Stone walls, rock gardens and narrow crevices can be excellent places for ferns, but they are very different from an ordinary shaded border. Root space is restricted, water drains away quickly, soil can be shallow, and the same site may move from damp to dry much faster than the ground beside it.

The best ferns for these positions are usually small species adapted to rocky habitats, with roots that can occupy a confined but well-drained pocket. Rock-garden planting is the easier of the two. Growing a fern directly in a wall is more specialised because the plant has less growing medium and is more exposed while it establishes.

This guide focuses on choosing and establishing ferns in those small rocky spaces. For the wider selection process covering shade, soil, size and other garden uses, see our guide to choosing ferns for Irish gardens.

1. Understand the difference between a rock garden and a wall

A rock garden can be built around the needs of the plants. You can create a planting pocket several centimetres deep, improve drainage, choose the soil mix and position rocks so that they shade or protect the root zone.

A wall gives you much less control. The fern may have only a narrow crack or joint in which to root, and that pocket can dry rapidly before the plant has established. Ferns that succeed naturally in walls are therefore not simply miniature versions of ordinary woodland ferns. They are plants able to live with limited root volume and sharp drainage.

Crevices between large stones sit somewhere between the two. If there is enough depth behind the visible gap for roots to enter a cool, protected pocket, they can provide excellent fern positions. A shallow decorative crack with almost no growing medium is much harder to use successfully.

2. Look for drainage, root depth and protection around the roots

Rock-garden ferns generally need a root zone that drains freely while still holding enough moisture for the roots to remain active. The aim is not to create a dry pile of stone. It is to create a cool, aerated pocket where excess water can escape.

Rocks help by shading and protecting the roots. They also reduce disturbance around small ferns and can keep low fronds away from surrounding plants. In a well-designed rock garden, part of the stone sits below soil level so the planting feels stable and the root zone extends around the rock rather than sitting on top of a thin layer of compost.

A gentle slope is particularly useful because it allows excess water to move away while providing different exposures. Raised beds and troughs can serve the same purpose where the existing soil drains poorly.

For a fuller explanation of how soil structure, aeration and drainage affect fern roots, see our guide to fern soil and drainage.

3. Good ferns for walls, rock gardens and crevices

Maidenhair spleenwort, Asplenium trichomanes

Asplenium trichomanes is one of the clearest examples of a fern suited to rocky conditions. It is a small evergreen fern with dark stems and narrow fronds, and it is naturally associated with rock faces and crevices rather than deep woodland soil.

It prefers a moist but well-drained root zone and can be grown in partial to full shade. Different forms vary in their preference for acidic or lime-rich conditions, so it is worth matching the plant to the stone and soil rather than assuming every maidenhair spleenwort wants the same pH.

Its compact size makes it especially useful for gaps in walls, narrow crevices and the front of a rock garden. Once established it can tolerate some dryness, but the roots should not be left in stagnant, waterlogged conditions.

Hart's-tongue fern, Asplenium scolopendrium

Hart's-tongue fern is larger and more architectural than maidenhair spleenwort. Its evergreen fronds are broad and undivided, so it gives a strong contrast to the finely divided foliage people usually associate with ferns.

It prefers partial to full shade, slightly moist soil and good drainage. The species also favours more alkaline conditions, making it a useful candidate where the planting pocket is influenced by limestone or other lime-rich material.

In a rock garden it works better where there is a deeper pocket than a very narrow crack. It can also suit a shady wall where the root space is adequate. Allow room for the mature rosette rather than squeezing it between stones that will eventually be hidden by the fronds.

Common polypody, Polypodium vulgare

Polypodiums are well suited to rocky planting, and Polypodium vulgare is a useful example for a low, evergreen effect. Its creeping rhizome allows the plant to move gradually through a suitable pocket rather than remaining as one tight central crown.

It is particularly useful in shaded rocky positions where there is enough moisture for establishment but good drainage around the rhizome. On a wall, it needs a real rooting crevice behind the visible opening rather than a smear of compost on the surface.

Because the rhizome creeps, give it a little horizontal room. A narrow ledge, broad joint or space between stones can suit it better than a deep vertical hole with nowhere for the rhizome to extend.

