Sweet potatoes are actually roots that grow underground.
image by Vangelis Thomaidis/sxc.hu
Sweet potatoes--which are not the same as yams--are generally planted on a ridge so that harvesting can be done on the side, without damaging the plant. After harvesting, sweet potatoes need to be laid out and dried for several hours and then stored for 10 to 14 days in an 80 to 85 degree F area so the sweet potatoes can cure before using.
Harvest the bulk of your sweet potatoes about 100 to 140 days after planting, generally in mid to late fall.
Harvest a few sweet potatoes in late summer if you can't wait until fall. However, do not harvest too many of them. You can do this by carefully digging into the side of the ridge where the sweet potatoes are planted and remove them without damaging the plant.
Harvest the remaining crop just before the first frost. Carefully dig into the side of the ridge, using a spading fork or shovel. Do not to damage the sweet potatoes. For the first sweet potato, dig on the side of the ridge, a bit further down than you think the potato is located and work your way up until you find the root (potato). Use this to help you determine where to dig for the rest of the sweet potatoes down the line.