People have eaten tubers since time immemorial. Indeed, a small but ardent band of anthropologists argues that cooked tubers in general played the critical role in separating humankind from the rest of the primates. According to them, the (presumably African) tubers provided foodstuffs that did not have to be chased down and required little chewing. Cooking turned the starch into sweet, appealing foods and easily absorbed calories. In addition, the tubers needed to be kept in one place under protection, so they initiated “home life.” All of this—according to the proponents—prompted the evolution of large brains, smaller teeth, modern limb proportions, and even male-female bonding.
This may seem like a big indictment to pin on a few homely plants, but Harvard anthropologist Richard Wrangham and his colleagues are convinced that cooked tubers were pivotal in this way to human evolution. They don’t speculate on which species led to the creation of humanity, but the subject of this chapter seems a leading possibility.1
Even today, Africa depends heavily on root foods. Indeed, without the contributions from cassava, potato, sweet potato, and yam, hunger would spin out of control all across the continent. Of those four cornerstones of the current food supply, however, only yam is native.2 Yet Africa has a wealth of indigenous edible roots and tubers. Sadly, they are among the most “lost” of Africa’s lost food crops.
Elsewhere in this book we describe marama and yambean, both of which are African legumes grown at least partly for tubers. Here we highlight the so-called “native potatoes,” two plants grown solely for tubers. In culture
and usage, these two are more akin to the conventional root crops.
Despite the name, the plants of this chapter3 are neither potatoes nor potato relatives. Nor are they related to sweet potato, yam, or cassava. They are members of the mint family. This 3,000-member family graces human existence with numerous herbs and fragrances, including lavender, mint, spearmint, rosemary, sage, thyme, oregano, basil, and majoram, but no major root crops. Indeed, Africa’s native potatoes are the only mints producing human food below ground.
Both these native potatoes are herbaceous perennials. Speaking generally, they are distributed from the warmer eastern regions of South Africa northwards to Ethiopia and from there westward as far as Senegal. The occurrences mostly overlap, however one native potato (Solenostemon
rotundifolius) is traditionally produced primarily in West Africa while the other (Plectranthus esculentus) is a resource primarily of Southern and East Africa. S. rotundifolius is also cultivated in parts of Asia—notably India, Sri Lanka, Malaysia, and Indonesia. Its fellow species, as far as can be ascertained, is unknown as a crop beyond Africa’s shores.
Despite their age-old heritage in cultivation and cuisines, both these crops are lacking in details and certainties. The literature treats them under a (sometimes inaccurate) mix of common names. The “northern” species (S. rotundifolius) is most often referred to as Hausa potato, Sudan potato, Zulu round potato, fabourama, and frafra potato. The “southern” species (P. esculentus) is most notably referred to as Livingstone potato, Madagascar potato, and scrambled eggs.
The literature also treats them under a (sometimes inaccurate) mix of scientific names. Taxonomists peering at the flowers and other parts have accorded these plants at least a dozen different scientific epithets.4 Some of their claims may well be valid, and there may be hundreds of species of just Plectranthus, but for simplicity sake (considering that our audience is made up largely of non-botanists) this chapter presents the native potato resource as if there were just two species, which is most likely the case.
Smaller than the modern commercial potato, the tubers of S. rotundifolius are small and oval shaped, while those of P. esculentus are longer and thinner, extending from the bunch at the base of the plant like fingers. The tubers of both crops are mostly boiled, but they can also be roasted, baked, or fried. Indeed, they can probably replace potato in each and every recipe—even potato salad. The flavor, at least of P. esculentus as served in South Africa, is described as “quite a pleasant minty taste.”
As far as industrial processing goes, little is known, because the quantities produced in any one location are generally small. However, given larger production, prepared food products seem quite possible. Flour milled from dried native potato (S. rotundifolius, there known as fabourama) is already produced in Burkina Faso, and it is reportedly turned into popular breakfast gruels. As of now, no one has reported on what sort of French fries or chips the tubers of this ancient crop yield.
Given their Mint-Family connection, it is no coincidence that the leaves of the plant are aromatic, but the tubers are neither fragrant nor flavorful. The tubers of P. esculentus have the kind of blandness that is preferred in a staple. The tubers of S. rotundifolius are much sweeter, by comparison.
Indeed, people are attracted to them for this very quality. Some liken the taste to sweet potato or parsnip. Probably in both species, however, the taste varies with locality and the individual clone of the plant. At least one observer notes that they are “an acquired taste, being rather bitter.” In general, though, both tubers are well liked by both Africans and Europeans.
Currently, native potato is exclusively a smallholder crop. Indeed, it is almost exclusively a women’s crop. Those who produce, collect, and process the tubers on their farms are female, both young and not so young. The tubers, overwhelmingly employed as subsistence food, make versatile family fare. They are good as a food-security insurance policy. They can, for example, be dried and put away for use during times of shortage. Although native potato is not a cash crop in the modern sense, part of the harvest is commonly put up for sale in the villages. Collectively, African women derive considerable income thereby.
