The Humble Mexican fleabane, latin name Erigeron Karvinskianus, looks beautiful on steps and walls. Once established it is very easy to grow and a great self seeder. It grows easily in walls and crevices with delicate white flowers which tinge purple and pink as they age. It is a tiny low growing plant which likes sun, good drainage and flowers for months. To get established conditions need to be on the dry side and sunny which is why Erigeron grows so well on walls, crevices, and steps. It prefers sheltered conditions although it is hardy to H4.
The image left shows Erigeron to great effect softening steps and also it shows how prolifically it self seeds. The image of Erigeron growing on steps is to be found in the garden of Sizergh Castle near Kendal in the Lake District.