Wall flowers in Weardale

Dry stone walls, and old stone walls held together with mortar, acquire a flora of resilient species that thrive in their damp crevices. In Weardale these vertical gardens are at their best from mid-May to early June, before conditions become too dry.

This beautiful display, with yellow Alyssum saxatile in the foreground, cascades down a wall in Stanhope, Weardale

Fairy foxglove Erinus alpinus covers large areas of old walls in Stanhope.

Ferns colonise walls too, but usually on the side that's shaded from the sun. This is brittle bladder fern Cystopteris fragilis, growing  in the wall around St. John's Chapel churchyard in Weardale.

Wall rue Asplenium ruta-muraria and maidenhair spleenwort Asplenium  trichomanes sharing the same crevice in a wall at Eggleston in Teesdale

Aubretia and dandelions in a retaining wall near Daddry Shield in Weardale.

Yellow Alyssum saxatile clinging to the narrowest of crevices in Stanhope.

Wallflower Cheiranthus cheiri, living up to its name in Stanhope.

Fairy foxglove Erinus alpinus, on a wall in Stanhope.

Snow-in-summer Cerastium tomentosum on a wall in Stanhope.

A Golden Wall

Thursday's Guardian Country Diary is about this remarkable 'golden' wall at Hexham, in Northumberland's Tyne valley. It's a retaining wall beside the main Carlisle to Newcastle railway line, that crosses the Tyne just beyond that bridge in the distance and passes this point at head height.

The golden covering is caused by an alga called Trentepohlia aurea.

It spends the drier months of the year as a  powdery deposit on rocks and tree trunks but when it's wet the alga grows into a forest of minute filaments, forming a dense mat on the surface.

You can see the filaments in this close-up, grouped together to form golden cushions that are a few millimetres in diameter. This wall is constantly wet, due to water percolating through from the railway track bed, so conditions are near-perfect for the growth of the alga.

When the tufts coalesce into a golden carpet they provides a very striking back-drop for other plants that are colonising the wall, like this ivy and .....

The liverwort Conocephalum conicum, also known as snakewort. Its surface is divided into small polygons, each with an air pore at its centre, giving it a resemblance to green snakes' skin.

There are other liverworts on the wetter sections of the wall, including this one which I think is Pellia endiviifolia.

The crevices are home to mosses and this little fern with leathery fronds - wall rue Asplenium ruta-muraria.

The alga seems to thrive particularly well on the cement but there are patches of lichen on some of the stones. A close look at this one revealed ....

.... these fungal fruiting bodies, which look like tiny pink toadstools. I think this is a species in the genus Baeomyces.

Another lichen, this time .....

... with fruiting bodies (apothecia)  that look like minute disks of liquorice. A species of Lecidea?

This lichen is a Cladonia species, probably C. fimbriata

Lichens are formed by the symbiotic association between a fungus and an alga and it's very likely that the algal symbiont of some of the lichens on this wall is Trentepohlia. 

Back now for a closer look at the alga Trentepohlia, this time under the microscope.

Under low magnification (c. x40) with a stereo-microscope you can see the forest of algal filaments that make up those orange cushions, while ....

....... here, under a compound microscope (x100) you can see the cells that make up the filaments, and .....

.... at a higher magnification still (c. x400) you can see the granular, pigmented contents of the cells.

There are animals living in the crevices in this wall, including the fearsome snake-back spider - but that's another story.

Home is a hole in the wall

I found this little ruby-tail wasp exploring a crevice in a south-facing house wall near Durham yesterday.

First in had a look inside to check that there was nothing menacing lurking ....

......... then it turned around and reversed in...

........ surveying passers-by .....

..... and occasionally coming out into the sunshine .....

.... to soak up some warmth and display its wonderfully iridescent colours, like a living Faberge jewel 

For more on the intriguing habits of ruby-tail wasps, click here

Wildlife on Walls 11: Fairy foxglove, Erinus alpinus

This attractive little plant growing on the walls of Stanhope Castle in Weardale is fairy foxglove Erinus alpinus, a native of Central and Southern Europe that was first recorded on this spot in 1934. 

