I have spent the summer watching my crosnes (Stachys affinis) grow and just recently went and did some diggin in the soil and all I can say is...BOY am I glad I planted the darn thing in a container!!!
The thing sent stolons all through the pot and even above the soil line. I have yet to dig to see how the fruiting portion of the root is doing...I'll report back when that happens.
By the way it was trying ot flower though appears the flowers will be insignificant at best.