There are two main sweet potato (Ipomoea batatas) varieties, those with white flesh and those with orange or purple flesh. They're roots, not the same as the tubers or thickened stems that store carbohydrates for Irish potatoes (Solanum tuberosum). Although supermarkets sometimes label sweet potatoes with colored, moist flesh as “yams,” they're not the same either.
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Sweet Potato Climates
Sweet potatoes will grow as perennials in U.S. Department of Agriculture plant hardiness zones 9a through 10b, and as annuals in colder climates. They grow from sea level to altitudes of 15,266 feet.
Irish potatoes are most often grown as annuals in USDA zones 2 through 11, but can be grown as perennials in zones 10 and 11.
Purple yams (Dioscorea rotundata, USDA zones 9a through 10) and white yams (Dioscorea alata, USDA zones 9 through 11) are both perennials.
White and Colored Sweet Potatoes
In developing countries where sweet potatoes are used as a food staple, the varieties have cream-colored to white flesh and a bland flavor. They’re dry in texture and are not sweet. Their starchy, dry content provides energy to staple diets. When baked, the starch in sweet potatoes breaks down into maltose and other soluble sugars.
Sweet potatoes eaten as a vegetable or dessert in developed countries are moist, have a distinct flavor, high sugar content, and a yellow or deep orange color.
Green or purplish sweet potato stems in the form of vines grow along the ground and are less than 6 inches tall. They have a milky juice and grow roots at nodes. They are not branched, instead growing short lateral stems from their sides. Their oval to round leaves are roughly 5 inches long with stems from 1 to 4 inches long. These tops die back to the ground each winter. They bear pale violet to white funnel-shaped flowers.
Sweet potatoes have slender, pointed ends, not rounded ends as do Irish potatoes. They mature in three to four months. Each plant will yield four to five sweet potatoes from 3 to 9 inches long and about 1 3/4 inch wide. They have thin skins and bruise easily when they’re dug.
The Vegetable Research and Information Center at the University of California-Davis reports that 1 cup of sweet potatoes contains roughly 103 calories and 3 grams of dietary fiber, or about 12 percent of the recommended daily amount. Sweet potatoes contain iron, potassium and vitamin C. Yellow or orange sweet potatoes contain beta-carotene that yields vitamin A.