Oh they become monstrous beasts! Lots of thorns, one short bloom season, and they're highly susceptible to Rose Rosette Disease. I had one show up near where another rose had grown and I at first thought a piece of root had survived. I made the mistake of leaving it there for a year before realizing it was a multiflora- was that ever a pain to dig out! The roots had gotten massive in only 1 year, so it's a good thing you found out what it is before it became a thug
