One way to get a jump on your growing season is to use a cold frame. While this can be done with almost any plant, if you want to plant sweet potatoes you must create a "hotbed" in your cold frame, because sweet potatoes need a warm soil to grow in. You can make a hotbed by using horse manure which will heat up as it decays and warm the soil allowing you to plant your potatoes up to eight weeks before they would normally be planted out in the garden.
Prepare the horse manure six to eight weeks before you want to plant the sweet potatoes by turning fresh manure two or three times to mix it thoroughly. Add water to it if it appears dry so that the manure is fairly moist.
Ready your cold frame by clearing the soil and any spent plants from along the bottom. Add a foot deep of moist, prepared horse manure to the cold frame and pack it down well to create a "floor" in the frame.
Add three to four inches of garden soil over the manure. Close the hotbed and allow the manure to heat the soil for about a week until it maintains a constant 80 to 85 degrees Fahrenheit.
Place your seed potatoes on the soil, spacing them about 10 to 12 inches apart. Cover the potatoes over with a 3-to-4-inch layer of garden soil.
Keep the hotbed closed through the evenings and overnight, but prop the lid open during the day from about 9 a.m. to 4 p.m. When the outside temperatures warm over the summer you can remove the lid entirely.