The name Dioscorea opposita Thunberg (Fl. Jap. 151. 1784) has been widely used in the sense of Dioscorea polystachya, (which is an earlier name and therefore the valid name for Dioscorea batatas). It was based on a misidentification of Japanese material as the strictly Indian species Dioscorea oppositifolia Linnaeus, which was cited as a synonym in the protologue, thus rendering Dioscorea opposita nomenclaturally superfluous and an illegitimate name[].

Common Name: Chinese Yam

General Information

Dioscorea polystachya is a perennial climbing plant growing from a large, vertical, cylindrical tuberous rootstock that can be up to 100cm long. One to several twining shoots up to 300cm long are produced, these shoots twine to the right[].
The plant is a commonly cultivated food crop, especially in eastern Asia. It also has a range of medicinal uses.

Known Hazards

Edible species of Dioscorea have opposite leaves whilst poisonous species have alternate leaves[].

Botanical References

,

Range

Temperate E. Asia - the exact range is not clear but was likely to have been the Russian Far East, northern and eastern China, Korea

Habitat

Prefers sunny slopes in the wild[]. Forests, scrub forests, herb communities, mountain slopes, along rivers, roadsides, also commonly cultivated; at elevations from 100 - 2,500 metres[].

Properties

Edibility Rating
Medicinal Rating
Habit	Perennial
Height	3.00 m
Self-fertile	No
Cultivation Status	Wild

Cultivation Details

Dioscorea polystachya is a very cold-hardy plant and can tolerate winter temperatures falling to at least -18°c[].
An easily grown plant, succeeding in a fertile well-drained soil in a sunny position or light shade[, , ], though it is best in full sun[].
This species of yam is much cultivated in China for its edible root which can be up to 1 metre long[]. It has a great potential to be a commercial crop in temperate regions, though a satisfactory method of harvesting the root needs to be found[].
Plants take 3 - 4 years to reach full maturity[], though one year roots of well grown plants can weigh more than 500g. There are many cultivated forms with different root shapes in China and Japan[].
The yam is a climbing plant that supports itself by twining around the branches of other plants[]. It can be grown successfully into small bushes or, perhaps simpler when being grown as a root crop, it can be grown up a frame in a similar manner to growing runner beans[].
Plants produce tubercles (small tubers that are formed in the leaf axils of the stems), and can be propagated by this means[].
The small white flowers have a pleasant scent of cinnamon[].
A dioecious species - both male and female forms must be grown if fruit and seed are required.

Edible Uses

Tuber - cooked[, , , ]. A floury texture[] with a very pleasant flavour that is rather like a potato[]. The tubers can be boiled, baked, fried, mashed, grated and added to soups[]. They store well and for a long time[, ] and can also be left in the ground and harvested as required in the winter[]. This is a top quality root crop, very suitable for use as a staple food[].
An arrowroot can be extracted from the root[], though this is not as good at binding other foods as the starch from Dioscorea japonica[]. The root contains about 20% starch. 75% water, 0.1% vitamin B1, 10 - 15 mg% vitamin C[].

Fruit. A starchy flavour, it is said to be very good for the health[]. We wonder if this report is referring to the tubercles[].

Medicinal

The Chinese yam, called Shan Yao in Chinese herbalism, is a sweet soothing herb that stimulates the stomach and spleen and has a tonic effect on the lungs and kidneys[]. The tuber contains allantoin, a cell-proliferant that speeds the healing process[]. The root is an ingredient of 'The herb of eight ingredients', traditionally prescribed in Chinese herbalism to treat hyperthyroidism, nephritis and diabetes[].
The tuber is anthelmintic, digestive and gently tonic[, , , ]. It is used internally in the treatment of tiredness, weight loss, poor appetite, poor digestion, chronic diarrhoea, asthma, dry coughs, frequent or uncontrollable urination, diabetes and emotional instability[]. It is applied externally to ulcers, boils and abscesses[, ]. The tubers are harvested in the autumn and can be used raw or baked[].

The leaf juice is used to treat snakebites and scorpion stings[].

The roots of most, if not all, members of this genus, contains diosgenin[, ]. This is widely used in modern medicine in order to manufacture progesterone and other steroid drugs. These are used as contraceptives and in the treatment of various disorders of the genitary organs as well as in a host of other diseases such as asthma and arthritis[].

Other Uses

None known

Propagation

Seed - sow early spring to mid spring in a sunny position in a warm greenhouse and only just cover. It germinates in 1 - 3 weeks at 20°c[]. Prick out the seedlings as soon as they are large enough to handle and grow on in a greenhouse for their first year. Plant out in late spring as the plant comes into new growth.
Basal stem cuttings in the summer[].
Division in the dormant season, never when in growth[]. The plant will often produce a number of shoots, the top 5 - 10 cm of the root below each shoot can be potted up to form a new plant whilst the lower part of the root can be eaten[].
Tubercles (baby tubers) are formed in the leaf axils. These are harvested in late summer and early autumn when about the size of a pea and coming away easily from the plant. They should be potted up immediately in individual pots in an unheated greenhouse or cold frame, covering them with about 10mm of soil. Protect them from mice etc and keep the soil moist but not wet. They should come into growth in the spring, plant them out in early summer when in active growth[].