Cire wrote:

My advice is, DON'T.

Just wait and watch, it will expand at an alarming rate once it is ready
and, unless you are able to contain it, you will wish you had never let it.

Brambles are OK. Thornless varieties with nice big brambles are handy.
The thorny ones make a pretty good robust anti-climbing decoration for a
rough fence. That, rosa rugosa and honeysuckle can fight it out.

They are absolutely taking over my garden and I am quite powerless to stop
them. They are growing at an alarming rate all amongst thick shrubs, on
steep banks, where I can't possibly get at them. Once they start they will
put out long leaders which root and start the process all over again so
that, in a year, one bush can became twenty, ad infinitum!

I can't poison them without killing everything around them; I can't dig them
out because I cannot get into them without cutting other plants down. If
anyone has advice on how ho kill them in this situation, would they kindly
let me know.

Keep cutting them back every time they regrow. Paint the leading new
growth stems with dilute glyphosate. Don't miss. Never let it root down.

Regards,
Martin Brown