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.

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.

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.