|Sweet potatoes are propagated by suspending a tuber in a glass of water or burying one part-way in sand or other porous media; letting shoots grow from the tuber; then rooting the shoots. Illustration from Sweetpotato Culture and Diseases, Agriculture Handbook No. 388, USDA Agricultural Research Service, 1971.
By Roberta Bailey
Sweet potatoes can be grown in the North. They need about four frost-free months and a little extra attention, but you can grow them just as well as you can grow other tropical plants, such as cantaloupes and sweet peppers.
Native to South America, the sweet potato, Ipomea batatus, is a member of the morning glory family, and its leaves and flowers closely resemble that familiar flower. It is not related to the African yam, which is in the genus Dioscorea.
Unlike our common potato, Solanum tuberosum, sweet potatoes are not tubers but fleshy roots, forming underground as enlargements of specialized roots. The vines are tender and spreading, often covering a 3- to 4-foot area.
Most varieties are suited for the Southern climate, but a few are well adapted for the North. The following have performed well over many years: Georgia Jet matures in 100 days, producing large, blunt, reddish roots with pale orange flesh; and Beauregard forms large, dark orange fleshed roots, although it is a weak sprouter and grower. Most likely, the roots in the supermarket are the Beauregard variety, which produces good, baking size roots. You can try sprouting them, but buy organic roots, since a sprout inhibitor probably was used on conventional roots. Beauregard bears slightly less than Georgia Jet does. Jewell, another orange fleshed variety with coppery skin, does well but does not bulk up as much as the aforementioned varieties. Suppliers of slips may have many varieties that do well in the North.
Start from Slips
Sweet potatoes are grown from slips – rooted sections of the sweet potato vine that are about 1 foot long. These slips are propagated by rooting sections of the vine or by sprouting a sweet potato root. One root can produce 15 to 50 slips. Slips can be purchased from a Southern grower or supplier, or they can be grown at home. Choose one or more large, sound potatoes. Suspend half to two-thirds of the potato in a jar of water, supported by three toothpicks. Keep the jar in a 75- to 95-degree F. location. The old fashion gas stoves with pilot lights were ideal sprouting areas. A cooler with a light bulb and a thermometer work as well. Take a day to regulate the temperature before setting the root and jar in the cooler. Slips will grow in four to six weeks. Colder temperatures greatly slow production. Laying a root in a bed of sand or vermiculite and keeping it moist and warm is another way to produce slips.
Once leaf clusters begin to form on the sprout, it can be twisted off and planted in a flat of potting soil. I have had success planting four slips per 8- x 12-inch flat or one each in 4- to 6-inch pots.
A plant can be maintained in a pot over winter, with shoots pruned to a manageable length. Once spring comes, sections can be cut and rooted in water for the summer crop.
The ideal transplant size is 12 inches. A 6-inch plant will not yield as well, while a larger plant gets gangly, is difficult to handle, and is more prone to transplant setback.
Transplant to Warm Soil
Temperature is critical when transplants are set in the garden: The soil must be at least 65 degrees F. I set plants out in early June unless spring is cold and late. Warm soil is the key to success with sweet potatoes. I prepare a bed in the fall and cover it with clear IRT (infrared transmitting) plastic. It starts to warm deeply very early, so that by June it is warmed about a foot deep. Clear plastic warms the soil more deeply than black plastic, although I do get some weed growth in the spring. Once the summer heat comes, sweet potato plants are fine.
Preparing a raised bed that is at least 15 inches wide in early May also works well. Cover the bed with plastic and seal the edges of the plastic into the soil to prevent heat loss. If more than one bed is being prepared, space them 5 feet on center.
Sweet potatoes need full sun. In tropical areas, some varieties yield better in partial shade, but in the North they need all the heat they can get.
Sweet potatoes are very adaptable to soil conditions and will grow in poor soils, but they yield far more in a well-drained, slightly acid, fertile loam. Heavy clay impedes root growth, so lighten heavy soils with organic matter. Roots spread easily in a well-prepared raised bed that is at least 15 inches wide. I turn a 6-inch layer of compost into the bed but avoid heavy applications of nitrogen. Fresh manure is said to blacken the sweet potato skin, but turning in manure or green manure the fall before planting is an excellent option.
Potassium promotes root growth, but in sweet potatoes high concentrations of potassium produce more but not larger roots. In the North, plants produce baking size roots with average potassium concentrations.
Once the soil is warm, plant your potted slips about 15 inches apart, slitting the plastic and setting them deeper than they were in their pots, but don’t bury the entire crown. Protect the slips from cutworms.
You may want to cover the beds with poly-tunnels, because the more 100-degree F. days, the better sweet potatoes yield. For optimal heat, I immediately cover the row with hoops and slitted row cover. If the bed is wide, I use two arced hoops side-by-side, forming an M to create more width. Plants should not be exposed to temperatures below 50 to 55 degrees F.
Drought slows root production. Plants will tolerate one or two dry weeks, but in the North we don’t have time for setbacks. Set a dark bucket or barrel near the bed to have a ready supply of sun-warmed water, and provide each plant with 1 gallon of warm water when the soil is dry. In large plantings, cold hose water is OK: It produces less of a setback than drought.
Since the sweet potato is a Southern crop, few of its pests live in the North. Ground hogs and rabbits love the foliage, but I find that the row cover greatly deters them. Wireworms can get into the roots; rotation and cultivation help control wireworms.
Harvest and Cure
Sweet potatoes don’t mature: They just keep getting larger until the soil cools to 50 degrees or lower. Under row covers, the soil cools by mid- to late September, when you should harvest the vegetables carefully. The firm roots can bruise, hastening rotting.
Once the roots are out of the ground, curing, which heals nicks and cuts, determines the success or failure of the crop. Have a curing plan ahead of time, because, until they are cured, the roots lose moisture rapidly.
To cure sweet potatoes, spread them out in a very warm space – the optimal temperature range is 85 to 90 degrees – and cure them for one week. At 75 to 85 degrees, curing takes two weeks. I cure my roots in a closed greenhouse for about 10 days. I have also heard of people using second bathrooms or saunas, heating them to 90 degrees for a week. A pilot lit gas oven is 85-90 degrees. An old refrigerator or freezer with a light bulb in it – especially one on a dimmer switch – can be adjusted to maintain 90 degrees. The optimal 80 to 90 percent relative humidity can be created with pans of water or a humidifier. The roots give off huge amounts of moisture while curing, creating high humidity in a small space.
Curing stimulates an organic reaction in the root that forms a protective outer layer in the skin, which greatly reduces dehydration. A properly cured sweet potato can sit on the kitchen counter for months and lose very little weight. Some sprouting may occur; in that case, rub off the sprouts.
Once cured, sweet potatoes should be stored at 55 degrees. If that temperature is not possible, go for warmer rather than cooler, since roots are damaged structurally by cold temperatures. Sweet potatoes should never be refrigerated. Brown paper bags regulate humidity and prevent moisture loss.
Allen, Ken, Sweet Potatoes for the Home Garden, Green Spade Books, Kingston, Ontario, 1998; 204 pages.
Bubel, Nancy, The Seed-Starter’s Handbook, Rodale Press, Emmaus, Penn., 1978.