If you could find a space or two, Verbena bonariensis is also good at seeding itself, and it's tall. Not only that, it's a butterfly magnet. Smaller annual plants which also self-seed might help you - like Nigella (Love in a mist) and Escholzia (Californian Poppies). They all find crevices in concrete and do well in that kind of situation. The annuals could be sprinkled on the area - and let them do the rest!

Thank you so much for your information and prompt reply. Sounds ideal. AK