I am living in Ottawa Canada, zone 3-4 and was wondering if all perennials are all ok if we divide and transplant in the fall. What kind of fertilizer should I use for transplant and when should I do this? My Dalhia are all in flower right now and are huge. The stem is almost as big as a shrub. I was wondering when I shall remove the root from the ground? Could I just leave it in the ground if I cover it well with mulch? If I remove the root, can I break it in numerous roots in the spring in order to get more plants? How should I care for it in the winter time?
Gosh, I don't "do" dahlias, they are a more specialized type of situation. I don't really consider them perennial, at least outside of their native Mexico! I know that you cannot leave dahlias in the ground after frost; the freezing will kill them. My best advice to you is to subscribe to Dave's Garden for $20 (it's a bargain) and ask someone on the Dahlia Forum, who will actually be able to give you a specialist's answer.