by Lisa Johnson, horticulture educator for Dane County UW-Extension
Did you know you could grow sweet potatoes in USDA Zone 5a, 5b, and 6 in Wisconsin? Sweet potatoes, or Ipomoea batatas, are tender, warm-weather plants related to morning glories. They come from Central and South America, and need a long, warm growing season. They grow on trailing vines, with bush-type varieties for smaller spaces.
Usually you buy what’s called sweet-potato slips, which are unrooted cuttings. They arrive by mail, 4-6 inches long, for planting in late May to early June. Soil temperatures should be a minimum of 55º F, 3 inches down for best rooting. Test this with a soil thermometer.
Strip off the bottom leaves, leaving one node (growing point) and a couple of leaves on top. Plant node-end up, 3 inches deep in mounded rows about 18-24 inches apart. Leave 36 inches between the center of each row, or plant in hills with three or four slips per mound.
If the soil is dry, water it before and after planting. Keep slips moist while rooting, but drier once established. Don’t water the last three to four weeks before harvest. Watch out for voles because they love the roots!
Harvest sweet potatoes before danger of frost in late September or early October. Handle roots carefully; they bruise easily. Gently remove dirt clods without rubbing. Don’t wash them before curing.
Curing sets the skin, heals wounds, and converts starches to sugars. Curing allows the roots to store much longer than those uncured. Ideally, cure them in a warm area for 10 days at 80º to 85º F and high humidity (85 to 90 percent), or next to a running furnace for two to three weeks at 65º to 75º F. Once cured, store in a dark location at 55º to 60º F. Don’t refrigerate! Wrap cured sweet potatoes in newspaper and store in a cool closet or basement. Store properly, they can last six to 10 months!
Sweet Potatoes Recommended for the Lower Half of Wisconsin
Cultivar Name Days to Harvest Root & Flesh Color
- Beauregard 100 light purple skin, dark orange flesh
- Bush Porto Rico 110 copper skin, orange flesh
- Centennial 100 orange skin, flesh
- Jewell 100 orange flesh
- Vardaman 110 golden skin, orange flesh