commonly called Loosestrife..It can be divided now, so you can get rid of or move as much as you like right now.

Once you've said Lysimachia a few times it'll be second nature, better to remember the proper name, if you can, because you'll never be misunderstood that way.

All new gardeners suffer far less confusion if they've remembered the proper name - we see all manner of confusions which are put down to this topic :-)

This plant, though pretty, really is a bit if a thug and I know what you mean by 'invaded'. I inherited it and tried, in my last garden, to keep it under control, but every last little bit of root has to be found and I'm sorry to say I never won the battle with it.

Oh dear !

One way to stop thugs is take a large plastic flower pot, cut off the bottom, and bury it up to the rim in the garden. Plant your thug inside it - it can't escape! You will still need to dig it up every third year or so as it will have filled the pot and be congested. but at least it can't take over the whole garden