Dioscorea bulbifera—Potato Yam, Aerial Yam, Bulbil-bearing Yam. Widely grown
in the tropics of Africa and Asia. Subterranean tubers are small, hard and bitter (or
may be absent), while the aerial tubers are good to eat, though they may need to be
detoxified. Stems twine to the left.
Dioscorea hispida—Asiatic Bitter Yam. Similar to the African Dioscorea
dumetorum. Large tubers, near to the soil surface, which are very toxic and are used
for hunting or criminal purposes; the toxin can be removed, and the tubers are used
as a famine food in the tropical East. Stems twine to the left.
Dioscorea nummularia—grown in Indonesia and Oceania. Similar to Dioscorea
Dioscorea opposita (or D. batatas)—Chinese Yam, Cinnamon Yam or Vine. Used
medicinally in China. The most resistant to cold of all the yams. Grown in China,
Korea, Taiwan and Japan.
Dioscorea pentaphylla—occurs both wild and cultivated in the warm, moist parts of
Asia. Non-toxic tubers. Often grown as hedges around fields.
PLANTING
whole small tubers. Rich topsoil and compost should be put in the planting hole.
About 2 MT/ha of tubers are needed. Many wild yams reproduce freely by seeds,
but many cultivated varieties rarely or never produce viable, fertile seed.
Germination: yams are monocotyledons, though some botanists believe that some
varieties have two cotyledons, one of which remains within the seed.
Soil: should be fairly fertile, loose, deep and well drained. Responds well to
Nitrogen but very often does not respond to Phosphorus. Normally, yams give an
economic response to NPK fertilisers at about 600 kg/ha, and manure. The plants
also benefit from mulching.
Plant spacing: 10–15,000 setts/ha. For sub-species with larger tubers, either in a
1×1m square grid or with 50 cm along the ridges and 1.5 m between ridges, giving
about 10,000 setts/ha. Planted somewhat closer together when the tubers are small.
Stakes: yams should be allowed to grow up stakes at least 2 m high and with
branches, which can increase yields by 60% or so.
Rotation: often the first crop planted during shifting cultivation, or after a fallow.
Yams yield well after a leguminous green manure crop.
Intercropping: almost always done, with one or more crops such as maize, okra or
cucurbits.
GROWTH CONDITIONS
Growth period: D. alata and D. rotundata produce the first edible tubers in 8–10
months. D. cayenensis needs about 2 more months, and can then be harvested
continuously.
2 3 years before they are dug up. Stems twine to the right.
cayenensis, but tubers are formed deep in the ground, and are allowed to grow for
Propagation: normally done vegetatively, using 100 g –2 kg parts of a large tuber, or