Posted: 25/04/2014 at 23:40
The clay, rocks and soil aren't particularly the problem. The roots would be, as they're likely to grow into plants you don't want, so I'd get as many of those out as you can.
You could give it over to wildlife, in which case there are 3 plants I know that are pretty much indestructible - at least in my garden. These are Aquilegia, Californian poppies, and (holding breath here) Nettles. The Aquilegia and Poppies are loved by bees - at least the yellow poppies are, and the Nettles are loved by butterflies amongst others. See http://www.nettles.org.uk/nettles/wildlife.asp.
I keep the nettles and Aquilegia under control, but let the poppies seed themselves.