Small Woodsia, Cystopteris and Gymnocarpium species

Rock gardens can also suit small ferns from genera such as Woodsia, Cystopteris and Gymnocarpium. These are more appropriate for a prepared rock garden or raised rocky bed than for assuming every small fern can simply be pushed into a wall joint.

The practical advantage of these smaller ferns is scale. Their root systems and fronds are easier to accommodate between stones, and they are less likely to crowd neighbouring alpine or rock-garden plants. Their individual moisture, soil and light preferences still need to be checked before planting.

4. Choose the position before choosing the fern

Rock gardens often contain several microclimates within a very small area. The top of a stone can be bright and exposed while the shaded side remains cool for much of the day. A north-facing crack can behave very differently from a sunny south-facing pocket only a short distance away.

Many rock-garden ferns appreciate better light than large woodland ferns, but that does not mean every one belongs in hot midday sun. A position with good light, some protection and dependable root moisture is often easier to manage than a crevice that bakes for hours and then dries completely.

Walls magnify this effect. An exposed face receives more wind and can lose moisture quickly. A shaded wall usually gives a more forgiving environment for polypodiums and hart's-tongue fern, while specialist wall ferns adapted to brighter rocky habitats can take more light.

Choose by the conditions inside the root pocket, not simply by whether the wall looks sunny or shady from a distance.

5. Prepare a proper planting pocket in a rock garden

Do not plant a fern into a thin layer of loose compost sitting between two stones. The roots need enough depth and width to establish securely.

• Create a pocket several centimetres deep and wide. The exact size depends on the fern, but the roots should be able to spread beyond the nursery root ball.
• Use a loose, well-drained mix. A loamy or suitable potting mix can be combined with coarse mineral material where extra drainage is needed.
• Match the soil reaction to the plant. Limestone-loving ferns should not automatically be planted in an acidic mix, and acid-loving species should not be surrounded by strongly alkaline material.
• Firm the fern securely without compacting the root zone. Small stones can help hold the plant and growing medium in place.
• Top-dress the surface. Gravel or small stones help reduce erosion and prevent soil splashing onto low fronds.

The fern should sit at its natural growing level rather than being buried deeply for stability. Secure the surrounding material instead of covering the crown or active growing point.

6. Plant wall ferns small and keep them moist while they establish

Wall planting is easier with young, small plants than with a mature fern and a large root ball. A small plant can be fitted into a genuine crevice with enough growing medium around its roots and has a better chance of adapting to the confined space.

A lean organic mix combined with coarse grit can provide both a little moisture retention and the sharp drainage wall ferns need. Moss can be used around a newly planted fern as temporary support and to help keep the immediate root area from drying too quickly.

The establishment period is the most demanding stage. Until the fern has rooted into the crevice, do not allow the small pocket to dry completely. Once established, suitable wall ferns usually need much less attention because their roots have moved into the protected spaces behind the visible opening.

This is particularly important in Irish gardens because a wall can experience very different moisture conditions from the surrounding soil. Rain may wet the face thoroughly, while a shallow crevice can still dry quickly between wet periods if it is exposed to wind.

7. Do not confuse good drainage with drought

Rock-garden advice often emphasises grit and drainage, which can make it sound as though these ferns want permanently dry soil. Most do not.

Sharp drainage means that excess water can leave the root zone and air can remain around the roots. It does not mean withholding water from a newly planted fern. Even species that tolerate drier conditions once established need enough moisture to produce roots and settle into the site.

The best rocky positions combine both qualities: water does not stagnate around the roots, but stones, depth and shade protect the root zone from drying instantly.

If the fern repeatedly wilts or the young fronds dry before opening fully, check whether the crevice is losing moisture too fast. If the crown stays soft and the planting pocket remains saturated after rain, the problem is the opposite and the drainage needs attention.

8. Use the stone itself to guide soil choice

The material surrounding the fern can influence the root environment. Limestone-based rock gardens naturally suit ferns that tolerate or prefer more alkaline conditions, while other species are better in neutral to acidic pockets.

