There is a perennial type of wallflower - Siberian - which lives for a few years and can be propagated by cuttings. They have slightly smaller flowers than the more usual type, usually in oranges and yellows. Ordinary wallflowers normally need to be grown from fresh each year to flower the next, but I suppose it would be worth cutting some back to see if they did come back again the next year. More probably though they are self-seeding to reappear in the wall.