Wall-rue

How to recognise it

Habit

Small evergreen tuft, 3–15 cm, with its leaves in a loose rosette growing out of a wall joint.

Leaf

Dull blue-green to dark green and slightly leathery; two to three times divided into small wedge- or diamond-shaped segments, finely toothed at the tip. The stalk is green, dark at the base, and usually longer than the blade.

Spores

No flowers: in summer and autumn, elongated spore clusters lie along the veins on the underside and eventually turn almost the whole underside brown.

Roots

A short creeping rhizome with fine roots that work their way deep into the mortar between the stones.

Confusion with

Maidenhair spleenwort has simply divided leaves with small round leaflets on a glossy black stalk. Rustyback has deeply lobed leaves with rusty scales underneath. Hart’s-tongue has undivided, tongue-shaped leaves.

Why grow it?

• One of the few plants that can live on a bare wall: it brings old walls to life.
• Evergreen: even in January you can see its small blue-green tufts in the joints.
• It indicates old lime mortar and a healthy wall habitat, along with mosses, lichens and other wall plants.
• Small creatures such as spiders, woodlice and springtails shelter among the little ferns and mosses.
• It stays small and has fine roots, so it does far less harm to a wall than woody seedlings such as elder or buddleia.
• Lovely to observe: in summer you can see the brown spore clusters on the underside of the leaves.

Wall-rue: where does it grow best?

• In the wild: joints of old lime-mortared walls, bridges, quays and churchyard walls; in Wallonia also crevices in limestone rocks.
• Sun to partial shade: it copes with a sunny, dry wall better than many other ferns.
• It needs lime: soft lime mortar or limestone. Modern hard cement joints give it little foothold.
• In the garden: an old garden wall, a dry-stone wall with some lime, a rock garden or a block of tufa, always well drained.

Goes well with

• Maidenhair spleenwort: Another small wall fern, with fine round leaflets on black stalks.
• Ivy-leaved toadflax: Trails over the stones with small lilac flowers all summer.
• Biting stonecrop: Little yellow stars in June on the sunny top of the wall.
• Yellow corydalis: Flowers yellow for months in wall joints and likes lime.
• Hart’s-tongue: For the shady side of the same wall.

Wall-rue: when and how to plant it?

1. Never take wall-rue from a wall in the wild or in a village: it rarely survives and you damage the wall. Buy a plant from a specialist fern nursery.
2. Plant in April–May or in September, when the wall will not dry out or freeze.
3. Choose a joint or crevice and fill it with a mix of sand, grit and a little crushed lime mortar or limestone.
4. Push the small root ball deep into the crevice, roots inwards, and wedge it in with a little moss or grit.
5. For the first few weeks, mist gently in dry weather; after that, rain is enough.
6. You can also sow spores: sprinkle ripe spores onto a damp brick or a limy compost under glass. The young plants are only large enough after a year or more.

Care

Watering

Not needed once established: it lives on rain and dew. Only in the first year, mist during long dry spells.

Feeding

Never: it grows in a wall precisely because there is little food and little competition.

Repointing

If the wall needs repair, use lime mortar and do not repoint all the planted sections at once, so that some plants remain to colonise the new joints.

Competition

Keep ivy and large seedlings off the wall: they take the light, and their roots really do damage the wall.

Cleaning the wall

Do not use a pressure washer or weedkiller on the wall: ferns and mosses will disappear.

Tip: If you want wall-rue in your garden, simply leave an old lime-mortared wall alone: no cement repointing, no pressure washing and no brushing. The little ferns often appear by themselves.

Pruning and mowing

When: Not needed: old leaves die off by themselves.

How:

• Leave the plant alone; brown leaves drop away by themselves in spring.
• At most, gently pull away a dead leaf by hand without tugging at the roots.
• Never scrape out the joints to make a wall look ‘tidy’.

Caution: Do not use a string trimmer or wire brush along the wall: you would rip out the little fern, roots and all.

Pruning calendar and basic rules

Propagation

Spores

In summer or autumn, let a ripe leaf with brown spore clusters dry in a paper bag. Sprinkle the spores onto a damp, limy surface (for example a brick standing in a saucer of water) under glass, in good light but out of direct sun. First a thin green film appears (the prothalli), and only months later the first leaves.

Self-spreading

On a wall it likes, it spreads by spores into new joints by itself.

Wall-rue: how does it get through the winter?

Hardy to −25 °C.

• Evergreen: the small leaves stay all winter and stand out on bare walls.
• Fully hardy, to about −25 °C: even hard frost on a bare wall does not trouble it.
• Old leaves die off in spring, as the new ones unfurl.

Diseases and pests

• Repointing and cleaning: The biggest threat: repointing with cement, sandblasting or pressure-washing walls. Repoint in stages with lime mortar and spare the planted sections.
• Drying out: A newly planted fern in a dry, sunny joint can shrivel. Start in a spot with morning sun and mist in dry weather.
• Smothered: Ivy, moss or large plants can smother the little fern. Cut back ivy covering the wall.
• Fails to establish: In ordinary garden soil or in a pot it often dwindles from too much moisture and too little lime. Choose a limy, well-drained crevice.

Through the year

• January: Evergreen: the blue-green tufts stand out on the bare wall.
• March: Old leaves die off. Preferably do not clean or repoint the wall.
• April: New leaves unfurl. Good month for planting.
• May: Plant in a wall joint. Mist in dry weather.
• July: Spore clusters form on the underside of the leaves.
• August: In long dry spells, mist young plants.
• September: Spores are ripe: collect them for sowing. Good month for planting.
• October: Sow spores under glass.
• December: Resting. Evergreen in the wall.

Did you know?

• The scientific name ruta-muraria means ‘rue of the walls’: its small leaves resemble those of rue.
• Asplenium comes from the Greek word for spleen (splen): people once believed these ferns worked against ailments of the spleen.
• Ferns do not make seeds: a spore first grows into a tiny, heart-shaped green plantlet, the prothallus, and only after fertilisation does the actual fern grow from it.