Plant profile- Dioscorea alata (winged yam, water yam)
Staple crops that are capable of providing the energy we need to stay alive are a major emphasis on my experimental farm. Most gardeners don’t bother with them since industrially produced grains or potatoes are extremely cheap and there isn’t a major difference in quality between bought and home grown. Vegetables offer a much better economic pay off under these conditions. In a resource constrained future this situation is likely to change.
One crop that has shown enormous promise under zero input management is the winged yam, so named because of the winged ridges on its stems. The name water yam comes from its preference for wet places, though it does not like boggy ground. The vines bear heart shaped leaves and can grow about 10 m tall but they will produce well on shrubs and trellises about 2 m tall. At the base of the plant a lumpy tuber is formed that can be up to 50 kg under ideal conditions such as where they are grown as a major staple crop in central Africa. Under my more difficult conditions I usually harvest them at about 3-5 kg, but one specimen recently caught my eye that was much bigger than normal. The vine had climbed to about 5 m tall up a dead tree and the roots were situated near the outlet for a storm drain.
I set about carefully harvesting this monster. Using a shovel is a recipe for snapping off the brittle tubers and leaving half of them in the ground. The damaged tubers will dry off and heal but need to be cut a fair way below the cut during preparation. A better approach is to use a digging stick or crow bar to dig around the tuber, eventually levering it out once the hole is deep enough. My scales were too sensitive to weigh the tubers but I estimate they are about 15 kg in total. The tubers will store for a few months in a dry shady spot, before the drive to sprout in late spring/early summer gets the better of them. The tubers are peeled with a knife and can be cubed and boiled. They retain a bit more firmness than a waxy potato so can be added to soups, but are also easily mashed into a strangely satisfying sticky mass that is traditionally eaten in small balls by hand in Africa. The cubes can also be roasted and produce a magnificent golden crust. I would rate them only slightly below waxy potatoes in this regard.
This tuber is a good focal point to discuss the idea of EROEI, or energy returned on energy invested. This metric is estimated for fossil fuel sources by counting all the energy used by the industry compared to how much energy is produced. Hunter-gatherers typically operate at an EROEI of 2:1 at most, while agricultural societies have around 5-10:1. The early oil industry was as high as 100:1 when oil shot out of the ground, but now we are only left with the difficult reserves it has dropped to around 15:1. Renewables are generally lower again. EROEI is important because the excess energy left over is needed to support all the complex and specialised functions of a society. This is why hunter gatherers have almost no specialists, while industrial society has thousands of them.
When applying this idea to subsistence staple crops and easier way to think is in terms of time. The 15 kg of yam tubers have about 2500 kJ of energy per kilogram. Allowing for peeling and storage losses there will probably be around 10 kg of finished tubers, so 25 000 kJ of energy. Humans consume about 1500 kJ of energy per day. This means the tuber I just dug up has enough energy for 16 days of energy. I am unlikely to rely completely on yams for my diet, so a more reasonable estimate of 50% of my calories from yams means that large tuber will support me for 32 days, or about a month. In this simplified system the time returned to me then is around 30 days. On the input side it cost me around 1 hour to harvest the tuber, generously one hour to plant (including plants put in other spots that didn’t produce enough to bother harvesting yet), peeling would take about 6 hours as it is a pretty fiddly and slow process due to the irregular shape of the tubers. At this stage the time equivalent EROEI would be looking pretty good, with about one day’s worth of labour (leaving time for sleeping and other tasks) returning 30 days of subsistence calories for an EROEI of 30. Currently I would cook the yam on an electric stove in a metal pot. In a deindustrialised future I would need to spend a couple of days labour collecting fire wood and maybe another day worth of time creating a ceramic pot for cooking (or I could roast the tubers but that would require more firewood). Suddenly the EROEI drops from 30:1 days to 30:4 days, or an EROEI of 7.5, smack in the middle of the range of typical agricultural societies. This would leave enough time spare to secure the other 50% of my calories from other sources, construct and maintain shelter, tools and clothing.
