Motivated by last year’s successful fern forays, and spurred on by my new membership of the British Pteridological Society, I have decided to try something different this year: a personal challenge to see as many of Britain’s fern species as possible in a single year. The idea being that, by seeking out our ferns, I’ll learn more about them, their habitats, and the subtleties involved in identifying them.
The premise is simple: any fern growing in a wild situation counts, as do subspecies, hybrids, and species accepted by the major authorities. I will do my utmost to document each new find here, beginning with easily accessible species and, hopefully, progressing to trickier ones further down the line.
Urban Ferns – 16th January
It seemed reasonable to kick-start this year’s pteridological challenge with something low-key and ordinary. Thus, a search for wall ferns was on the cards.
On a typical street in Newcastle, there are four or five ferns that commonly occur on walls, most of which belong to the genus Asplenium. What I find especially interesting about Asplenium is that it is a genus of polar opposites. Perhaps half a dozen species are so widespread as to seem almost mundane, while others rank among our most elusive ferns, known only from a few carefully guarded sites. When it comes to the former group, the good news is that you don’t need to travel far to find them.
Skulking along some Heaton terraces, it wasn’t long before we came across our first species: Wall-rue Asplenium ruta-muraria, growing snugly in the mortar. Around fifteen plants clung to the wall, many stunted but unmistakable on account of their small, fleshy, club-shaped fronds. It is an easy species to recognise, and one of the commonest of our ferns.
Mixed in among them were examples of Maidenhair Spleenwort Asplenium trichomanes, its fronds borne on dark, wiry stems and clustered in tight tufts within the mortar. While easy to recognise at first glance, there is more to this little fern than meets the eye. What we collectively know as Maidenhair Spleenwort is, in fact, an aggregate of three subspecies, each occupying its own ecological niche. Two of these are rather rare; the third is incredibly common.
Distinguishing between the subspecies requires attention to detail: the shape of the pinnules, the point of insertion of the stalk, the colour of the rachis, and the angle of growth against the substrate. Truth be told, it is a fiddly, detailed, and at times frustrating affair – though it is possible to cheat. The simplest shortcut is habitat. The commonest subspecies, quadrivalens, is a calcicole: a lover of lime. In the wild, it grows on calcareous rocks, but it is equally at home in the lime-rich mortar of brick walls. The second, trichomanes, is a much rarer plant of acidic rocks, typically confined to upland areas and occasionally encountered on dry-stone walls. The third, pachyrachis, is so scarce that it barely warrants consideration here, though for completeness it favours damp limestone and is restricted to just a few corners of the British Isles. By process of elimination, our plant could only be subsp. quadrivalens – a conclusion quickly confirmed by the insertion of the stalk at the corner of each pinnule.
Where these two species occur, a third often appears as well, though it rarely looks healthy when growing on walls. Hart’s-tongue Asplenium scolopendrium may tolerate such habitats, but it much prefers the shade and humidity of woodland, where it typically grows on mildly alkaline soils. Perhaps our most recognisable native fern, with its glossy, lime-green, tongue-like fronds, it presents few challenges. It is also a particularly useful Asplenium for demonstration purposes. The specific epithet scolopendrium is derived from the Greek skolopendra, meaning centipede – an allusion to the arrangement of the sori on the underside of the frond in parallel lines, a key feature of this genus.
Moving on to another terrace, along another street lined with delightfully neglected student front gardens, there was one more species on the day’s agenda. Another Asplenium, though one that, in my experience, is typically harder to come by: Black Spleenwort Asplenium adiantum-nigrum. Unlike the wall specialists already encountered, this species is not a strict calcicole. Instead, it favours base-enriched substrates containing modest amounts of calcium or magnesium. For this reason, it is often found on mortared walls, though it grows equally well on natural rock faces and on ruins built from basic stone. It is common in many parts of Britain, but here in the North East, it is by no means abundant.
While it took some finding, after a short while, we managed to locate one small, rather pitiful example peeking out from a crevice. It will do for now!
Running Total
Seeing as I rarely carry a notebook, we’ll log our running total for the year here…
| #1 Wall-rue | Asplenium ruta-muraria | 16 January, Newcastle |
| #2 Maidenhair Spleenwort | Asplenium trichomanes subsp. quadrivalens | 16 January, Newcastle |
| #3 Hart’s-tongue | Asplenium scolopendrium | 16 January, Newcastle |
| #4 Black Spleenwort | Asplenium adiantum-nigrum | 16 January, Newcastle |