The sweet potato (Ipomoea batatas) belongs to the family Convolvulaceae (morning glory family). While typically considered as a vegetable suitable for vegetable gardens in the southern United States, sweet potato is a vegetable that can thrive in the hot and humid Iowa summers. In this week’s Yard and Garden article, horticulturists with Iowa State University Extension and Outreach answer questions about growing sweet potato.

Is there a difference between sweet potato and yam?

Yes, they are different. Yam is a monocot (single cotyledon), dry and starchy, rough-skinned, grown for its tuber, originated from West Africa, and belongs to the Dioscoreaceae family. Sweet potato on the other hand is a dicot (two cotyledons), moist and sweet, smooth-skinned, grown for its roots, originated from Central/South America and belongs to the Convolvulaceae family.

What are optimal growing conditions for sweet potato?

Sweet potato is a warm season crop that requires a long, frost-free growing season to mature. Plants require full sunlight and should receive at least six to eight hours of full sun each day. Sweet potato plants vine out and trail along the ground. Well-drained, sandy or loamy soils are the best for sweet potato. Heavy clay or rocky soils will result in misshapen sweet potato roots. Soil that does not drain well may result in lower yields or root rot. Sweet potato is tolerant of a wide range of soil pH but will grow best in soils with a pH of 6.0 to 7.0.

What is the planting material for sweet potato?

Sweet potatoes are started from a vegetative stem tip cutting, also known as “slip,” which is propagated from the roots of the previous season’s crop. Slips should be disease and insect free and bought from reputable vendors. A good sweet potato slip should be firm, green and 8 to 12 inches long, preferably with one or two leaves. Slips may or may not have roots when they are cut.

What is the proper way to plant sweet potato?

Plant in mid- to late-May or early June after the danger of frost has passed. Since sweet potato is a vining plant and prefers to spread, plant slips in rows that are 36-48 inches apart. Plant spacing within the row should be 12 inches. More than 12-inch in-row spacing will lead to a higher proportion of jumbo-sized roots at harvest that are difficult to handle and process. Less than 12 inch in-row spacing will lead to a higher proportion of smaller and slender roots, which are also undesirable.

How do I care for sweet potato?

Before planting, apply 1 to 2 pounds of an all-purpose garden fertilizer, such as 10-10-10, per 100 square feet. At the time of vining, sprinkle another 1 to 2 pounds of an all-purpose garden fertilizer around each plant. Keep plantings weed-free early in the season after which the vining nature of sweet potato will by itself smother and suppress many weeds. Ensure even irrigation throughout most of the growing season; however, don’t keep the root zone constantly wet. It is better to limit water during the last four weeks before harvest to protect the developing roots from cracking.

What are some good sweet potato cultivars for Iowa?

Several sweet potato cultivars are available to home gardeners. These cultivars vary in color, size and to some extent in shape. In terms of color, there are cultivars that have light orange, yellow or even white flesh color. Cultivars that are suitable for Iowa include Beauregard, Centennial, Covington, Evangeline, Georgia Jet and Jewell.

When and how do I harvest sweet potato?

Sweet potato cultivars vary in days to maturity, but most range between 90 and 120 days. Typically, they are harvested late summer/early fall before soil temperatures drop below 60 degrees Fahrenheit. One can wait until the maturity date of the cultivar (90-120 days) or harvest immediately after the first frost. There is no rush to harvest before the first frost, but it is not recommended to expose the root to colder soils for long periods. Cool soil reduces root quality and storage life. Harvest by digging, using a spading fork or shovel, carefully around the plants without damaging the roots.

How do I cure and store sweet potato?

Sweet potato roots require curing to heal cuts, bruises and reduce rots, and to convert some of the starch in the roots to sugar. Commercial growers cure their sweet potatoes in special rooms at 85 F and 80% to 90% relative humidity for seven to 10 days. Duplicating this process would be very difficult for home gardeners. A middle ground approach would be to air dry sweet potatoes for 10-14 days in a warm and shady location like a garage. After curing, the roots should be stored in a dark, cool place with temperatures between 50 to 55 F. Sweet potatoes can be stored under good conditions for four to six months.

Ajay Nair
Extension Commercial Horticulture Specialist

Iowa State University Extension & Outreach

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