So you have mare's tail (also called horse tail). It has been around since
dinosaurs roamed the land!

I have found the weedkiller "SBK" to be very effective in getting fid of it
and if it's among other plants use a paintbrush very carefully - it comes up
my asparagus bed sometimes so I have to be careful.

The roots of mare's tail go very very deep so unless you're a miner . . .
..!!

Regards

Ron