The old way of getting moss etc to grow on new stonework was to paint with a
solution of Cow dung, failing that a slower way was mil or as said Yoghurt,
but if your walls are south facing then none will work to any degree.
When I built a bridge over our stream I left some joints open to the backing
soil and planted cliff break fern into them, all took but none have any
sunlight and are damp most of the time.
No fern will be able to root through cement to find soil.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk