Ferns on Walls
What are they?
Ferns are primitive plants that evolved before the flowering plants and reproduce sexually by means of spores rather than seeds. This page covers the species of ferns that are most frequently to be found growing on walls in our region. With no natural, rocky substrate available in our region, ferns have done well to colonise the next best thing - human structures made of brick or stone! It is quite possible that the first ferns to do this arrived as spores on the rocks that were brought in from elsewhere for construction and a few species of fern - most notably the spleenworts - grow almost exclusively in these places in our region.
Where are they found?
The range of walls that attract ferns is vast, but especially includes the older walls of churches, churchyards, older town centres and stone or brick bridges. Plants generally grow better on the shadier, north facing sides. Several species also regularly grow below ground in towns - in basement or cellar entrances, beneath gratings and even inside abandoned buildings.
Identification
This is an easy group to identify, based on overall leaf shape. All species are evergreen. For ease of describing the various parts, a couple of technical terms are used; the side arms that form the first division of a leaf are called pinnae (singular, pinna), while the secondary divisions of the pinnae are called pinnules, which make up the smallest parts of each leaf.
Intermediate Polypody Polypodium interjectum
Native. Widespread and by far the commonest of the polypody species in East Anglia, being found on shady hedgebanks, ditch sides, shady churchyards and commonly on old walls, where it is more tolerant of limey mortar than is Common Polypody. Ripe spores present in late autumn or early winter. The leaf shape tends to be broadest about the middle and tapers toward each end and the pinnae are somewhat pointed at the tip. Under high magnification, the spore cases can be seen to consist of two halves, held together by a ribbed 'strap'. In Intermediate Polypody, this strap has on average 7-9 (extreme 4-13) ribs or bands, while Common Polypody has on average 10-14 (extreme 7-17) ribs. Note: A polypody on a wall is likely to be this species, but other species should be checked by visiting the Polypodies page.
 |
 |
 |
 |
| |
|
|
and ribbed straps |
Hart's-tongue Asplenium scolopendrium
Native. Occasionally found in woodland especially along ditch sides and shady banks, but increasingly more common as a plant of dank corners, old walls, cemeteries and other human-influenced habitats. Commonly found beneath gratings in towns. A very distinctive fern, being our only species with highly glossy leaves that are entire and not at all divided. The spore-bearing sori on the underside of the leaves appear as a series of narrow, dark, parallel lines. This plant is evergreen and the old leaves can get browned and very tattered over winter before the new leaves emerge in spring. Plants may form large clumps with leaves up to 60cm long in shady woodland, but are much smaller when growing on walls.
 |
 |
 |
 |
| |
|
|
|
Black Spleenwort Asplenium adiantum-nigrum
Native. Widespread but rather local. Almost exclusively found on churches, bridges and other stone-built, man-made structures. Can be very common where it occurs, often forming colonies on stone walls and attaining great heights on church towers! A delicate fern, unusual in this group in having deeply divided leaves with long-tapered tips. Plants in sunnier places often turn golden-bronze in colour.
| |
 |
 |
 |
| |
|
|
|
Maidenhair Spleenwort Asplenium trichomanes
Native. Widespread but rather local. Almost exclusively found on churches, bridges and other stone-built, man-made structures. The long, narrow leaves with rounded pinnae and black midribs are distinctive.
 |
 |
 |
 |
| |
|
|
|
Maidenhair Fern Adiantum capillus-veneris
Native in the UK but only an escape from cultivation in East Anglia and occasionally found in urban habitats. The leaves bear fan-shaped pinnae, carried on fine, hair-like, blackish stems are very distinctive. This plant os probably much better known as a houseplant in the UK.
 |
 |
 |
 |
| |
|
|
|
 |
 |
 |
 |
| |
|
|
|
Wall-rue Asplenium ruta-muraria
Native. Widespread but generally uncommon on walls, bridges and old buildings, becoming a little more common northward and westward in Norfolk. A very distinctive fern that has a blue-green colour to its leaves, which resemble the leaves of Common Rue (Ruta graveolens).
 |
 |
 |
 |
| |
|
|
|
Rustyback Fern Asplenium ceterach
Native. A rare species in East Anglia, recorded from fewer than 20 sites in the region on old walls, cemeteries and other human-influenced habitats. The leaves have distinctive pinnae that are broadest at the base and merge into the midrib without stalks. The underside of the leaves are covered in papery scales and turn rust-coloured as the spores ripen.
 |
 |
| |
|
Brittle Bladder Fern Cystopteris fragilis
Although native to northern and western parts of the UK, the rare occurrence of this species on walls in East Anglia may perhaps be due to accidental introductions. There may currently be no known examples of this species in our region. A small, delicate fern with widely-spaced pinnae and prominent veins on the pinnules.
 |
 |
 |
| |
|
|