Mulch the mounds
Cover the soil with mulch to conserve moisture and discourage weeds. In the North, laying black plastic mulch on the ground a week or two before planting is an effective way to warm the soil. To plant the slips, poke a hole in the plastic and set the plant in the hole. Mulch also helps produce bigger sweet potato yields by preventing the plant from rooting at spots where the vine and soil meet. These side roots draw energy away from root production on the main plant, Drowns explains. In southern zones, mulch protects plants from the sweet potato weevil (Cylas formicarius), a widespread pest that gains access to the roots through cracks in dry soil.
Harvest after frost
“Remove the vines and dig immediately after the first light frost,” says Richard de Wilde, of Harmony Valley Farms in Viroqua, Wisconsin. Cure sweet potatoes for about three weeks in a warm, humid location, such as your kitchen. After curing, store sweet potatoes in a dark spot at moderately cool temperatures (55° to 60°F). Healthy roots that have been cured and stored properly can last four to six months in storage.
Where to Order Sweet Potatoes
Sweet potatoes need about four frost-free months to mature. If your growing season is shorter than that, you don’t have to give up on growing your own sweet potatoes. With one or all of these strategies, you can harvest your own sweet potatoes no matter how far north you live.
Start with a short-season variety. Try an heirloom proven in short seasons, such as ‘Frazier White’, ‘Continental Red’, or ‘Ivis White Cream’, or the more widely available ‘Centennial’.
Use plastic mulch. Cover your sweet potato mounds with black plastic two to three weeks before you plant—the soil temperature beneath the plastic is typically several degrees warmer than uncovered soil. When you are ready to plant, leave the plastic in place and cut slits in it through which you will put in your slips.
Don’t rush planting. Researchers at North Carolina State University found that sweet potatoes grow faster when planted later. “Even if you have a short season, you’re far better off waiting until the early part of June to plant,” says Glenn Drowns, a seed saver and sweet potato preservationist in Iowa.
Grow in containers. Sweet potatoes produce well in large containers such as whiskey barrels. Remember that they need to be well watered and fertilized regularly. (Use an organic liquid fertilizer, such a fish-and-seaweed mix, which is rich in micronutrients as well as the major ones.)
Extend the season. When the first light frosts of fall threaten to end your sweet potato season, you can keep them growing a bit longer by bringing containers inside or covering the plants with old sheets or blankets.