Hi Jacquetta. Leylandii hedging takes all the goodness out of the soil, so I doubt there's anything that would grow successfully. But see what others say.

I agree with Rosie. There is little that will grow through Leylandii except brambles and other tree seedlings all of which will grow anywhere anyway.

Also if what you grow keeps much light from the hedge it will go brown underneath whatever it is and the brown will not recover.