|Dioscorea subsp. var.|
- For the vegetable sometimes called yam in the United States, see sweet potato
|Standard Cyclopedia of Horticulture|
Yam. The name yam properly belongs to the members of the genus Dioscorea, although unfortunately it has been applied for many years in the United States to the large varieties of the sweet potato, Ipomoea Batatas. The name as applied to the sweet potato is thought by some to be a corruption of an African word pronounced "nyam," brought by the negroes who were transported to America in the early days. This word was used for the true yam or other large roots or tubers used for food in Africa and was bestowed by the negroes on the large sweet potatoes in America.
The botany of the cultivated yams has not been cleared up. Many varieties are of mediocre quality, while some are excellent, being superior in flavor and mealiness to almost any other starchy vegetable. Single roots or tubers of some varieties, particularly of D. alata, attain great size, occasionally even reaching 100 pounds or more in weight, and several feet in length. The best varieties, however, are of small size, often less than a pound in weight. Among the best known of these are the yampis of Jamaica and the Isthmus of Panama, and other varieties of similar quality, such as the cush-cush of the Island of Trinidad. The Chinese yam or cinnamon vine, D. Batatas, is of excellent flavor, but on account of its deep-growing habit is very difficult to dig. Several kinds of yam are grown scatteringly in Florida. For further discussion, see Dioscorea.
Yam is the common name for some species in the genus Dioscorea (family Dioscoreaceae). These are perennial herbaceous vines cultivated for the consumption of their starchy tubers. There are many cultivars of yam.
The sweet potato (Ipomoea batatas) has traditionally been referred to as a yam in parts of the United States and Canada, but it is not part of the Dioscoreaceae family.
Yam tubers can grow up to 2.5 m (8.2 ft) in length and weigh up to 70 kg (154 lb).
The vegetable has a rough skin which is difficult to peel, but which softens after heating. The skins vary in color from dark brown to light pink. The majority of the vegetable is composed of a much softer substance known as the "meat". This substance ranges in color from white or yellow to purple or pink in ripe yams.
The tubers can be stored up to six months without refrigeration.
Pests and diseases
Major cultivated species
Dioscorea rotundata and D. cayenensis
Dioscorea rotunda, the "white yam", and Dioscorea cayenensis, the "yellow yam", are native to Africa. They are the most important cultivated yams. In the past they were considered two separate species but most taxonomists now regard them as the same species. There are over 200 cultivated varieties between them. The Kokoro variety is important in making dried yam chips.
They are large plants; the vines can be as long as 10 to 12 meters (35 to 40 feet). The tubers most often weigh about 2.5 to 5 kg (6 to 12 lbs) each but can weigh as much as 25 kg (60 lbs). After 7 to 12 months growth the tubers are harvested. In Africa most are pounded into a paste to make the traditional dish of "pounded yam" (Kay 1987).
Dioscorea alata, called "water yam", "winged yam" and "purple yam", was first cultivated in Southeast Asia. Although not grown in the same quantities as the African yams, it has the largest distribution world-wide of any cultivated yam, being grown in Asia, the Pacific islands, Africa, and the West Indies (Mignouna 2003). In the United States it has become an invasive species in some Southern states.
In the Philippines it is known as ube (or ubi) and is used as an ingredient in many sweet desserts. In Vietnam, it is called khoai mỡ and is used mainly as an ingredient for soup. In India, it is known as ratalu or violet yam. In Hawaii it is known as uhi.
Uhi was brought to Hawaii by the early Polynesian settlers and became a major crop in the 1800s when the tubers were sold to visiting ships as an easily stored food supply for their voyages (White 2003).
Dioscorea opposita, "Chinese yam", is native to China. The Chinese yam plant is somewhat smaller than the African, with the vines about 3 meters (10 feet) long. It is tolerant to frost and can be grown in much cooler conditions than other yams. It is now grown in China, Korea, and Japan.
It was introduced to Europe in the 1800s when the potato crop there was falling victim to disease, and is still grown in France for the Asian food market.
The tubers are harvested after about 6 months of growth. Some are eaten right after harvesting and some are used as ingredients for other dishes, including noodles, and for traditional medicines (Kay 1987).
Dioscorea bulbifera, the "air potato", is found in both Africa and Asia, with slight differences between those found in each place. It is a large vine, 6 meters (20 ft) or more in length. It produces tubers; however the bulbils which grow at the base of its leaves are the more important food product. They are about the size of potatoes (hence the name "air potato"), weighing from 0.5 to 2 kg (1 to 5 lbs).
Some varieties can be eaten raw while some require soaking or boiling for detoxification before eating. It is not grown much commercially since the flavor of other yams is preferred by most people. However it is popular in home vegetable gardens because it produces a crop after only four months of growth and continues producing for the life of the vine, as long as two years. Also the bulbils are easy to harvest and cook (Kay 1987).
In 1905 the air potato was introduced to Florida and has since become an invasive species in much of the state. Its rapid growth crowds out native vegetation and is very difficult to remove since it can grow back from the tubers, and new vines can grow from the bulbils even after being cut down or burned (Schultz 1993).
Dioscorea esculenta, the "lesser yam", was one of the first yam species cultivated. It is native to Southeast Asia and is the third most commonly cultivated species there, although it is cultivated very little in other parts of the world. Its vines seldom reach more than 3 meters (10 feet) in length and the tubers are fairly small in most varieties.
The tubers are eaten baked, boiled, or fried much like potatoes. Because of the small size of the tubers, mechanical cultivation is possible; which, along with its easy preparation and good flavor, could help the lesser yam to become more popular in the future (Kay 1987).
Dioscorea trifida, the "cush-cush yam", is native to the Guyana region of South America and is the most important cultivated New World yam. Since they originated in tropical rain forest conditions their growth cycle is less related to seasonal changes than other yams. Because of their relative ease of cultivation and their good flavor they are considered to have a great potential for increased production (Kay 1987).
Dioscorea dumetorum, the "bitter yam", is popular as a vegetable in parts of West Africa; one reason being that their cultivation requires less labor than other yams.
The wild forms are very toxic and are sometimes used to poison animals when mixed with bait. It is said that they have also been used for criminal purposes (Kay 1987).
- Standard Cyclopedia of Horticulture, by L. H. Bailey, MacMillan Co., 1963
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