Dioscorea batatus is native to the mountainous regions of Japan and China, where it was discovered by Decne in 1854. It has spread out throughout the world as cultivated plant. It's a robust twining perennial and its shoots die down to the ground every winter. It attains the height of 6 m in its native habitat, and about 3,5 m in Poland. Its shoots twine clockwise round the supports (unlike to Dioscorea sativa, sweet potato widely cultivated in tropical countries). Bisexual flowers are small, white-green-yellow with a pleasant cinnamon scent and appear in June-August. Attractive leaves are dark green, lustrous, cordate, 7 to 9-veined and slightly lobed.
The cultivar 'Variegata' has green leaves mottled with white and, when young, the shoots, leaf petioles and leaf veining are crimson. Lower parts of the stems are often bare, both in the species and in its cultivar. In late summer and autumn small brown aerial bulbils, the size of a pea or a hazelnut, appear in leaf axils. They can be used either for propagation or eaten - they are tasty and have a nutty flavour.
Requirements.
Chinese yam requires a sunny site with fertile, moist, well drained humus soil. It tolerates frosts up to -20oC and if you sprinkle the soil round the plant with bark or wooden chips, it will sustain even much harsher winters.
Application.
It can grow up various kinds of supports, such as poles or trellises; you can plant it on the southern site of a grove and let it scramble up the bushes or trees (if the plants of the grove are not too valuable). If you decide to grow it in a garden for consumption, you can plant it densely (even with only 25 cm interval), since the bulbs are formed vertically deep under the plant. In order to extract the tubers you need to dig deep holes, so you shouldn't plant Chinese yam next to valuable plants, so as not to risk damaging their roots.
Underground large bulbs, known as sweet potatoes, can reach even 50 cm in length (in very favourable conditions they can grow as long as 1 m) and weigh up to 1 kg (max 2 kg). Pleasantly fragrant and nutty flavoured - they are a lot tastier than potatoes.
They can be eaten raw, cooked, baked or fried. When you bake or boil them, it's best to leave the peel (in the same way as when you make jacked potatoes) and only prick them with a fork. You can add them to various dishes, including soups and salads.
Chinese yam tubers contain 20% starch, 75% water, 0,1% B1 vitamin and 15 mg% vitamin C, tonifying and energizing substances and alkaloids that stimulate the healing of wounds.
They are also used in Japanese and Chinese medicine for stimulating appetite, treating stomach disorders, as a tranquilizer, in arthritis, lungs diseases, asthma, dry cough, kidney, spleen and bladder troubles. They can be also applied externally for treating burns and ulcerations. Leaf juice is used for poultices in case of venomous snake stings.
Tubers should be dug out at the end of the second year of cultivation, when the plant has already spread and the bulbs are big enough. They can be kept in a cool place (but sheltered from frost) even for several months. During this time they should be protected from drying.
Dioscorea batatus can be propagated from seed (you need to have both female and male specimens, as the plant is bisexual), aerial bulbils (gathered in autumn after ripening, but before they fall) or from the upper parts cut from the ground tubers (with dormant buds). In spring they can be planted into pots indoors and, once the danger of ground frosts is over, directly into the ground. The simplest way, however, is to buy a plant in a container from a garden centre or a nursery.
