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Botanical name: Dioscorea batata
Common name: Dioscorea, Chinese yam
Literal name translation: mountain herb
Family: Dioscoreaceae, yam family
Part used in Chinese medicine: rhizome
Major Chinese medicine actions:
Tonifies Spleen and Lung qi and yin, tonifies Kidneys
and secures the essence
Shan Yao, or Chinese wild yam, is both a nutritious food and a medicinal herb, used in cooking as well as Chinese herb formulas. The tubers are similar in texture to a floury potato, though they are superior in both taste and nutrition to the potato. They store well for an extended time, but can also be left in the ground to be harvested as needed during the winter season. In some regions of Asia, they are eaten in quantity and grown as a staple food crop, as well as grown for harvest to be dried and sold in herb stores.
Dioscorea opposita is a native species found growing wild in the mountains of northern Japan and on the sunny slope of hillsides in many regions of China. This species, and the similar Dioscorea batata (pictured above), tolerate temperatures down to about -15 F, are hardy in a wide range of climates from USDA zones 5 to 10, therefore easily grown outdoors in most parts of North America. There are several species of Dioscorea grown as the medicinal herb Shan Yao, including Dioscorea opposita, D. oppositifolia, D. batata, D. persimilis, D. alata, and D. fordii
Growing and Propagation
Dioscorea is very hardy, growing and propagating in a variety of different habitats and conditions. It prefers sun, though will do fine in part shade, and does best in fertile, rich, loamy soil where the roots can grow down deeply. In good conditions, the tuberous roots can grow up to 3 feet long and weigh 4 pounds or more.
The plant is a vigorous twining herbaceous climber reaching 8 feet or more in height, sending out long shoots each year. It can be grown like runner beans up a frame or bamboo trellis, or if there is a good deal of space the shoots may be left to grow on the ground. Plants should be watered regularly and soil kept moist, but not over-watered.
Care should be taken that the plant does not overgrow other garden plants as it can create a thick blanket, competitively excluding light. Without occasional thinning and care it may also weight down and break branches of large trees and shrubs. In the wild, an entire stand of native shrubs may become covered by the vine, causing total shade conditions and eventually killing the stand.
Precautionary practice requires harvesting the tubers in the fall within the first few years of growth, and deadheading plants to keep the leaf tubercles from propagating volunteer seedlings, creating the next season’s runaway crop. If these measures are taken, the plant is easily controlled and provides a good nutritious medicinal food.
Propagation is quite simple, achieved by either replanting the top portion of the root, planting the small tubercles formed on older plants, or by stem cuttings taken in the late spring. In the first method, after harvesting the root in the fall, cut off the top few inches of the root and store in a cool, dry place for the winter, replanting the tuber piece in early spring. It is best to wait to remove the top portion until you want to eat the remainder of the yam, as it will store better for food use if kept in one piece.
Another easy method of propagation is to harvest the small tubercles (pea-size baby tubers that look a little like small bulbs) formed in the leaf axils along the stems (see photo below). These can be treated like seed. Collect them in late summer or early fall, once they are easily detached and fall freely from the plant. Store in a cool, frost-free place over the winter being careful not to let them dry out, or pot them immediately and leave in an unheated greenhouse or cold frame, keeping the soil moist but not wet. They will remain dormant in the winter and come into growth in the spring. Plant seedlings in the ground in the summer when they are in active growth.
Harvesting and Preparation
The tuber grows in the shape of a club, about as thick as an adult’s finger at the top, thickening to the size of an arm at its base. Harvesting the root is the most difficult challenge with the plant as the roots are large and grow quite deep. The tubers can be dug in the fall, like most root crops, since at this time the plant has stored it’s nutrients in its roots for the winter. Or it may be left in the ground to be harvested as needed during the winter season. It is recommended to leave plants in the ground to grow for a second year when possible, as the tubers will become considerably larger, achieving full maturity in 3-4 years.
For medicinal use, good quality Shan Yao comes from solid, heavy, thick tubers and is white and powdery in appearance.