Wall rue, Asplenium ruta-muraria is the second commonest wall fern in this area after A trichomanes. It is found in exactly the same locations in the town although A trichomanes can grow much larger and luxuriantly and virtually cover the face of the stone. Slightly out of town and amongst the old railway bridge abutments and retaining walls, A trichomanes dominates. However Grime (1988) says
‘The species is the only common fern in which the distribution shows a S- facing bias….here is no doubt that A.r-m. is the most drought tolerant…‘ however ‘..the frond canopy, or even the whole plant, may be periodically destroyed during summer drought.’ (p110)
Both ferns undoubtedly owe their widespread distribution to man made structures and without mural habitats they would be much less common
‘I know of no single natural wild sites, for example, in which Ceterach officinarum, Asplenium trichomanes or A ruta-muraria are as abundant as they are on wall sites,….In studying and preserving mural fern habitats, we are undoubtedly dealing with a living legacy which was begun by the Romans..‘(Page 1988 p153)
The habit and triangular leaf shape mean that this is a fairly straightforward fern to identify.