Terroir North writes:

A suprising number of plants are tenacious enough, and versatile enough, to successfully colonise all sorts of nooks and crannies in what appear to be very inhospitable environments, including man-made structures like stone and brick walls.

For obvious reasons, this sort of ‘wall flower’ needs to be able to withstand drought. Such flora can also be typical of horizontal environments such as dry, bare ground, where there is no competition from other plants. White stonecrop, a native succulent, has fleshy, waterstoring leaves, which enable it to grow in the free-draining conditions of crevices as well as rocky soils.

Similarly, plants which thrive in the lime mortar, which holds many walls together, may also be typical of more horizontal limestone outcrops.

Where mosses have established they can provide humus and a more favourable seed bed for many subsequent wall colonisers.

Some wall plants are true natives to the UK such as wall speedwell and wall lettuce. But others may have been introduced some time ago from beyond our shores and have ‘naturalised’ and become accepted as part our native flora. Ivy-leaved toadflax (below left) probably started life in Britain adorning the walled gardens of large country houses, following introduction from the Mediterranean as far back as the 17th century. Pellitory of the wall (below centre) is another incomer, thought to have been introduced to Britain for its (doubtful and unproven) herbal medicinal uses. Old cottage garden plants, such as common red valerian (below right) and purple toadflax, have escaped from gardens and also naturalised in the wild.

In centuries-old castles, such as Caerphilly in South Wales, or Thirlwall Castle near Hadrian’s Wall in Northumberland, a diverse range of plants have successfully established themselves all around the walls. Over time the lime mortar between the stone blocks has crumbled, providing a more suitable foothold for these plants. Seeds, either carried on the wind or excreted or dropped by birds, may land in these spaces and, where a crack or crevice in the mortar has developed, they can germinate and become established. In the damper and more shady parts of the walls the spores of ferns carried on the wind may also establish themselves.

Thirlwall Castle:

At Caerphilly Castle in June 2021, the wall flower display was dazzling. Ivy leaved toadflax and pellitory of the wall were present in abundance, but the other star of the show was the navelwort (also called pennywort), in various stages of its spectacular development.

The Caerphilly supporting act was diverse.

Of course the grasses get in on the act with their wonderfully evocative names. Here is a selection:

The former induustrial buildings of Cornwall provide a lush wall habitat with an eclectic mix of mosses and shrubs, reflecting the local rainfall statistics, the shelter of surrounding landforms and seed from the adjacent scrub regeneration.

No review of wall botany would be complete without a mention of lichens. Terroir is far better at taking photographs of lichens than in identifying them, as illustrated by the uncaptioned images below. If you can assist us, please use the comment box at the bottom of this blog. We would be very appreciative. Sadly a quick trawl of lichen related websites demonstrated a preponderance of sites dedicated to removing lichen from stone work, paths and many other substrates. As we assume the search engines list sites in order of popularity/hits/willingness of product manufacturers to pay, this is deeply worrying.

On the whole, lichens, mosses and herbaceous plants don’t seem to do significant harm to their host structures and can certainly add a great deal to local biodiversity, provide a source of food for pollinators, and enhance the interest and appearance of old walls.