural mulch.

How:

• Leave the dead fronds where they are: they protect the rhizomes and feed the soil.
• Fronds that dry up in a dry summer can be cut off; the plant regrows the following spring.
• If the carpet gets too wide, cut off the edges with a spade in spring.

Pruning calendar and basic rules

Propagation

Division

In March–April, cut a piece of the carpet with rhizomes and some soil and replant it elsewhere at once. Water well.

Spores

In summer collect ripe spores and sow them on sterile, moist compost under glass. It takes patience: small ferns only appear after several months.

Oak fern: how does it get through the winter?

Hardy to −35 °C.

• Very hardy, to about −35 °C: no protection is needed in Belgium.
• The fronds die back in autumn; there is nothing to see in winter. Mark the spot so you do not dig into it by mistake in spring.
• Leave fallen tree leaves on the carpet as a natural winter blanket.
• In a pot: stand it against a sheltered wall and do not let the compost dry out completely.

Diseases and pests

• Dried-up fronds: Brown, shrivelled fronds in summer from drought or too much sun. Water, mulch and if necessary move it to a shadier spot.
• Yellow fronds: Pale, yellow fronds in limy soil or from limy tap water. Use rainwater and work in plenty of leaf mould.
• Smothered: Vigorous neighbours or weeds grow over the delicate carpet. Give the fern a place of its own and weed by hand.
• Slugs: Sometimes eat the young fronds in spring. Pick them off in the evening or lure them under a plank as a hiding place.

Through the year

• March: Plant. Spread leaf mould.
• April: New fronds unfurl. Plant or divide.
• May: Plant. Keep an eye on slugs.
• June: Water in dry spells, ideally with rainwater.
• July: Water in dry spells. Collect ripe spores.
• August: Water in dry spells.
• September: Plant. Collect ripe spores.
• October: Plant. The fronds turn colour and die back.
• November: Leave fallen tree leaves as a winter blanket.

Did you know?

• Gymnocarpium means ‘naked fruit’: the spore clusters under the frond have no protective membrane.
• The species name dryopteris means ‘oak fern’, but the plant does not grow especially under oaks.
• The young frond unfurls in an unusual way: its three parts each uncoil separately, like three little balls on a stalk.