Dioscorea opposita Seeds | Rhizoma Dioscoreae
LATIN NAME: Dioscorea opposita
COMMON NAME: Chinese Yam
Plant Profile
Chinese yam is a twining herbaceous climber of the yam family, raising soft, wiry stems a couple of metres up whatever support it can find and dying to the ground each winter. Its tuber is produced on a long narrow neck that joins the crown, and once the neck is broken the tuber cannot be lifted, which is why the harvest is dug rather than pulled. Growers on light soils open a narrow trench beside the row, expose the tuber along its length, and lift it whole, backfilling afterwards so the crown can crop again.
The vines are monocotyledons, yet they carry broad, network-veined leaves unlike the strap foliage of a grass or a lily, with heart-shaped blades and seven to nine prominent veins fanning from the base. The name opposita refers to the arrangement of those leaves, which sit in opposite pairs on the lower stem and often become alternate higher up, an unusual pattern in a genus where most species are alternate throughout. Twining is counter-clockwise, and the stems need a thin, rough support rather than a smooth pole, since they grip by friction.
Plants are dioecious, with male flowers in narrow upright spikes and female flowers in looser, drooping racemes on separate vines. In cultivation the male plants are usually the more vigorous, and where the species has naturalised abroad it spreads not only by seed but by small aerial bulbils carried in the leaf axils, which drop from the vine and grow away as independent plants. Anyone growing it outside its native range should watch for those bulbils, because they are the mechanism behind its weedy reputation in parts of eastern North America.
Young tubers are eaten directly: peel them, grate the flesh into cold water, and cook the resulting slurry into soups and porridges where the mucilage thickens the liquid. Older, larger roots are washed, trimmed, sliced and dried for storage, either in the sun or in a low oven, and the dried slices keep for a year or more.
Science & Biotechnology
The genus Dioscorea contains diosgenin, a steroidal sapogenin that can be opened chemically to yield the steroid skeleton, and this chemistry is the reason yams entered industrial organic chemistry in the middle of the twentieth century as the raw material for synthesising cortisone and the first oral contraceptives. The species that carried that industry were largely tropical, but diosgenin is present across the genus, and the cultivated Chinese yam is studied for its own sapogenin content alongside its food value.
Uses & Benefits
Cut a fresh tuber and the surface immediately runs with a slippery, stringy juice that clings to the knife and the hands; this mucilage is a mixture of water-binding polysaccharides rich in mannose, and it is what thickens the tuber when it is boiled. Behind the mucilage lies a dense starch, finer and stickier than potato starch, together with allantoin and a range of minor phenolic compounds. The mucilage is also why cooks grate the tuber under water rather than onto a dry board.
The nomenclatural history of the cultivated Chinese yam is tangled and worth a sentence, because seed lists and dried-root catalogues do not agree. Thunberg’s Dioscorea opposita was applied for many years to the crop, and the name persists in horticulture and in the medicinal trade, while botanical treatments now generally place the cultivated plant in Dioscorea polystachya.
Growing from Seed
Sow in warmth, into a coarse, open mix of two parts sieved compost to one part perlite, setting the seed on edge and covering it with about twice its own thickness of soil. Yam seed needs steady warmth rather than a cold shock, so a heated propagator or a warm windowsill suits it far better than an outdoor frame. Once the shoot is up and a true leaf has opened, move each seedling into a deep pot, because a shallow pot produces a kinked root that never straightens. Plant out when the stem is twining and the weather is settled, giving each vine a tall, slim support and spacing the crowns a good half metre apart in rows that can be trench-dug at harvest.
Prepare the bed by loosening the soil to the full depth of a fork plus a spade, working in coarse sand and well-rotted leaf mould but no fresh manure. Mulch after planting, water steadily through dry spells, and let the vine ramble; the moment to lift is read from the yellowing leaves, not from any count of days. Lift the tubers after the foliage fades, handling them like eggs, and store them cool and dry in sand.
Our Take
Give it a deep, sandy bed and a tall stake and it will reward you with tubers that come up like pale logs, provided you dig rather than pull. The genus is broad, and its relatives make interesting comparisons: Japanese yam grows in cooler ground, while Dioscorea polystachya is the same crop under its botanical name. If you want a root crop without the trench work, yam bean offers a tuber that lifts with one fork.
Related Reading
Gardeners who grow Chinese Yam often pair it with these related subjects: