Does anyone know what this is? I spent an awful lot of time trying to eradicate it from the perennial beds last year - clearly to no effect, because suddenly, in the last week, it's everywhere.
Maybe it's something beautiful or useful and I've been battling it for no good reason. Or maybe (and this is my suspicion, since anything that spreads that quickly can't be good) it's a vile, invasive weed that I will be fighting until the day I put down my gardening tools.
It's about two inches high (and getting bigger every day) with pointed green leaves and purple stems.
Help! What is it?