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White potatoes and sweet potatoes are not cousins. They are not even close friends. White potatoes are in the same family as tomatoes and eggplants, the nightshade family. This is important to know because the tubers are the only part of the plant that are safe to eat. Sweet potatoes are from the morning glory family and the entire plant is edible. There are also differences in preparation, cultivation, and harvesting.
Soil preparation
White potatoes prefer well-drained, sandy or loamy soils that are rich in organic matter with a soil pH between 4.8 and 5.5 to prevent diseases like scab. Sweet potatoes like a similar soil texture that is low in nitrogen. Too much nitrogen and your sweet potato plants will produce more leaves and fewer tubers.
Plant preparation
Select healthy, disease-free seed potatoes, sweet potatoes, or slips (the sprouts that grow from mature sweet potatoes). Order from a reputable provider or buy at an organic food store.
White potatoes do best when chitted. Place your seed potatoes eyes up in a container filled with shredded newspaper or an egg carton. Place them near a sunny window but not in direct sunlight. The eyes will wake up and start to grow. It takes about two weeks. Start in March or April.
Cultivation
Sweet potatoes are tropical plants so wait until the danger of frost is over and soil temperatures are consistently above 55°F, generally in May. When planting, ensure that only the top leaves are above ground; bury the rest of the plant. Use a marker to show where you planted the slip. Keep moist but do not over water.
In Illinois, plant potatoes from mid-March to early May. The University of Illinois Extension Home Vegetable Gardening site recommends soil temperatures of 60-70 degrees for maximum tuber formation. Plant seed potatoes after two or three dry days. Plant approximately 4-5 inches deep, with eyes facing up. When watering, deep water, but avoid overwatering especially during the rainy season. When plants are 6 inches tall, hill soil around the base of the plants to support tuber development and prevent greening from sun exposure.
Harvesting
Harvest new potatoes about 65 to 75 days after planting, and mature potatoes 2 to 3 weeks after the vines die back. Pull the entire plant from the ground. Dig around in the soil for loose potatoes. After harvest, let the potatoes dry for a few hours in the sun and brush off any soil. Wash them when you are ready to cook them. Store them in a cool, dark place to prevent sprouting and greening. Paper bags are good for storage.
Sweet potatoes take about 90 to 140 days to mature depending on the variety. Harvest when they reach a usable size, before first frost. Cut back the leaves and start digging near the initial planting location. Let them dry in the sun.
The University of Illinois Extension site recommends curing for 10 to 14 days at 85 degrees with high humidity. This can be accomplished with a laundry basket, plastic bag(s), and intermittent space heater use. Then store in a cool area. Sugar levels will rise after 7 to 10 days at 55 degrees.
Interested in learning more about growing potatoes and sweet potatoes, visit the Extension website here for potatoes, and here for sweet potatoes. Also, check out the University of Illinois Extension Horticulture YouTube Channel for videos on other horticulture topics.
• Evonne Waters is a University of Illinois Extension Master Gardener serving Kane County. Water’s primary areas of expertise are deconstructed square-foot gardening, winter sowing methods, and chaos gardening.