Wall rue
Asplenium ruta-muraria
Family · Aspleniaceae · Harmless
The minute fern growing in mortar joints and limestone cracks, its fronds divided into small fan-shaped paddles that look like rue leaves rather than fern.
Walls and rocks · Uplands
Also known aswall spleenwort
How to tell it
Fronds three to twelve centimetres, in a dense tuft. Divided twice or three times into small fan or wedge-shaped segments, slightly toothed at the edge, leathery and dark green. A mostly green axis, unlike the other black-stemmed spleenworts. It grows only on lime-rich substrate.
What it is confused with
Maidenhair spleenwort
Asplenium trichomanes
Maidenhair spleenwort has a shiny black axis with round leaflets paired like a ladder; this has a green axis and fan-shaped segments divided several times. Both on walls, neither dangerous.
Open sheet →What it is for
No use. It indicates lime mortar and limestone, and a wall covered in it is nearly always an old wall, built before cement.