From The Old Farmer's Almanac for Kids Volume 5
Sweet potatoes are grown from slips—sprouts that develop their own roots. Most varieties require long growing seasons. If you live in the north, try ‘Centennial’, which needs 90 to 100 days to mature. You can buy sweet potato slips from garden centers, nurseries, or local farmers. Or, you can grow your own!
How to Grow Some “Sweet” Sweet Potatoes
1) Go to your supermarket and select untreated sweet potatoes with no blemishes or cracks. One potato should yield about 12 slips.
2) Store the sweet potatoes in a well-lit room with a temperature between 70 and 80 degrees F. Place them in a box about 2 inches apart and 2 inches deep in a good-quality potting soil. Water regularly to prevent them from drying out. Do not overwater. In 2 to 4 weeks, you should see sprouts (the slips) growing up past the soil.
3) When the slips are 6 to 12 inches tall and all danger of frost has passed, carefully remove them from the potato and plant them outdoors in fertile, well-tilled, well-drained soil deep enough to cover their roots and about ½ inch of the stem. Set them 12 to 18 inches apart.
4) Water the slips generously for a few days to make sure that the plants root well. Water regularly thereafter.
5) Fertilize with a 5-10-10 mix 3 to 4 weeks after transplanting.
6) Weed occasionally. Do not prune the vines.
7) Harvest 3 to 4 months after planting, when the leaves begin to yellow.