Jennifer Schultz Nelson
Extension Educator, Horticulture
This Thursday, among all the other tasty dishes crowding our Thanksgiving tables, many of us will find sweet potatoes, while some families refer to them as yams. Which is it? What are you actually eating?
While we sometimes use the name interchangeably, yams and sweet potatoes are two totally different plants. The yam includes various species of the genus Dioscorea that are native to Africa. The Sweet Potato, Ipomoea batatas, is native to tropical portions of Central and South America, including the Carribbean.
Botanically, yams and sweet potatoes are not very close relatives. The yam is more closely related to lilies than the sweet potato, and the sweet potato is more closely related to morning glories than to potatoes! A very significant demonstration of their differences comes in the fact that yams are monocots, and sweet potatoes are dicots. In the plant world, this is a fundamental difference not only in how certain structures appear, but also includes differences in the biochemical reactions that occur in plants from each group.
Reportedly Columbus was responsible for spreading the sweet potato throughout the "New World" including the present day U.S. Spanish explorers carried the sweet potato everywhere, even as far as Asia. The Portuguese carried sweet potatoes even further, into India. Of course, Columbus and other Spanish explorers did bring the Sweet Potato to Europe, but since it is of tropical origin, it never caught on as a basic food crop like it did in warmer parts of the world.
In regions of the world where the sweet potato could be grown easily and successfully, it rapidly became a staple food crop, both for humans and livestock. Given the necessary mild temperatures, sweet potatoes will produce a crop even on very poor soils. They have few pests, the fast spreading vines usually shade out any weeds that emerge, and the crop stores well after harvest. A 1992 study by the Center for Science in the Public Interest compared the nutritional value of sweet potato to other vegetables. It ranked highest, with Irish or white potatoes a very distant second.
So for people like the English colonists in what is now the Southern U.S., sweet potatoes were a great find, not only could they be grown and stored easily, they also offered a great deal of nutritional value. A relatively short time later some confusion of the name sweet potato and yam was largely credited to a not-so-bright spot in American History.
Since the sweet potato was such a staple food source in the Southern colonies, it was also commonly fed to African slaves kept on plantations. They called the sweet potato "nyami", from a Fulani word meaning "to eat" or from the Twi word "anyinam" referring to the true yam, a root crop common to Africa and Asia. Over time, many markets began to advertise sweet potatoes as yams–the name stuck.
What we eat on our Thanksgiving tables this Thursday is most likely the sweet potato. The name yam and sweet potato used to be used interchangeably in stores, but in recent years the USDA has tried to regulate use of the name. Any use of the word "yam" to describe sweet potatoes must be accompanied by the name "sweet potato".
There are over 600 different species of true yam, and probably just as many uses for them. They grow underground from a vine, and vaguely resemble sweet potatoes, but tend to be more cylindrical, and often have "toes" sprouting from them. Many of them grow to astounding sizes, up to seven feet long weighing 150 pounds! Their flesh can be white to bright yellow, and the tuber is covered in a very tough skin that is difficult to remove.
Many yams contain more sugar than sweet potatoes, but they must be prepared properly before they are safe to eat. Yams contain chemicals including oxalates that can have adverse health effects if eaten. Typically yams go through cycles of boiling, pounding and otherwise leaching out these harmful compounds before they are eaten.
Cultures that depended on yams for food also developed legends about associations of yams with magic or divine origin. In Samoa, Tonga, the names of the days of the week and moon phases were geared towards yams and growing them.
On a more scientific note, in Mexico during the 1940's, a plant biochemist named Russell Marker developed a process to extract chemicals from Mexican yams and convert them into progesterone, a steroid hormone. Later, the process was refined to enable synthesis of the male hormone testosterone, and the female hormone esterone. This discovery paved the way for pharmaceutical companies to be able to provide inexpensive sources of steroids, which could be used to manufacture products like oral contraceptives and cortisone drugs.
Yams are becoming more commonly available in the U.S., particularly in ethnic markets. They grow much like a sweet potato, but need up to a year of frost-free weather before harvest, where a sweet potato will take 100-150 days to harvest. So unfortunately, growing true yams is best left to those living in climates much warmer than central Illinois.
On the other hand, sweet potatoes are a great choice for the home gardener in central Illinois. Some of the best sweet potatoes I've ever had were grown right here in Decatur, Illinois by Operation Green Thumb, a long-time community gardening group.
Sweet potatoes are grown from small sprouts taken from the eyes of the potato called "slips". They will tolerate a wide range of soil conditions, but produce the best quality tubers in loose soils amended with sand and organic matter such as compost. Fertilizers should contain low levels of nitrogen and high levels of phosphorus to encourage tuber formation. 'Beauregard' and 'Georgia Jet' are two cultivars commonly grown in Illinois and elsewhere in the U.S.