Last year I grew some nasturtiums in a brick planter. They self seeded and soon there were baby nasturtiums scattered to about a foot and a half from the base. I left them to see what would happen. Winter came in and killed them all...except for some that had rooted up against the bricks. A nice little example of a beneficial microclimate.
Nasturtiums are funny. They are annuals and die back in the summer. They like cool dry conditions. The seeds that drop will lay dormant until conditions for growth are good again and then they sprout. I have nasturtiums coming us regularly now. I just have to keep them in the beds. Mine are just starting to flower now, all from last years' seed. I did not plant any new ones.