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.