Tough plants for tough places

Once of the best gardening lessons I ever had was from the esteemed designer, Jill Billington, who pointed out sagely that plants don't read books, so the only real way to see if they work in a particular spot is by trial and error. I find this a useful, comforting and entertaining gardening attitude (as long as you're not spending tons on very expensive plants). It's particularly liberating when it comes to guerilla and community gardening, because anyway the books never take into account the added complications of dog and human abuse, the  usually horrendously poor soil, lack of watering, feeding and any real tlc at all. Plants out there in the city need to be very thick-skinned and able to fend for themselves.
In my encounters with community gardening I have found out the following: penstemons, billed by the RHS website as "easy to grow in any fertile, reasonably moist, free-draining soil in full sun or light shade" will also flourish in the poorest of rubble-laden soil (yes, it is free-draining), that is prone to drought; in fact, just to really annoy me, they do much better than the same cuttings in my own garden.
Some plants positively thrive on neglect. I have battled with rosemary for years, trying different varieties, in pots, in soil, new cuttings. But my biggest success is a tiny cutting stuck into rubble-soil on top of a neighbourhood wall; it's going great guns now, clearly happy in the illusion that it has escaped to some craggy mediterranean hillside.
Meanwhile, the oleander I donated to the same wall-garden, hoping it too might think it was at  home on some mediterranean roadside, is sick and struggling when it's supposed to be 'tolerant of poor soils and drought'. Hollyhocks in the same spot thrive unravaged by snails, unlike those in my garden and look quite healthy (apart from the bindweed).
Another other top performer is red valerian, although it doesn't seem to self-seed quite as merrily outside my garden as it does inside - I'm hoping for the same look around my neighbourhood as those pink and white festooned lanes in the countryside, where valerian has found its way into every nook,cranny and crevice. But for sheer persistent flower-power you can't beat it - it literally doesn't stop from early May right through to September (in fact I almost get sick of it) and the bees and butterflies love it. Lychnis coronaria on the other hand will self seed anywhere, and manages to look rather exotic in a sparse guerilla border with its velvety leaves and blooms. My other sexy stalwart is the  fabulous geranium psilostemon in the picture - it never seems to stop producing gorgeous bright magenta pink flowers all summer in sun or shade.