Joe Muellenberg, Dane County UW-Extension Horticulture Educator
The sweet potato (Ipomea batata) is a delicious, nutritious, and high-yielding vegetable crop. Prepared as pies, fries, or in African stew, this “super food” is high in fiber, vitamins A and C, and low in fat. Some varieties are grown ornamentally in containers or hanging planters and produce smaller roots not used for consumption. Not be confused with the yam, the sweet potato is orange in color and native to the Latin American region. The true yam (Dioscorea sp.) is unrelated genetically, less nutritious, white or yellow in color, and native to the Asian and African regions.
Sweet potatoes are propagated vegetatively rather than by seed. Old potato seed is used to grow slips, or transplants. Slips can be purchased, but it may be more economical to sprout your own slips. Do not plant until early June until all risk of even light frost has passed. Remember that the slips are very fragile because they rely on establishing new root systems rather than obtaining nutrients directly from the tuber like the standard potato. Do not store slips in water before planting. The photo to right shows slips that have prematurely sprouted roots and are weakened having spent precious energy before being planted. Keep the slips in a warm, dry space for no more than 5-7 days before planting. Each node will sprout a sweet potato. Remove leaves from the nodes except for 1-2 at the upper-most growth point (meristem). Be sure to plant the slip vertically in the ground with all leafless nodes buried. Remember that each node will sprout a sweet potato so planting less nodes will yield larger, less numerous sweet potatoes.
It is very important to give your plant the proper spacing. A member of the morning glory family, the sweet potato will spread for many feet. Slips should be planted at least 12 inches apart with at least 24 inches between rows. If you are planted in raised beds, give each plant at least 4 square feet. More space is optimal. If your space is limited, consider vertically trellising the vines. Your slips will wilt from transplant shock. Water for several days after planting and they will recuperate as the roots establish. After initial planting, sweet potatoes will need 2 inches of water weekly, twice the average vegetable plant. Proper mulching with improve water retention, keep competing weeds down, and increase soil temperature. Thickly spread marsh hay or grass-clipping or use black plastic sheet mulch (biodegradable is best).
To harvest, dig the main crop of sweet potatoes before or directly after the first “hard” night frost (below 32 degrees) in the fall. Most varieties need 90-100 days to grow, so most Wisconsin grower leave the roots in the ground to mature until first frosts in October. To harvest, remove the top growth by snipping the stem at the base of the plant. Dig 1-2 feet around the sweet potato cluster with care not to damage the roots as you dig. Use a spading fork or stout shovel and be careful not to bruise, cut, or otherwise damage the roots. Dig below the level of the ridge and gradually move closer toward the plants, removing soil until the fat roots are exposed. Sweet potatoes should be handled as little as possible to avoid damage. To clean off stuck-on soil, lightly spray with them water and try to avoid rubbing.
For best quality, use the potatoes as soon as possible after they have been stored. If you are would like store them overwinter, you will need to allow them to cure. Curing them in a warm, humid space (85 degrees; 90 percent humidity) such as a porch or garage. After 1-2 weeks, the starches will convert to sugars and thicken the skin for longer storage and will make the potato sweeter. Good luck growing!