Well it used to grow on bombsites in my younger years. It spread by little parasols of floating seeds. So I guess it could be invasive. It is also called 'Fireweed'. Rose bay willow herb too, and not often seen, unless we have another war. Rather a pretty flower and grow it if you will. Spreads by rhizomes and those wafting seeds...so may not be popular with the neighbours.

In this part of the country the wild version is well settled in as a weed and spreads uncontrollably. The garden version is surely much more polite in its habits, but I expect it would spread, too.

Funny re neighbours, I was just about to plant it in the back garden and thought "ey up I have seen those leaves before!" Sure enough for the last few years I have spent time pulling them up from my front garden as they have grown through from the neighbours!! I have had a quick look on line and they do say they are invasive . May plant it up in a large pot to stop the rhizomes. I have done this with 2 large bamboos about 3 years ago and so far so good.

Quite honestly I wouldn't touch it with a bargepole. You only need one plant and next year it will be everywhere. Planting it in a pot won't stop it seeding and its the seeds that spread it far and wide. I pull out all willowherbs on first sight!