Sweet potatoes (botanically, Ipomoea batatas) are tropical plants that need a relatively long growing season—usually four to five months—so they're ideally suited for warmer climates.
Gardeners in the north have greater success growing sweet potatoes when warming the soil in spring by covering it with black plastic. That technique also prevents the sprawling vines from rooting—which results in smaller potatoes—and helps keep the vines uniformly watered.
If you grow sweet potatoes every year, move them to a new location every three or four years.
When planting sweet potatoes, create ridges or dig raised beds 6 to 9 inches deep and about 18 inches wide. Space ridges 3 to 4 feet apart. Sweet potatoes need well-drained soil so they don't rot. Scratch a 5-10-10 time-release fertilizer (or a blend with a lower nitrogen number; nitrogen promotes leaf growth at the expense of the roots) into the soil before planting. The first number of the three shown on a fertilizer package is for nitrogen.
Wait to plant sweet potatoes until three to four weeks after your region's last frost-free date to be sure the soil is warm enough for good growth.
Sweet potatoes start as tiny rooted vines called slips. Place the rooted parts in the soil 12 to 18 inches apart. Keep the soil evenly moist but not waterlogged to encourage growth.
Buying bare-root slips is an advantage over container-grown slips. Slips started in pots may become root-bound before a gardener plants them, which may inhibit them from filling out properly. If your slips are root-bound in a pot, snip the sprout above the soil line and tuck it into the ground so it can form new roots. Keep those slips well watered for their first week or two of life.
Once the slips are established, mulch them with an inch or two of organic, biodegradable material, such as straw or grass clippings from a lawn that hasn't been sprayed. Mulch helps conserve water and cut down on weeds. As the mulch degrades throughout the summer, add another inch or two.
Once established, well-mulched sweet potatoes need supplemental deep watering once a week during drought conditions.
When planting sweet potatoes in pots, choose a variety labeled as a bush or bunch type. These produce shorter vines better suited to small gardens or pots.
Choose a 20- to 30-gallon container such as a terra-cotta pot, half whiskey barrel, or grow bag. Plastic and metal containers don't work as well because the material doesn't allow the soil to breathe. Garden soil is too dense to use in containers, so choose a soilless potting mix or a combination of sand and compost. Be sure the pot has several drainage holes; sweet potatoes rot if they are waterlogged. Plant the slips 12 inches apart.
Growing sweet potato plants in a pot allows you to start the plants indoors up to 12 weeks before moving them outdoors after the last frost.
Water each sweet potato plant deeply every week, depending on rainfall. Overwatering can lead to rot.
Sweet potatoes are ready to harvest just before or after frost kills the vines. Depending on the variety you choose, the roots should be ready to dig 130 to 170 days after planting. Pull or cut the vines two to three days before you dig to help toughen the skins.
Use care when digging sweet potatoes. Wear soft gloves, and handle them as little as possible to prevent bruising and nicking the skin. Carefully knock off excess dry soil, but don't wash the roots. Allow the roots to dry and cure before you do anything else.
The curing process is necessary to let the sweet potatoes convert their starch to sugar. Freshly harvested sweet potatoes are not sweet!
Place them in a warm, humid place (85 degrees F at 90 percent humidity) for 10 to 14 days. Here are two ways to achieve this:
When the sweet potatoes are cured, wrap them in newspaper and keep them in a cool, dark closet (no cooler than 50 degrees F) for several months. Never refrigerate sweet potatoes before cooking.
Although the names are used interchangeably, yams and sweet potatoes are entirely different, unrelated plants. True yams are rough, scaly, starchy tropical vegetables low in beta-carotene.
Most of the yams sold in the United States are actually sweet potatoes, even when they have white or purple flesh instead of the more common orange. The U.S. Department of Agriculture requires that a label with the word yam must include the words sweet potato.