Plants on walls

 Chasmophytes are specialised plants that colonise crevices in bare rock faces - ivy-leaved toadflax is a notable example, that even produces seed capsules that grow towards dark cavities, planting their own seeds back into the wall. But there are a surprising number of plants that colonise the man-made equivalent of rocky cliff faces - either drystone walls or walls constructed with mortar. 

Here are a few of the accidental wall colonists that I've encountered recently.

Native wild strawberry on a wall top at Stanhope in Weardale, undoubtedly arriving as a seed in a bird dropping

Native dog violet in a damp retaining wall beside the railway line at Hexham in Northumberland. It has a ballistic seed dispersal method, firing out seeds when its capsule splits and contracts

Native shining crane'sbill in a drystone wall at West Blackdene in Weardale. This plant, a wall specialist, has leaves that turn bright crimson as summer progresses, perhaps induced by stress as conditions become drier.

Non-native trailing bellflower, on a wall top beside Quarry Heads Lane in Durham city. Escaped from a nearby garden.

Native wall lettuce, another wall specialist with plumed seeds that are carried on the wind. Growing here on a ledge below the parapet of Prebends bridge, spanning the river Wear in Durham city.

Moth mullein, a non-native, well established in the wild. Here, on old walls beside Sunderland Marina. 

Wallflower, doing exactly what its name dictates, growing in a wall beside Sunderland Marina. Cultivated since medieval times but well established on walls in the wild.

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: 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: 5. Wall Rue

There aren't many plants that are so intimately associated with man-made walls that this tough little fern, wall rue Asplenium ruta-muraria.

It's likely that human constructions in brick and stone have extended its range because it's a species that prefers an alkaline substrate - and the mortar between bricks and rocks provides and excellent substitute for limestone in areas with a more acidic natural geology. Stretching the point a little, you could argue that the Romans helped widen the distribution of this fern because they introduced mortar as a means of constructing walls. Without this inadvertent human assistance, this would be a much rarer fern.

It produces spores under those leathery, drought-resistant fronds which have a distinct coincidental similarity to rue Ruta graveolens. 

Wildlife on Walls: 3. Viviparous Lizard

If you are a viviparous lizard (aka common lizard, aka Lacerta vivipara), on the lookout for food, safe shelter and warmth, then a dry stone wall has a lot to recommend it. 

The sunny side of the wall is the perfect spot to bask in the morning after a cold night, until your blood warms up....

... and those nooks and crannies between the stones are a safe retreat when dangers - like moorland kestrels, for example - hove into view. Add to that the fact that the wall is probably home to plenty of small, tasty invertebrates that you can snack on, plus the way in which the sun-warmed stones tend to act as a night storage radiator after darkness falls, and its easy to see why dry stone walls are amongst the best places to look out for these lovely reptiles.

I took these photos (which are scanned colour transparencies) a few years ago now, on a dry stone wall on the moors above Wolsingham in Weardale. Must go back soon for another look ......


More on the wildlife of walls here.


More on local reptiles here.

Wildlife on Walls: 2. Cladonia macilenta

In the beginning, when the glaciers melted, there was nothing but bare rock. And lichens thrived there. Then came the grasses and flowers in newly weathered soils, followed by scrub and finally forest whose shade cast out the bare rock-loving lichens. Then came humans with their axes and fire, cutting and burning the forest to grow crops and graze animals that often wandered away. So their owners quarried rocks and built walls. And then the lichens - like this Cladonia macilenta (I think - correct me if I'm wrong) found a new habitat on the rocks that humans piled one on top of another, in an unconscious act of habitat creation. Full circle.

Under close scrutiny, lichens are extraordinarily beautiful organisms - an intimate amalgam of alga and fungus, with each different combination producing a lichen of a distinct shape, colour and texture. The granular columns here, rising from those grey-green scales,are called podetia. They're tipped with red apothecia that produce fungal spores that will blow away, land on a rock, germinate and - if they happen to have landed close enough to a compatible alga, form a new lichen, just as they did on bare, broken rock left by the glaciers.

More on walls and their associated wildlife here

More lichens here

Dry stone Walls

Dry stone walls are the most conspicuous, ubiquitous man-made structures in the north Pennines. There are thousands of miles of them and they've been used to enclose land for centuries, especially in areas where sheep are farmed. Building them is a highy skilled craft that was delightfully described in a booklet by Arthur Raistrick called  The Story of the Pennine Walls, published by Dalesman Publishing back in 1946 (see below).

Raistrick described a dry stone wall as being ".. a structure in a state of equilibrium", thanks to a method of construction that ensures that all the internal forces bear down through the wall faces that are assembled in a way that prevents the wall bulging outwards. The stretch of wall in this photograph, alongside the old railway line that ran from Tow Law to Consett in Co. Durham, is eight feet tall in places and as straight and robust as the day it was built.

This photograph of a wall in Teesdale shows some of the key features. After digging out the footings to a width of about four feet the wall was built on a foundation of large stones and is actually a double wall with infilling. In the best walls the infilling of small broken stone isn't simply tipped in; the stones are wedged in individually, to support the outer wall layers which are built with overlapping stones, of decreasing size from the ground upwards, on either side. In a typical wall there are two courses of 'throughs' near the middle and near the top. These thinner flat stones run right through the wall, often protruding on either side, and help to tie the two faces of the wall together. The top layer of throughs also provides the base from the capstones.

Walls built in this way stretch far across the landscape, snaking around boundaries and following the rise and fall of the land. As Raistrick said, "The extent of walling in the Pennines represents many lifetimes of patient skill spent in hard manual work. We benefit today by the work of these generations and it is incumbent upon us to maintain the walls in good repair". Fortunately, there still seem to be enough skilled craftsmen around to do just that in many parts of the dales. They have their own professional Dry Stone Walling Association, with an excellent web site here.

In addition to gateways, many walls incorporate these 'cripple holes', just big enough for sheep and a sheep dog to squeeze through, so that sheep could  be easily moved between pastures and separated, before the hole was stopped up with a flat slab

These hundreds of miles of stone walls are a wildlife habitat in their own right, representing as they do a vast expanse of inland cliff faces, with shaded or sunlit aspects, that plants can root in and animals can shelter in ......... but that, as they say, is another story..............

Arthur Raistrick's booklet on dry stone walls carried this very fine cover design by Edward Jeffrey. Well worth looking out for a copy if you can find one in a second hand bookshop. Jeffrey was the illustrator of the Toby Twirl children's adventure stories, which I remember vividly from my childhood.