I think it's been around in my garden for a while but I've always pulled it up when tiny. I've just removed 4 fairly mature specimens that conned me whilst hiding in a carefully sown flower bed. I spaced out 5 different seed types in groups of 5 or 7 with the plan that they would intermingle nicely. Didn't bank on a dud packet of seeds (or dud planter!) and the willowherb stepping in to grow in exactly the spots where the seeds should've been germinating.
I uncovered a massive couple of them whilst weeding my neighbour’s garden a few weeks ago. The seeds went up into the air and the wind blew them aaaaaalllllll into my garden. Fast forward to Monday afternoon and I had about 80 little seedlings popping up in the gravel.
I’ve started taking a plastic bag over and pulling them out in that and then carefully depositing them in a big sack.
Posts
https://extension.umaine.edu/cranberries/grower-services/weeds/hairy-willowweed/
That flower bed has been thinned out
Each plant can produce up to 1,000, 000 seeds.
I live in west central Scotland - not where that photo is...
Not only a thug, but a cunning devil as well