As I was cleaning the patio behind my bedroom, I noticed that a petunia had sprouted from seed between the paving stones. I left it there, and it is growing and blossoming and seems happy in its unlikely location.
On the same patio I had several cleome (also known as spider flowers) sprout between the stones. I left them, and they are now growing around a flower pot planted with annuals. The tallest one, to the right of the flower pot, has begun to bloom.