Beyond the pocketbook issues, this crop is a reasonable contributor to dietary improvement. A standard serving provides a large percentage of the daily requirement of calcium and vitamin A (in the form of β-carotene), as well as more than the daily complement of iron. The tubers contain 5-13 percent protein (calculated on a dry weight basis), or up to twice the amount found in potatoes (5 percent). In addition, the protein of P. esculentus is well endowed with essential amino acids (threonine, tyrosine, methionine, valine, leucine, lysine, etc.)5 A serving thus contributes a fair portion of the daily protein requirement. The food-energy content is good as well—almost 400 calories per 100g dry matter in S. rotundifolius tuber.
Not only are the native potatoes nutritious, they are productive. Even in their current, horticulturally fairly primitive form, they can yield a lot of food from a small area. The recorded yields are 4 to 7 tons per hectare for P. esculentus and 15 tons per hectare for S. rotundifolius. And by employing the best plants and best cultivation practices under highly conducive conditions over 50 tons per hectare is supposedly obtainable, at least on experimental plots.
Despite these heartening signs of high production and good nutritional contributions, this is hardly a big resource in terms of geographical area or nutritional importance. Indeed, most Africans have never heard of it. Perhaps because the plants are basically hidden in plain sight no national or international research or extension organization has accorded it major support. This is unfortunate considering that calcium, vitamin A, iron, and protein—all of them vital to life—are typically deficient in rural diets in countries where this crop is found. And it is doubly unfortunate because these are big, bland staples that are eaten in bulk and can deliver quality nutrition in both a broad and a powerful manner across the society.
But without outside help a spontaneous adoption on a wide scale is unlikely. Working against these tuber crops is the popular impression that they are inferior, old-fashioned, outmoded foods. That impression resides in the minds of officialdom rather than of the village. It is primarily because of official (as opposed to consumer) neglect that the crop suffers from a lack of research support. And that lack is leading to sad consequences: In many locations this is another age-old resource that is dwindling toward obsolescence. And in Chad, where it is known as ngaboyo, it is said to be facing outright extinction.
Although much remains to be learned, native potatoes represent a pool of indigenous germplasm awaiting the specialists’ plunge. They may never prove food-supply superstars, but they are well worth investigating. A recent report from the CGIAR declares that: “root crops will be many things to many people by 2020.”6 Driving the authors to this deduction is the root crops’ adaptation to marginal environments, their vital role in promoting food security at the household level, and their flexibility in mixed farming systems. The authors note that root and tuber crops are often preferred over cereals both by farmers and consumers, and that they should be important components of programs, policies, and strategies aimed at improving the rural poor’s welfare.
In this regard the native potatoes seem good candidates for pan-African attention. They are clonal crops that are easy to handle and propagate. They are found in the areas of low agricultural potential across the most needy regions of the continent. They occur where a shortage of suitable vegetable crops now results in endemic malnutrition. They produce large amounts of nutritious food from a small land area. And they seem primed for rapid advancement.
This last point may not at first be obvious. But because the native potatoes have not been subjected to intensive or extensive horticultural science, the immediate application of knowledge garnered from counterparts such as potatoes and yams—and even fellow mints—can likely bring a quick payoff. And in the long run native potatoes may have an even bigger payoff than anyone now could anticipate. Indeed, small agronomic improvements could well bring big jumps in yield. Also, more detailed research could solidify and enhance the nutritional and commercial gains to be reaped from the greater production.
Taken all round, then, these ancient native foods could prove good tools for reducing malnutrition and hunger, while improving farm profitability and providing African families with greater food security.
With these crops no one can foresee what the future holds for production or use. Not enough is known about the fundamentals to be sanguine about any future trajectory of supply and demand. The long term possibilities will become clear only as researchers burn away the mists of uncertainty and expose whatever potential is hiding behind today’s doubt. But that could come quite quickly and here’s what we think of the prospects for the different climatic zones.
Humid Areas Seemingly excellent. Tolerance to high temperature and rainfall is a feature of S. rotundifolius, which is widely (if thinly and irregularly) spread across Africa’s tropical lowlands. P. esculentus prefers dryer conditions but is also in moist regions parts of South and East Africa.
Dry Areas Good but uncertain. In West Africa, to mention just one area, S. rotundifolius is grown from humid coastal areas to the dry interior woodlands. Nonetheless, during excessively dry periods irrigation will likely be essential to create an outcome that is commercially satisfying. P. esculentus produces reasonable yields in South Africa with annual rainfall as low as 450mm, although the rain must be well distributed through the season for the plants to produce under such parched conditions.
