Growing Sweet Potatoes

Nutritious and high-producing, sweet potatoes are a rewarding root crop to grow.

Molokai

MOLOKAI PURPLE Sweet Potatoes have broken up with their long term partner, marshmallows, to become the next great super food.

Photo courtesy www.RareSeeds.com

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The sweet potato is a delicious and generous root crop. It is a plant with the potential to prevent measles and childhood blindness in Africa, and has been referred to as “food that ends famine quickly” in a Hawaiian proverb. It is a heroic plant, with an illustrious future ahead of it as an affordable and nutritious food source that can withstand harsh growing conditions, yet much of the history of this valiant root crop remains a mystery. Could this humble root, most famously found hiding under a blanket of marshmallows on the Thanksgiving dinner table, have a super-food secret identity?

Although they are native to the same geographic location, the sweet potato and the “Irish potato” are not closely related. The sweet potato is actually a member of the morning glory family, while the “Irish potato” is a close relative of tomatoes and eggplants. Many refer to the sweet potato as a tuber, which is a thickened stem much like the tubers of a regular potato plant. In fact, sweet potatoes are the swollen and starch filled roots of the sweet potato plant, not a stem at all.  Finally, the most muddled sweet potato myth: the yam, sweet potato or not? Often a fleshy southern variety of sweet potato is incorrectly referred to as a yam. The yam is actually an African native tuber that is related to lilies and grasses It has a much thicker stem, it cannot tolerate temperatures below 68 degrees Fahrenheit and it grows much larger than sweet potatoes. The confusion began when southern farmers decided to call their sweet potato variety yams in order to set their varieties apart from Northern varieties. 

Much confusion surrounds the history and origins of the sweet potato. Carbon dating has taught us that the sweet potato has been domesticated in Central America for at least 5,000 years. However, a wild ancestor of the domesticated root has never been found. The exact native range of the wild sweet potato ancestor remains a mystery. However, there is now strong evidence that the sweet potato may be native to the Yucatán Peninsula of Mexico and the Orinoco River of Venezuela. This theory stems from the fact that there is a great deal more molecular diversity in sweet potatoes from this area than in South America, where the sweet potato was first observed by Europeans.

Sweet potato and the “regular” potato, as well as corn managed to catch the eye of Spanish conquistadors in the 1400s. The sweet potato was immediately recognized as an exotic delicacy when introduced to Europe. Conversely, the starchy white potato was not embraced in Europe until well after the sweet potato was established. After its 1492 introduction to Europe, the sweet potato spread to Africa by the 1500s, and to North America and Asia by the 1600s.

Europeans are credited with the dispersal of the sweet potato and many other crops from the Americas to the rest of the world. However, an entirely different group of people have been found to have visited South America and brought this botanical jewel home for cultivation. It has been discovered that Polynesians had been cultivating a South American variety of sweet potato at least four hundred years before Europeans.

Captain James Cook visited Polynesia in the 1760s. He was fascinated to find civilizations inhabiting far flung islands far into the Pacific Ocean. Cook returned to London with samples of the sweet potatoes that he had found Polynesians cultivating. The genetic makeup of these 18th century sweet potato remains were recently examined and found to be from AD 1000 to AD 1100. The potato remnants are believed to have descended from Peruvian and Ecuadorian varieties. This piece of living evidence suggests that Polynesians made an incredible 5,000 mile journey to South America, and returned to the Pacific Islands to cultivate the crop. There is also compelling linguistic evidence to back up this almost unbelievable theory. There is a close relation between the words “Kuumala”, the Polynesian name for sweet potato, and “kumara”, a native South American word for the same root.

While a 5,000-mile journey by canoe sounds like an impossible feat, remember that ancient Polynesians boasted an impresive maritime history. Upon his arrival, Cook wondered how their people reached these astonishingly remote islands. The native Polynesians explained that they used the stars and the moon to navigate in open water on long journeys. The sweet potato has become a crucial piece of evidence in the theory that Polynesians, and not Europeans were the first to make contact with South America.

