Same sort of problem in our garden with the nettles, I was strimming them like a demon and they'd be back, Jaws music in background
Then I saw loads of capertillars on them, that's what my youngest called them and saw a program about leaving them for Butterflies, so I do and they eat a lot of it and when the Bufferlies (young son again) have gone I go strimmer mad again, oh and get OH to pee on them :-D