Work in plenty of compost, avoiding nitrogen-rich fertilizers that produce lush vines and stunted tubes. In the North, cover the raised rows with black plastic to keep the soil warm and promote strong growth.
It's best to plant root sprouts, called slips, which are available from nurseries and mail-order suppliers. Or you can grow your own, by saving a few roots from your previous crop or by buying untreated roots (store-bought sweet potatoes are often waxed to prevent sprouting).
About six weeks before it's time to plant sweet potatoes outdoors in your area, place the roots in a box of moist sand, sawdust, or chopped leaves in a warm spot (75º to 80ºF). Shoots will sprout, and when they reach 6 to 9 inches long, cut them off the root. Remove and dispose of the bottom inch from each slip, as that portion sometimes harbors disease organisms.
Sweet potatoes mature in 90 to 170 days and are extremely frost sensitive. Plant in full sun 3 to 4 weeks after the last frost when the soil has warmed. Make holes 6 inches deep and 12 inches apart. Bury slips up to the top leaves, press the soil down gently but firmly, and water well.
Harvesting: You can harvest as soon as leaves start to yellow, but the longer a crop is left in the ground, the higher the yield and vitamin content. Once frost blackens the vines, however, tubers can quickly rot.