From mysterious beginnings, the sweet potato has been lauded as a life saving crop that will tolerate harsh growing conditions. The sweet potato is also one of the most nutritious root crops that can be stored during leaner months. In Africa, there is an ambitious campaign to fight Vitamin A deficiency, which can cause childhood blindness and sometimes death. The secret weapon to fighting this ailment is the orange sweet potato. In Africa, the white sweet potato reigns supreme, it is less sweet and does not contain beta carotene like its orange counterpart. Humans need beta carotene to make vitamin A. The effort to encourage farmers and consumers to switch from white to orange sweet potatoes has been surprisingly successful. Convincing local farmers to embrace more colorful, nutritious crops is cheaper and more sustainable than trekking to remote villages to dole out vitamin A capsules.

The sweet potato has been warmly embraced just about everywhere it can be grown. Once introduced by the Polynesians, the sweet potato was grown extensively in Hawaii due to its heat tolerance and vigorous growth habit. Ancient Hawaiians not only relied on the root as an important food crop, they also ate the leaves (all parts of the plant are edible)  and fermented the plant into an alcoholic beverage. There were also medical uses for the plant; as a tonic for pregnant women, and the raw plant was ingested to induce vomiting. On the Hawaiian islands, purple sweet potatoes are an antioxidant- rich favorite. The Molokai Purple is a deep purple sweet potato that boasts an incredible flavor and is rich in anthocyanin, a purple hued antioxidant appreciated for fighting cardiovascular disease and other chronic illness. The Okinawan Purple sweet potato is another nutritious variety. Originally grown by Japanese immigrants, it remains a popular Hawaiian variety and grows exceptionally well in the volcanic soil there.

The sweet potato has saved people in the past and it shows promise to save many people in the future. Surely there are few foods more deserving of the super-food title, but what makes the sweet potato so special? It’s a high calorie starch food, however it contains no cholesterol or saturated fats. It’s substantial and filling, yet highly nutritious. It raises the blood sugar slowly, which makes it safe for diabetics. The sweet potato is also a great source of dietary fiber which aids in digestion. The bright colors, ranging from orange, pink, yellow, and dark purple, proudly represent the high levels of flavanoids which are antioxidants capable of helping the body fight certain cancers. Obtaining these important plant nutrients can be difficult in off-season months when gardens have gone by, however, the sweet potato’s ability to be stored for months ensures a supply of crucial vitamins and minerals when people often need them the most. The foliage of the sweet potato is even higher in vitamin C and folate than the tubers.

What really makes the sweet potato an indomitable force, is its ability to flourish with little care. The sweet potato is grown from slips or stem cuttings, which are shoots grown from a mature sweet potato. Once planted, the vines take off, making a beautiful ground cover. Since the sweet potato is a heat loving crop, southern growers will find that there are few limitations in growing the plants. For northern growers, a layer of black plastic mulch to warm the soil in spring is highly recommended. It is wise to purchase tissue cultured sweet potato slips which are much less likely to carry disease.

When preparing the soil for sweet potatoes, keep mind they need loose, aerated soil to develop good tubers. Sandy soils are ideal for tuber development. They do not like overly rich soil, so a thin layer of compost is all you will need. Plant slips about 3-5 inches deep and about 12-18 inches apart. From here, the growing is a breeze: just keep plants well watered and let the vines cover the ground. They grow vigorously and are seldom out competed by weeds.

The tubers can be dug and eaten at any time once they are large enough, however, a nip of frost will sweeten them up and allegedly increases the vitamin content of the roots. Wait until the vines have yellowed or blackened from frost. Harvest as soon as vines begin to blacken, as the roots will rot very quickly after leaves turn black. Dig with a spade, then leave the fresh tubers out in the sun to dry for the afternoon. Next you will want to cure the sweet potatoes to ensure good storage quality and a reported better taste than uncured roots. The best way to cure is to place dry roots in a box lined with newspaper in a warm room at about 85-90 degrees. The humidity should be at around 85 percent. Let roots cure for about 10 days. After curing they can be kept for months in a root cellar at about 55-60 degrees.

The sweet potato is truly amazing! From mysterious beginnings it has become a life saving root crop with limitless potential for feeding future populations.


Shannon McCabe is a gardener and writer for Baker Creek Heirloom Seeds. She is a graduate of the University of Rhode Island with a bachelors in horticulture and sustainable agriculture. She grew up on Block Island where she was a market farmer, in addition to being an orchard keeper for the University of Rhode Island. In her spare time she likes to make cheese, ride horses, and listen to psychedelic rock music.