Wall Ferns

In our moist Devon environment, lots of our old walls have ferns growing on them. This note is partly to help you to know which ones you may have on your wall, and also to encourage you to preserve them and look after them.
The illustration shows five of the commonest species:

On the right is the Hart’s-Tongue Fern Phyllittis scolopendria. This is an extremely common fern in Devon, growing on the ground in woods and in hedges as well as on walls, where it really never seems particularly happy and is usually stunted. The fronds form neat clumps, and the new fronds are produced in the spring, uncurling neatly from the tip. The frond shown here is dwarfed because the wall on which it was growing is really too dry for it to develop well. In woods and hedges the fronds can be almost a metre long, but on walls, fronds of the size shown are common. Even at this size, however, they form spores, in neat chestnut-brown lines beneath the frond, sloping slightly upwards. This is the only wall fern with undivided fronds.

Second from the right is something of a rarity, the Rusty-back Fern Ceterach offinicarum. This is a very neat fern, with fronds that are dark green above but which are easily recognised because the undersides of the fronds are covered in rusty-coloured scales. It grows on sunny walls, usually on limestone or on lime mortar. It is not a particularly common fern in Devon. Look after it if you have it!

Third from the right is what is probably our commonest wall fern, Maidenhair Spleenwort Asplenium trichomanes. This forms dense tufts on the lime mortar in your wall, and can also be seen here and there on natural rock outcrops such as those on the Undercliffs, although it dislikes deep shade. The leaflets are neatly arranged on each side of the black frond axis. The spores are borne on the underside of the leaflets, in two to five patches on each; each group of spores is partly covered by a little green flap.

Top left is the Wall-Rue Asplenium ruta-muraria. This is a fairly common wall species, usually growing on the lime mortar between the stones. The little fronds are dark green, and the dark chestnut-brown spores almost completely cover the underside of the fertile fronds. It takes its name from the similarity of its frond shape to the leaves of Rue.

Lower left is Black Spleenwort Asplenum adiantum-nigrum.  It is badly named because there is little or nothing black about it! The fronds and the frond axis are both green, usually a rather pale olive-green, and the spores are brown. It is usually a scarce species although it is probably present in most parishes. Like the other wall ferns, it tends to grow on the lime mortar between the stones of the wall.  The frond shown is a small one, looking rather like that of the Wall-Rue, but Black Spleenwort fronds always have a clear axis with leaflets arising alternately on opposite sides.

All these ferns, with the exception of the Harts-Tongue Fern, are virtually confined to walls. Outcrops of limestone are rare in Devon, so they depend on the walls that we provide for their habitat. They are mostly calcicoles (limestone-lovers), and so they enjoy the lime mortar that binds most of our older walls. Lime mortar stays a little bit damp for much of the year, and it also takes many years to harden completely, so that the roots of the ferns can penetrate it and become established. 

Cement mortar is much harder and less yielding, and does not absorb water, so that ferns do not generally grow on cement-mortared walls. So – if you value your ferns (and they are very much a feature of Devon’s lanes and walls) – then rebuild your walls using lime mortar. If you must use cement, leave some gaps in the pointing here and there so that ferns can establish in the crevices.