Sweet potato is a hot weather plant, but to be able to plant them when the ground is warm the slips need to be growing now. I start them indoors with potatoes grown the year before in our garden. The potatoes store very well in my basement in a laundry basket all winter. Even now I still have potatoes that are firm and fine for culinary enjoyment. I really love eating sweet potatoes and we have a lot of fun in the fall digging them up to see what treasures have been growing underground during the hot summer months.
To start slips it is good to use potatoes that are free from any mold or shriveling. If there are any potatoes that have sprouts already coming out, then you have a head start. Place the potatoes in a jar or container vertically and fill with water. Their bottom ends should be wet, but at least one to two inches of the tops need to reach above the water. Place this in a warm sunny window or a shelf in the kitchen that stays moderately warm. Be sure to add water when necessary. Changing the water occasionally is important, too, because rot can occur. If you notice a potato rotting, remove it and either dispose of it, or cut off the rotten part. Soon vines will start to grow. They will grow out of many different nodules on each potato. When any of these vines grow to about 5-6 inches, pick it off at the base and place in a jar filled with water. It is amazing how quickly these little guys will root. When a potato starts producing slips, it usually becomes very prolific and many slips can be picked off of one potato! Once the ground is warm and the slip has a good set of roots you can plant them. We usually get all our sweet potatoes out the last part of May.
Other methods exist to start slips. I have a friend that remembers that her mom started slips in a box of horse manure. The heat from the manure helped the plants get started. I also have another friend starting them in potting soil on a heated bed with florescent lights.
Slips can be bought in bundles from nurserys and online as well. They need plenty of room for growing. Their beautiful dense green vine takes up at least a five foot wide area, but can be set out as close as two feet apart in rows. We occasionally grow giant football sized sweet potatoes that can fill 3-4 pies worth (though were prefer a good baking sized potato). They are so good to eat so many ways. Sweet potato pie, fried sweet potatoes, sweet potato fries, candied yams, sweet potato casserole, baked sweet potatoes with sorghum...