Hart's-tongue fern is a useful example of a fern associated with lime-rich conditions. Maidenhair spleenwort is more complicated because different forms have different soil preferences. Polypodium and other rocky ferns also vary in the conditions they prefer.

For that reason, do not choose a fern solely because it is described as a rock-garden plant. Check its soil preference as well as its size, moisture requirement and light tolerance.

9. Keep maintenance light but regular

Once a suitable fern has rooted into a stable rocky position, it should not need constant intervention. The main jobs are keeping the planting pocket open, preventing neighbouring plants from overwhelming it and checking moisture during establishment or prolonged dry periods.

• remove damaged or exhausted fronds when they have finished;
• keep aggressive neighbouring plants from shading a small fern completely;
• replace displaced gravel or small stones if erosion exposes the roots;
• avoid repeatedly disturbing the soil around a creeping rhizome;
• check wall pockets after unusually dry or windy periods while plants are still establishing.

Low ferns can also be damaged by soil splashing onto the fronds. A gravel or pebble surface around the plant helps keep foliage cleaner while protecting the planting pocket from erosion.

Common mistakes with ferns in walls and rock gardens

• Choosing an ordinary large woodland fern for a tiny gap. Mature frond size and root habit need to match the space.
• Using a shallow smear of compost. A fern needs a real rooting pocket behind or between the stones.
• Making the mix free-draining but impossible to keep moist. Newly planted ferns still need dependable moisture.
• Ignoring soil reaction. Rocky-site ferns differ in their tolerance of acidic and alkaline conditions.
• Planting a wall fern too large. Young plants are easier to establish in narrow crevices.
• Assuming every fern wants deep shade. Many rock-garden ferns tolerate useful light, provided the roots remain suitably cool and moist.
• Letting a small fern become buried by neighbours. Rock-garden species are easy to lose beneath vigorous ground cover or larger perennials.

Frequently asked questions

What is the best fern for a stone wall?

Asplenium trichomanes, maidenhair spleenwort, is one of the strongest examples because it is naturally associated with rocky crevices and remains compact. Polypodiums can also suit shady walls, while hart's-tongue fern can work where the crevice is larger and the conditions suit its preference for a moist, well-drained, more alkaline root zone.

Can ferns grow in mortar joints?

Some wall ferns naturally establish in cracks and joints where enough fine material and moisture collect for roots to grow. In cultivation, a genuine crevice with a lean, gritty rooting mix is more important than the visible width of the joint. A shallow surface crack with no protected root space is unlikely to support a fern well.

Do rock-garden ferns need full shade?

No. Many small rocky-site ferns tolerate brighter conditions than typical woodland ferns. The important point is to match the species to the exposure and prevent the root zone from overheating or drying repeatedly.

What soil should I use between rocks?

Use a loose mix that suits the particular fern and drains freely. A loamy or suitable potting mix can be combined with coarse sand, granite grit or other mineral material where additional drainage is needed. The pH should also match the fern rather than following one universal rock-garden recipe.

How do I stop soil washing out of a crevice?

Firm the planting medium enough to support the fern, use small stones to secure the edges and apply a gravel or pebble top dressing. This reduces erosion and also limits soil splash onto low fronds.

Can I grow ferns in a rockery if my garden soil is heavy?

Yes, provided the fern's immediate root zone is designed to drain properly. A raised rock garden, raised bed or trough can be easier to manage than trying to create one small free-draining hole within heavy, poorly aerated ground.

Choosing ferns for rocky spaces

Start with the space available to the roots. A wall joint needs a much smaller fern than a broad pocket between rocks, and a rock garden can accommodate plants that would never be suitable for a narrow vertical crack.

Then check drainage, moisture, light and soil reaction. Asplenium trichomanes is a strong small crevice fern, Asplenium scolopendrium suits larger shaded, well-drained pockets with suitable soil chemistry, and Polypodium vulgare gives a low creeping option where there is room for the rhizome to extend.

When you know what your site can support, you can browse our fern collection to compare suitable plants. For other compact plants that suit stony planting schemes, see our Rockery Plants collection.