Ask a Master Gardener: Sweet potatoes
Question: Can sweet potatoes and yams grow in our climate? Can you recommend some tips for growing them successfully in my backyard garden?
Answer: There is perennial confusion about the difference between sweet potatoes and yams. What most people don’t know is that sweet potatoes and yams are not interchangeable. In fact, they are from completely different plant families.
The yam, a resident of the Dioscoreceae family, is a starchy, dry tuber that grows in West Africa, the Caribbean and Asia. It would probably not grow well in our climate here. Sweet potatoes, on the other hand, are a member of the morning glory or Convolvulaceae family and originate in China. The sweet potato also is not related to the Irish potato, which belongs to the nightshade or Solanaceae family. Unlike potatoes, which are tubers, sweet potatoes are roots.
Sweet potatoes can be grown successfully in our area. Unlike other vegetables, sweet potatoes are propagated from slips, also called vine cuttings. Slips can be produced at home, purchased at a local garden store or ordered from online seed companies.
To produce your own slips, buy healthy, disease free sweet potatoes from a local market. Scrub them clean and then cut them in half. Suspend each half over a jar of water by inserting toothpicks so that half is submerged in the water; place the sweet potato near a window for warmth and sunlight. Over the next few weeks, shoots will form on top. Many slips can be picked from one potato. Once a plant develops into a 5-6 inch slip, they can be twisted off from the potato and placed into a glass of water to form more roots. Let the roots grow a couple of inches or more before out planting.
Sweet potatoes should be planted in full sun. They require lots of heat and humidity. They must be planted in a well-drained, fine loamy soil with a pH between 5 and 7. Add a couple of inches of compost or a complete fertilizer into the soil. This will be the only fertilizing needed. Over-fertilization causes vigorous leaf growth and long, skinny roots. If grown in clay soil, roots can be small or misshapen and will be hard to dig. Mound the soil into a slight ridge or raised bed 12-24 inches wide. Cover the soil with clear plastic pulled smooth and tight. Secure the edges of the plastic well enough to keep as much heat in as possible.
Let the soil warm until it gets to about 80 degrees. Don’t worry about getting the soil too hot; sweet potatoes thrive in conditions above 100 degrees. Make slits in the plastic about 15 inches apart. Pull back each slit to create a 12-by-12 inch opening and form a depression. Plant the sweet potato slips in the depressions. Fill up the planting holes with construction sand for drainage and to draw heat. Plant slips deeply into the raised bed up to the top of the leaves. Since hot days and warm nights are ideal, the use of low plastic tunnels or cold frames can also help maintain temperatures.
Water late in the day when the soil is at its warmest. Ideally, use a soaker hose under the plastic. To get the fastest growth, use water warmed by the sun in black plastic tubs.
For home gardeners, the best time to harvest sweet potatoes is immediately before or just after the first fall frost, or when the leaves turn yellow, growth has stopped and the roots have matured. The sweet potato root has a delicate skin that is easily bruised at harvest. Bruises, gouges or scratches can quickly lead to rot. Cure immediately. Place the sweet potatoes in a warm, humid spot for 5-7 days. Suggestions include a small green house or heated room. For best eating quality, it is important to wait a few weeks before eating the roots once they have been dug. Research has shown that the percent of sugars can more than double in the three weeks after harvest, dramatically increasing sweetness and overall eating quality.
Sweet potato varieties that perform well in our area are:
Beauregard: This is great for gardeners in cooler growing conditions. It is red skinned with orange flesh.
Georgia Jet: Early to mature, this variety has orange flesh, and rose colored skin. It does not store well.
O’Henry: This sweet potato has a creamy white interior. It is high yielding, has great flavor and stores well.
Murasaki: This is a popular variety with amethyst skin and pearly flesh that is sweet, and nutty flavored.
Purple: As its name implies, this variety is bright purple outside with vibrant amethyst, tie-dye streaked flesh.
Whatever variety you choose to try, sweet potatoes — especially the varieties with orange flesh — are an excellent source of beta-carotene, which is easily converted by the body into vitamin A. The plants also produce colorful flowers as well as trailing vines often used as groundcovers.