In article , Bob Hobden
writes

You're the first person I've known who's bought it, but when I was a
child our parents collected some seeds from the grounds of a stately
home and sowed them in our garden. Like you we realised our mistake
after a couple of years and had to pull them all out.
Tomorrow I'll be with a gang of birdwatchers trying to remove it from
the banks of the River Loddon. It's rather late in the season, I think
some of it will have ripened seeds already.

What we used to do along the River Mole is to simply walk on them earlier in
the season, crush them under foot. Unfortunately by now they will be so tall
and strong and thickly growing that won't work. As you say I would expect
some to have seeded already especially this year.

All ours seems to be in places where you can't walk easily - it's wet or
steep or overgrown. I expect people have already eliminated it wherever
they could! We were in time to avoid causing fusillades of ripe seeds.
--
Sue ]