Well, I just planted it last year, so it may be too soon to tell--
(it is really dark and gloomy out there today--been raining all week--but here is a not-so-great pic just snapped)
I've got it down on the street-corner edge, in a rockpile by the firehydrant, where it can duke it out with the horehound, just in case. So far, it has tripled in size from the little plug I planted. It hasn't produced any runners and popped up elsewhere, and hasn't melted down in all the rain, but it looks like the horehound is winning the spreading competition...