New garden with clay substratum - please help me!
 

We've acquired a 50-foot long, 4-foot wide strip of canal bank that has never been gardened before. Presumably as part of the original canal building the ground is comporised of a laughbale layer of top soil - about 8 inches - overlaying a hard-packed layer of clay, beneath which is aa porridge-like layer of gravel. There are some serious boulders too.
Can anyone suggest what I can plant straight into this garden that would be sufficiently vigorous and thuggish to survive.
Please don't suggest either trying to improve the composition (this would incur major expense and we're both pensioners) or hard landscaping it (ditto, and we hate that kind of gardeing anyway!).
Many thanks.