Sweet potatoes are a nutritious crop than can be grown successfully in Iowa, if given a little extra attention. Sweet potatoes have two basic flesh types, dry or moist. Moist-fleshed types convert more of their starch to sugars during cooking, becoming softer and sweeter than the dry-fleshed types. Moist-fleshed types are often called yams, although a true yam can only be grown in tropical climates. The skin and flesh of sweet potato roots range from white to orange to red depending on the cultivar.
The sweet potato is a trailing vine which roots down into the soil along the vine. Some of these roots then swell to form the storage root which we harvest and eat. To grow successfully, sweet potatoes prefer both warm days and nights. The sweet potato is a perennial plant in its native habitat, but in our area, it is grown as an annual. Sweet potatoes take up a considerable amount of space in the garden. Jewel and Georgia Jet are two moist fleshed varieties recommended for Iowa.
Sweet potatoes are planted from slips or transplants. These can be purchased through reputable mail-order firms or grown yourself. To produce your own slips, select a smooth, well-shaped root from the previous year's harvest. This root should be about 1 1/2 inches in diameter. Each root will produce several slips, so only a few roots are needed for the home gardener. Place roots in clean sand and cover with an additional 2 inches of sand. Water thoroughly. Water regularly to prevent roots from drying out. Keep temperatures between 75 and 80 for rooting. Bottom heat will promote sprouting. Slips should be ready for transplanting in about 6 weeks or when 6 to 10 well-developed leaves are present. Gently pull each sprout along with its developed root system away from the seed root. Although not recommended, slips can also be produced by partially covering seed roots with water in a jar or other container. If you do not have your own seed roots, store-bought sweet potatoes can be substituted. However, since store-bought sweet potatoes are not likely to be locally grown, the cultivar may not be adapted to Iowa growing conditions. Although yields will vary with cultivar, gardeners can expect 50 plants to produce about a bushel of roots for storage.
Sweet potatoes prefer an acidic, well-drained, sandy soil. Heavy, clay soils should be avoided. Before planting, prepare the soil by tilling and ridging the planting area. These ridges or raised areas will allow the soil to warm and dry faster. If your soil is heavy, ridges should be 12 to 15 inches high. On light soils, ridges 8 to 10 inches high are adequate. Space the ridges 2 1/2 to 3 1/3 feet apart. Allow the ridges to settle for a few days before planting. Slips should be transplanted into the garden when the soil has warmed, usually 2 to 3 weeks after the average last frost date. Space slips 12 to 18 inches apart in the rows.
Maximum yields are produced with good cultural care. Water plants once a week during dry weather to prevent the cracking of roots which can occur with wide moisture fluctuations. Do not cut vines back during the growing season. This practice can cause the sweet potatoes to sprout in the soil and reduce storage life.
Sweet potatoes should be harvested just before or after a vine killing frost. Dig roots carefully to avoid skinning or bruising the roots. Before storing cure the sweet potatoes to promote healing of wounds and improve flavor. Place the sweet potatoes in an area with a temperature of 80 to 85 F and high relative humidity for approximately 10 days. A high relative humidity can be provided by placing the roots in storage crates or boxes and covering them with paper or heavy cloth. Packing in perforated plastic bags will also keep humidity high. Roots can also be cured near a furnace for 2 to 3 weeks. Once cured, store roots in a dark location with temperatures of 55 to 60 F. Good storage results can be obtained by wrapping cured sweet potatoes in newspaper and storing them in a cool closet. Freezing or canning are other storage alternatives.
This article originally appeared in the February 7, 1997 issue, p. 10.
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