Ivy-leaved toadflax Cymbalaria muralis, also known as Kenilworth ivy, is a well-established introduced species in our flora that's an attractive feature of walls all over Britain. It's been grown in gardens here at least since the beginning of the 17th. century and was first recorded as a garden escape in 1640, but I suspect that if it was a new arrival today the newspapers would be full of hysterical predictions about how this alien species would be about to disfigure our built environment and destroy our buildings. It comes from a restricted area of rocky subalpine habitat in southern Europe but, thanks to the construction of walls and migrations of people, has spread throughout much of the Continent.
The delightful little snapdragon-like flowers bloom from early spring until late autumn and are visited by bees, but they self-pollinate and always set seeds prolifically.
Ivy-leaved toadflax tolerates the nutritionally challenging environment of wall crevices but if you grow it in soil with plenty of nutrients it will make rampant growth, as I discovered when I introduced it into my greenhouse, where I pull out handfuls of the plant from the gravel floor every summer.
Gilbert White, of Natural History of Selborne fame, also marvelled at the growth potential of this plant, which he knew as Antirrhinum cymbalaria, and commented on it at least twice in his journals.
Delighted by the plant, on April 30. 1780 he wrote: ‘A sprig of Antirrhinum cymbalaria, the ivy-leaved Toadflax, which was planted last year on a shady water-table of the wall of my house, grew at a vast rate, and extended itself a full nine feet; and it was in perpetual bloom ’til the hard frost came. In the severity of the winter it seemed to die: but it now revives again with vigor, and shows the rudiments of flowers. When in perfection it is a lovely plant. ‘
Then, five months later on 16th. September, came this excited journal entry: 'The Antirrhinum cymbalaria is grown to an enormous size, extending itself sideways 15 or 16 feet, and 7 or 8 in height!!'
You can read the rest of his fascinating journals at this simply brilliant web site..
Aside from its prolific seed set and potential for rampant growth, the other ace card that ivy-leaved toadflax holds is a mechanism for sowing its own seeds in the dark, damp crevices of walls. During flowering its blooms face the sun but as soon as seeds are set and the flower pedicel begins to elongate the seed capsule becomes negatively phototropic, growing away from the light and towards the nearest dark crevice before the capsule bursts open. The seed capsules in the photo above are just beginning their search for somewhere shady to shed their seeds.