In the cracks!

[Hoy]

This species needs only cracks in the rock to establish itself. The seeds are dispersed by ants. Cymbalaria muralis grows along the coast in southern Norway and is used as a garden plant.

[Carlo A. Balistrieri]

Moreover it does not need cracks...or rocks at all. It's currently smothering an Opuntia in my C&S bed. It is best in walls where it has been very reliable. It shows up in pots and moves itself around where it likes. My favorite plant of it was growing in a crack in a stucco wall of a building---no soil at all and returns every year.

[Todd Boland]

Doesn't appear hardy in my area.

[Kelaidis]

I must remember to take a picture of my colonies tomorrow: I've finally isolated it in a few spots where it can't do too much damage, but I can enjoy it's Kenilworthian charms!