At this point many people would be tempted to take these values and estimate the amount of land area needed to support a single person on yams. The big yam took up about 3 x 3 m, so about 10 square meters. Therefore if one yam supports a person for a month you just need to scale up by a factor of twelve, so 120 square meters is enough to feed a person for a year for 50 % of their calories, or double this at 240 square meters for some miserable hypothetical human who eats nothing but yams for eternity. This estimate would be a mistake for a few reasons. Firstly nutritionally a diet of 100 % yam would be a recipe for all sorts of deficiencies and imbalances. Additionally yams contain toxins (more below) that while easily reduced by cooking some residual amounts inevitably remain. That feeling of indistinct disgust when you eat too much of one kind of food is often related to these minor food toxins accumulating over time. On top of this it is worth remembering this yam was exceptional. I normally get about 1/5 as much yield per plant, so the area would need to be five times more, or 1200 square meters. It was also unusual in having a perfect trellis as the dead tree. If I needed to build trellises for all the vines it would take investment of a lot more time, plus I would need the area to grow the trellis material. The original calculation also didn’t have any measure of land used for fire wood to cook the yams and fire the clay pots. And the final problem is that yams are only available from late winter to late spring, so maybe four months of the year. They could be cut into slices and dried to extend the season, but this would also increase the amount of time invested and require extra land for more firewood. Like the original idea of EROEI calculations the whole idea suffers from a border problem, as in where do you stop counting something as an input. Does the energy used to educate the child that goes on to become an oil field worker count as an input?
Widening the scope of the calculation to include everything needed to support a person gets me back toward an estimate of 5-10 acres per person in a mature zero input system of subsistence agriculture. I would be delighted if my 40 acre parcel reached the point of supporting 2-4 people after another 20 years of work. At present it produces enough food for about one person, but with considerable remaining subsidies from industrial civilisation.
Getting back to the crop itself, winged yam has some big advantages and a few difficulties to consider. The tubers have excellent eventuality, the quality I made up that allows a crop to gradually grow through good and bad seasons until it reaches a point of harvest. Yam tubers will gradually increase in size over several years, with a new larger yam being produced beside last year’s one. This means tubers don’t get woody over time. Yam tubers are also exceptionally resistant to herbivores, like our local rats, mice and bandicoots. By contrast crops like cassava, with roots packed with cyanide, are torn to shreds. Yams contain a range of bitter and toxic chemicals, with some related species remaining toxic after boiling or roasting. Winged yam is easily detoxified by cooking.
A good indication that winged yam is a good subsistence crop in my area is the existence of a different locally native species, Dioscorea transvera, that was used extensively as a food source by indigenous tribes. True yams are one of the oldest domesticated crops and have been vegetatively propagated for many thousands of years. Aborigines knew to replant the top of the tuber to grow again, and burned prime yam habitat during their dormancy to maintain their dominance. This brings up one of the major disadvantages of the crop. They are propagated from small pieces of tubers produced on the previous crop. This means most of my plants are identical clones of one white coloured form, limiting the capacity of the crop to be bred and selected for local conditions. I also have a small amount of material of a purple skinned form, but each plant only produces a handful of seed tubers for next year so it takes many years to build up a large stock of plants. On top of this the plants are either male or female, and only flower sporadically from large plants. The white form shown here is a female but I haven’t flowered any other forms. I have a few seedlings coming on from various sources but I am not even sure if they are the correct species. With a bit of luck I may be able to flower a male from our local D. transversa and hybridise it with the winged yam, combining the size and productivity of winged yam with the local adaptation of the native species. It might require pollen storage and hand pollination if they don’t flower at the same time, which means needing freezers and very tall ladders to reach the flowers. It may be possible to develop a hybrid form that can grow in an outer zone in partnership with a useful shrub for support.
Overall this genus has potential to become an important part of my zero input agricultural system, providing calories at a time of year when they are in short supply after the bunya nuts are gone but before the big late spring potato harvests. They will never be more than one important piece of the puzzle I am solving bit by bit.