the solution is

1 - cut between where you want to keep the boo & where it must be removed.
If needed cut what you need to have good access the the drench.
= when done, place a plastic separation.
(eventualy none stay so nothing to do for point 1)


2 - cut ALL culms where the boo is to be detroyed

3 - Wait until spring growth. let is go up...
The boo is using it's stored energy to get new culms up
let it go;
the new culms start branching. they are still soft = let it go up.
The new culms makes thier first leaves = CUT at ground level.

wait again for new growth& go back to point 3.

The more you let the culms grow, the less you will have to do afterward.
After that you can start digging away... or chemicals...