Self seeding plant

I can't remember the name of this plant. Not even hazard a guess at the moment to search on the internet. Never mind, the point is that this plant is a very easy self seeder, but beware, although pretty, it can take over your garden...... It's easy to dig up the tubors when they take over. I think I dug up too many last year - it takes a bit of getting used to seeing gaps in the soil again.