Dioscorea alata L.

Tuber polymorphous, (i) cylindrical or clavate and deeply descending into the soil even to 1.5 m, (ii) globose and stout and short (D. globosa ROXB.), (iii) pyriform, (iv) lobed in various way, (v) fingered, (vi) fingered and fasciated (D. vulgaris MIQ.), (vii) losing its positive geotropism with a subsequent change of direction in the soil (D. spiculata Bl. only as to ref. to RUMPH); skin brown to black; flesh white or ivory coloured or purple, either superficially or throughout (for the complete range of shapes see Ann. R. Bot. Gard. 14, pl. 125), never poisonous. Plant glabrous. Stem unarmed though rather rarely rough or warted close to the soil, climbing to 10 m, quadrangular and as a rule conspicuously 4-winged above the very base at which the leaves are not decussate. Bulbils abundant, more so in some races than in others. Leaves generally a few alternate at the very base, thereafter opposed, herbaceous, subsagittately or sub-hastately ovate, rarely subhastately deltoid, shortly acuminate, usually to 22 by 15 cm, the upper surface bright green, 5-nerved; petiole about as long as the lamina, sometimes marginally frilled, sometimes with the pulvini suffused with purple. Male flowering axes 1-2 together, aggregated on leafless branches which only rarely exceed 30 cm in length, with upwards of 20 flowers spaced about their own diameter apart. Buds somewhat flattened at the base, otherwise nearly globose 1 mm long, at times the axis a little zigzag with the buds on the angles. Female flowering axes 1 from an axil, de-curved but rigid, to 60 cm long with about 20 flowers, angled or at the base narrowly winged. Capsules at the apex slightly retuse, at the base obtuse; wings 17-20 by 15 mm; stipe 3-4 mm long.

A tropical plant. It grows from sea level up to about 1800 m in the tropics. Yams are most important in seasonally dry areas. They need a well drained soil and it has to have reasonable fertility. The temperature maximum is >30°C while the minimum is 20°C. The optimum temperature range is 25-30°C. Rainfall is often seasonal in yam areas and the maximum to be needs 14-20 weeks rain with an optimum of 1,150 mm during the growing season. Yams can tolerate drought but give maximum yields with high rainfall. The critical rain period is during the first 5 months. Light influences tuber growth. A continuous exposure of tubers to light significantly reduces tuber yields. Day length-Yams are influenced by photoperiod, or hours of sunlight. Short days (less than 10-11 hours of sunlight) favours tuber development. It does not suit atolls. It suits hardiness zones 10-12. In Yunnan.

The tubers are boiled, baked or mumued. They can be roasted, fried or mashed. When they occur the aerial tubers of bulbils are also cooked and eaten.

Ceremonial yams have very specialised production techniques. For general food production, use top pieces of the tuber after they have sprouted, use a branched stick for support of the vine, space plants about 1 m apart and choose a smooth round cultivar. Given the large diversity of cultivars of greater yam, for efficient production varieties need to be chosen which have regular rounded tuber shapes for easier harvesting and preparation; also selection needs to be made for varieties with less leaf spot and virus susceptibility and stable yield. Colour, cooking quality, storage ability, texture and other qualities need to be considered to suit the growers demands. In most places the yam growth and maturation is integrated with seasonal rainfall patterns. They are mostly planted just before the first rains where a 8-10 month rainy season exists and give better yields in 6-8 month rainy season areas when planted 3 months before the rains. Earlier planting requires larger sett size to withstand desiccation. Pre germination of tubers which are cut and stored in shady places gives improved yields over tubers left whole then cut into setts at planting. Because yam tubers have a period of dormancy, tubers do not normally commence re-growth for up to 5-6 months. This enhances their storability but delays out of season replanting. Dormancy can be broken using Calcium carbide treatment for 5 hours or by covering tubers with leaves of Croton aromaticus or Averrhoea bilimbi.. Yams are demanding in their nutrient requirements are are therefore often planted first in rotations. They need a fertile free draining soil. They cannot tolerate water logging. It is normally grown from sections of the tubers especially top pieces. In some cultivars, it is also propagated by bulbils. Using staggered plantings of male and female plants and doing hand pollination it is possible to get viable seed set which can be used for establishing plants. It is common practice in many areas to plant the yam piece upside down. The probable reason for this is to give the shoot and roots time to develop and get established away from the sun and wind, so that the plant does not dry out. People in yam areas have their varieties classified as to whether they are planted at the top or the bottom of the hole, and whether the shoot is pointed up or downwards. This is a response to the diversity of tuber shapes and cultivars. A planting depth of 15 cm is optimum. Normally top pieces give higher yields than middle pieces of tubers and these are better than bottom pieces. Varietal differences in this occur. Top pieces give earlier and more reliable germination and mature earlier. They are also the less attractive part of the tuber for eating so are preferred for planting. The larger the sett the earlier the germination and the greater the yield. Increasing the seeding rate and plant density gives greater total yield but the extra planting material required means yield of food available is less. Normally on lighter soils closer spacing is used. Compact soil or hard pans or stones result in tubers being exposed which decreases the yield and needs to be avoided This is related to light as well as physical constraints. Good drainage is essential. Yams must have a well drained soil with plenty of air in the soil. So yams will not normally grow on heavy clay soils or in areas with a lot of soil moisture. The soil can be improved for yam growing by putting leaves and other plant material in the planting hole, by making a mound above the hole, or by planting on a hillside. In some very loose sandy soils yams can just be planted in flat unmounded soils without digging a special yam hole but these situations are not common. Yams should also have sticks to climb up. It is best to have a stick that is twisted or branched because the vine can slip down a very straight stick. Normally a stick 2 metres tall is sufficient. It needs to be a strong stick, firmly fixed in the ground. Yam varieties vary on the type of vine growth they have. This affects where the stick needs to be placed. The fat irregular yams can have the sticks near the mound as a thick clump of vines and leaves soon develops. But if the stick is put beside the mound of one of the long ceremonial yams the vine will often reach the top of the stick before it has produced more than a couple of leaves, and will then fall back down to produce its leaves on the ground. The stick often needs to be put at some distance from the yam hole. The tip can be picked off the vine if branching is wanted earlier. It may be that the long vine yams are more common in forest areas and the shorter branched vines in grassland areas. In some areas yam vines are allowed to creep over the ground and do not have sticks to climb. This method only works satisfactorily in dry places because diseases of the leaves and vine can cause serious damage in wetter places. Where yams do not have sticks to climb plants need to be more widely spaced. Under most circumstances the amount of food produced can be doubled by allowing yam vines to climb up sticks. In drier grassland areas mulching the mounds at planting has been found to improve establishment and yield.