Image: Paul Pellegrino/Shutterstock.comSweet potatoes, sometimes spelled as one word, sweetpotatoes, (Ipomoea batatas), are a herbaceous vine and part of the morning glory family. A long season root vegetable, they are native to warm, tropical regions and originated in Central and South America, where they have been cultivated for thousands of years. Riding a heightened surge of popularity in North America, sweet potatoes are often referred to as “yams” to distinguish them from potatoes (to which they are not related despite their name). Sweet potatoes, however, are not yams—as botanically they are quite different. Yams, which originated in Africa and Asia, belong to another cultivar group (Dioscorea). Yams have dry, rough, scaly skin and are starchy tubers few would ever describe as having a “sweet” taste. In contrast, sweet potatoes have smooth skin of varying colors, moist flesh, and a sweet flavor due to postharvest curing (which turns starch to sugar). They are also very rich in fiber, beta carotene, and chockful of vitamins A, C, and other antioxidants and nutrients. Size is a differentiator as well: yams can grow to enormous dimensions, weighing more than 90 pounds and measuring several feet in length; sweet potatoes tend to be slender, tapered, and petite in comparison. Because the terms sweet potato and yam are often used interchangeably in the United States (and Canada), the U.S. Department of Agriculture (USDA) requires yam labels to always be accompanied by “sweet potato” in an attempt to alleviate confusion. Actual yams are not commonly found in North American supermarkets, but specialty stores and ethnic suppliers often carry them. Sweet potatoes are popular both domestically and abroad, with a healthy portion of U.S. supply exported to European countries. North Carolina has long dominated U.S. production and is responsible for 50 to 60 percent of supply.
Types & Varieties There are up to a thousand varieties of root tubers (a few are poisonous), and hundreds of sweet potato cultivars. Outer skin can range in color from coppery pink, pinkish-red and red, as well as off-white and deep or light purple. When it comes to flesh, some varieties have purple, others are white, and many are variations of the familiar orange, found at most supermarkets, restaurants, and farmers markets. Among the most popular varieties is the Covington, developed by North Carolina State University professor and extension specialist Dr. Henry Covington. Affectionately known as “Mr. Sweet Potato” for his lifelong work with the root tuber, Dr. Covington is credited with not only raising the state’s profile as a sweet potato producer, but helping growers steadily increase yields. His legacy and influence continue today, as the Covington variety still dominates the majority of sweet potato plantings in North Carolina. The Covington is small or medium-sized, oblong, tapered, has smooth pink or rosy skin, bright orange flesh, and is generally considered the tastiest, creamiest, and sweetest type of sweet potato. Other varieties with varying skin and flesh colors include Bayou Belle, Bellevue, Beauregard, Bonita, Burgundy, Centennial, Evangeline, Garnet, Georgia Jet, Heart of Gold, Jersey, Jewel, Murasaki, O’Henry, Orleans, Stokes, Sumor, and Vardaman. CULTIVATION Sweet potatoes can be propagated by planting whole seed potatoes, slips (rooted sprouts from a mature sweet potato in soil or water), or vine clippings, and grow best in warm weather. Once the roots have spread, minimal weeding is required. Plants are not drought-tolerant, so irrigation is needed if extended dryness occurs. The average growth cycle runs from 90 to 120 days, as long as development isn’t interrupted by frost. Two to three weeks before mechanical harvesting is set to begin, irrigation is typically stopped so vines begin to dry out. Dug up sweet potatoes should be collected from fields quickly, to prevent sunscald during in the day and chilling injury from dropping temperatures at night. Careful handling will minimize damage to skin and reduce decay. Unwashed sweet potatoes are cured for 4 to 7 days after harvest. Temperatures ranging from 77 to 90°F, along with high humidity (at 90 to 95%) and ample ventilation, will stimulate the outer skin to heal, eliminating most bruises and wounds from harvest or handling. Pests & Diseases Sweet potato flea beetles injure leaves with narrow channels or grooves in the upper surfaces, turning them brown. Shallow tunnels can also be found on roots, which darken and leave scars. Several other types of beetles can pose a threat as well. The sweet potato weevil makes small holes on the stem-end allowing larvae to tunnel inside. White grubs leave large, shallow feeding scars. Other pests of concern include aphids, armyworms, caterpillars, leafminers, and wireworms. Nematodes in the soil will also affect growing plants. Various diseases and physiological disorders can plague the sweet potato growth cycle. Rhizopus soft rot is a common fungal disease infecting root wounds. Diseased sweet potatoes turn soft, become overly moist, and change color (which varies among cultivars). Black rot also attacks root wounds and is spread by wind, water, soil, harvesting baskets, some insects, and even human clothing. It can survive for several years in the soil, so crop rotation is recommended. Symptoms include small, sunken dark spots on the surface. Fusarium rot is common in long-term storage; lesions are circular with light and dark brown rings. Other diseases to look out for include several types of blight, mold, scab, stem rot, and scurf.
Storage & Packaging
Optimal storage is between 55 to 59°F with high relative humidity (greater than 90%). Under these conditions, sweet potatoes can be kept for up to year, providing for year-round availability.
Temperatures above 59°F can cause rapid sprouting and weight loss. Chilling injury (at temperatures below 55°F) can result in decay, pulp browning, and shriveled roots.
References: Alabama Cooperative Extension, Louisiana Sweet Potato Commission, Louisiana Dept. of Agriculture & Forestry, North Carolina Sweet Potato Commission, North Carolina Cooperative Extension, Oklahoma State University, Purdue University, University of Georgia, University of Illinois Extension, U.S. Sweet Potato Commission, USDA.
GRADES & GOOD ARRIVAL
Sweet potatoes are divided into five separate USDA grades: U.S. Extra No. 1, U.S. No. 1, U.S. No. 1 Petite, U.S. Commercial, and U.S. No. 2.
All have specific details and descriptors, as well as size requirements (with the exception of U.S. Commercial).
Sweet Potato Retail Pricing: Conventional & Organic