Call me crazy, but I've tried growing sweet potatoes twice here in the Michigan UP - both years were more or less the coldest summers we've ever had, so I didn't have the best results, although a couple of years ago I did get a few that were at least edible. I think the next time I'll just grow them for the leaves!
As far as starting the slips, that much I can do (it's really easy, trust me. All you need is a sweet potato and a jar of water...
next you can just break the sprouts off the potato and stick them in a different jar of water and they will grow roots, and they can then be planted in the ground.
I'm not sure exactly when I stuck the potatoes into the water, but the photo was taken 5/23 and it did take them a while to start sprouting, so I think it was probably mid April.
Hope this helps (and that you are more successful than I was!)