How and when it reached our shores is uncertain. Gordon Graham, in his Flora and Vegetation of County Durham (1988) quotes the following theory from W.J.Weston's The County of Durham, published in 1914;

"Round Edmondbyers the rare little pink flowers, a curious survival from the Roman occupation, may reward diligent search.It is supposed that the plant, which is found nowhere but in the neighbourhood of the camps was brought into the country by the Spanish legionaries"

For more wildlife on walls, click here.

Wildlife on Walls: 10. Brittle Bladder Fern

Brittle bladder fern Cystopteris fragilis is one of the characteristic wall ferns of North East England, growing in crevices in shady limestone walls or in mortar and often thriving in old industrial sites, such as the inner walls of railway bridges and the ruins of old lead mine workings in the Durham dales.

It owes its common name to the shape of the coverings over the clusters of spores on the underside of its fronds and to the brittleness of the frond stems, which are succulent and snap easily.

Click here for more plants and animals that live on walls

Wildlife on Walls: 9. Biting Stonecrop

Succulent leaves are common in drought-tolerant plants from arid environments but they are not a feature of many plants that are native to Britain's cool, wet summers. This plant, biting stonecrop Sedum acre is an exception. Its small but fleshy, water-retentive leaves allow it to survive long periods of drought on wall tops, roofs, gravel and sand. It also regenerates very easily from small pieces of stem or even from single leaves, so can spread quite quickly once it has established itself. 


It probably gets some of its water from nocturnal dew formation during long, dry periods (remember those?) and as long ago as 1776 the botanist and doctor William Withering speculated that this is how it obtained water. "This plant continues to grow when hung up by the root," he wrote, "which is proof that it receives its nourishment principally from the air, as is the case with most of the succulent plants".

Withering recommended its use in medicine, with some reservations, noting that " It is very acrid. Applied externally it blisters. Taken inwardly it excites vomiting. In scorbutic cases and quartan Agues it is an excellent medicine under proper management."


The acrid, peppery taste is where the 'biting' part of the common name comes from and in Withering's day it was known as pepper stonecrop. 

Biting stonecrop produces a very fine display of starry yellow flowers that are popular with bees and you can cultivate the plant on almost any hard surface where a little dusty humus accumulates. The plant below was growing on bare, rusty metal in the angle of a bridge support and the bottom image shows the plant on a harbour wall in Sunderland.

Wildlife on Walls: 7. Wall Pennywort

Wall pennywort Umbilicus rupestris is one of those plants that would be much less common if humans hadn't built the perfect habitat for it, althought its distribution is geographically restricted by climate. It's confined mostly to the milder south western part of the British Isles. This photograph was taken near the Pembrokeshire village of Dale in Milford Haven, where the distinctive style of dry stone walling, constructed from vertical flat rock, provides the perfect niche for the plant.


I have tried to introduce this plant in a stone wall in  my garden here in north east England and although it survived for a while our severe winters killed it. 


The leaf stalk is attached to the centre of the fleshy, circular leaves, like an umbrella shaft - an arrangement known as peltate in botanical parlance. That produces a belly button-like dimple in the upper leaf surface, which accounts for its other common name, navelwort. John Gerard also included the mildly erotic names lady's navel and hortus veneris (the garden of Venus) in his herbal.

Wildlife on Walls: 6. Wallflowers

Wallflowers Cheiranthus cheiri (or, to give them their more recent name Erysimum cheiri) have been known as naturalised plants in the wild at least since 1548, but were cultivated long before that and probably originated as a hybrid of uncertain parentage of other members of the cabbage family in the eastern Mediterranean. Whatever their origin, they are a feature of old walls over much of Britain and seem to thrive particularly well on crumbling ancient monuments.

The examples above were photographed on the walls of the lovely Jervaulx Abbey in Wensleydale, Yorkshire, which is noted for its diverse wall flora. Garden wallflowers tend to be grown as biennials but 'wild' plants are perennial and often develop a substantial woody stem. They have a remarkable ability to thrive in dry conditions and are at their best when growing on a sun-warmed wall.

The fragrant flowers appear in various shades, from red through to yellow with every hue in between and this example, on the walls of Bamburgh Castle in Northumberland, is a particularly attractive shade of orange.

Most cultivated wallflowers are spring flowering and attract early bumbleebees in search of pollen and nectar but the plants that are naturalised on walls often seem to have a longer flowering period.