I was in Lowe's earlier this year, and saw that they had sweet potato slips for sale, so I bought a container of them, and followed the directions someone gave me for planting them. And, in spite of how dry it was this year, they grew! Only problem being, this is the first year I've grown them. So, I haven't a clue as to how to tell when they're ready to harvest. Anyone else grow them? What do I look for to know whether or not they're ready to come out of the ground? Thanks.