HI Folks
Any suggestions for the best way to kill off the mint that's trying to
take over one corner of my polytunnel..

I planted it (some years ago) in a pot, as recommended, to restrain it.
It's escaped... and is now spreading.

I did lay some ground cover over it (old tarpaulin) - which seems to
have suppressed it in the centre, but now it's made its way around the
edges..

I try not to use weedkillers in the (vegetable) polytunnel - but
'organic' techniques (like vinegar) offer only a temporary respite.

I'm guessing that something systemic is required.
Recommendations, please?

Thanks