How to kill a Buddleia?

Calibos

Have a Buddleia growing between a wall and the premises behind. I sawed it off flush with the top of the wall last year but it's back as big as ever. It's still in leaf at the moment. Is now the time to spray it or does that matter. Should I instead cut it back flush with the wall again and drill into the trunk and/or paint the cut trunk with some chemicals?

Unusually, I actually have access to some of the deep roots. There is a sewage manhole closeby where the roots emerge. No wonder the thing is so tenacious with access to all those nutrients. I bring that up as I wondered if it means I can bypass the other application methods and spray the roots directly and thus hasten the plants demise?

looksee

If you are spraying with a systemic weedkiller to get the roots and all, you need to do it while there are growing leaves. Now if not sooner is the time to be spraying it. I would suggest you spray all the leaves now, cut it back in about a couple of months, then when new growth appears spray it a couple of time on the new leaves.

I don't know about the roots, I suspect it might not make any difference, but no harm in trying, and cut them back also in a couple of months.

Sonnenblumen

Cut back as much as you can, drill a number of long holes and add a little undiluted systemic weedkiller (eg Roundup) or alternatively diesel.

These plants are an absolute nuisance and can cause considerable damage to properties and underground services.