I agree, mid-March would probably be a good time for you to start sprouting the potatoes -- it would be better to start too early than too late, you can always prune them back a bit if need be. I do really love sweet potatoes so will probably try it yet again... when I grew them a couple of years ago I planted them in my (unheated) hoop house, so they certainly did better than they would have in the main garden, but I think they needed about a month more of (warm) growing season than we had.
My zone supposedly changed from 4b to 5a, but not sure I agree with that... although this year would certainly support it, we've only had a couple of nights with temps slightly below zero where I am, near the north shore of L. Michigan (Escanaba area). Not much snow here either (although a different story up by L. Superior), so a good thing it hasn't been terribly cold -- one year just about every water pipe in the area froze because there was no snow and the winter was so cold! Don't tell anyone, though, because if this kind of weather keeps up everyone will want to live here!