Upland Areas Uncertain, but possibly good. Native potato is generally regarded as a low-altitude crop, but some biologists have speculated that S. rotundifolius possibly evolved in Ethiopia. Likely, select types will be found that fit conveniently into upland habitat niches. In recent tests in South Africa P. esculentus has performed well at altitudes around 2000m.7
It is in Africa that the plants seemingly have their greatest potential, however S. rotundifolius is also grown in South and Southeast Asia, so there are opportunities for developing it there as well. There does not appear to be any reason why P. esculentus would not also be successful beyond Africa.
Unlike the promising resources dealt with in other chapters, native potatoes are essentially one-product plants (omitting certain purported medicinal properties, that is).
Roots The tubers are mostly eaten as cooked vegetables. Like potatoes, they may be boiled, baked, or fried. However, one northern Nigerian type (a form of S. rotundifolius said to be distinguished by dark leaves) has tubers that can be eaten raw—something not even potato generally claims. The light colored tubers of P. esculentus in South Africa, Zimbabwe, and Zambia can also be eaten raw, and are reportedly eaten this way in Malawi too.
The literature has so far provided few nutritional details from which to draw conclusions. One early report recorded a S. rotundifolius tuber sample as being 76 percent moisture. Its dry matter consisted of 91 percent carbohydrate, 5 percent crude protein, 4 percent fiber, 4 percent ash, and 1
percent fat. The nutritional energy was 392 calories per 100g.8
Recent studies on the nutritional value of P. esculentus in South Africa recorded (on a dry-weight basis): 81 percent carbohydrate, 13.5 percent crude protein, 4 percent ash, and 1 percent fat. In addition, the tuber material contained (in mg per 100g of plant material): vitamin A (0.2), thiamin (0.04), riboflavin (0.06), vitamin B-6 (0.3), phosphorus (337), potassium (1,721), calcium (140), magnesium (327), zinc (3.5), copper (1), manganese (1.4), sodium (73), and iron (50).9
This perennial is normally grown as an annual. It is, as we’ve said, a smallholder crop, and is probably grown in as many intercropping patterns as there are farmers. None of the cultivation methods has been investigated, but many are likely to be interesting and to reflect long-tested local experience.
Propagation is by tubers, setts, stem-cuttings, or suckers sliced from sprouted tubers. However, the standard planting method is using tubers or portions of tubers. At the beginning of the wet season these are planted into mounds, ridges, or rows on prepared beds.
Obviously, the plants need to be spaced to fit the site, the climate, and the mix of species in the plot. However, the recommended spacing varies from 50 cm to 90 cm between rows and 15 cm to 30 cm between plants. The propagation materials should, it is said, be placed (horizontally where appropriate) at a depth of 5-8 cm.
In practice, fertilizer is rarely applied, but organic material incorporated liberally into the ridges or mounds before planting, followed by a shot of fertilizer once the crop has become established, are reportedly worth the effort and expense. As in potatoes, piling earth around the base of the plants as they grow encourages greater tuber development.
Even though caterpillars of various kinds feed on the leaves, pests are rarely of economic importance. With S. rotundifolius weeding is said to be required only in the first stage (before the spreading foliage shades out the competing species). However, P. esculentus plants do not spread to such a degree, and weeds remain a problem to the end of the season.
HARVESTING AND HANDLING
Depending on the place and the plant, the tubers are ready for harvesting after 120-200 days. In the case of S. rotundifolius all the aerial parts have by
then flowered and died back. P. esculentus, on the other hand, does not flower at the end of the summer growing season. Instead, it drops its leaves and goes into a dormant phase during the winter. With the warmth of spring, flowers pop out of the leafless stems, after which these stems die back and new fresh growth emerges from the tubers underground. For this reason, the plant is seldom seen to flower when cultivated.
There are literature claims that the mature tubers must be dug up promptly and protected carefully—that their skin is easily damaged and they deteriorate rapidly. However, recent research in South Africa indicates that the tubers of P. esculentus form wound tissue extremely fast, within a few hours, so post harvest diseases are not as bad a problem as expected.10 S. rotundifolius tubers have a thicker skin and are even more resistant to damage. Nonetheless, post-harvest diseases and pests can be serious. Packing the tubers in dry sand and storing them in the shade has been found to extend shelf life.
Although pests are not normally problematic, the tubers may harbor diseases, notably including viruses and bacteria. Other clonal tuber crops commonly carry a load of such afflictions, which are passed on generation to generation. The presence of such microbes has not been shown in Africa’s native potatoes; however, the possibility that they are suppressing the crop at least in parts of Africa deserves consideration.
With such a neglected resource there may well be general marketing problems to resolve in each locality, even those that know it best. The possibilities for long-distance transport, for example, are limited by the tuber’s short post-harvest life as well as a lack of processing methods and storage facilities.
Sad to say, these indigenous resources are now among the most neglected of all edible plants. Few have begun to apply modern science and given them a chance to shine. Yet, as we’ve noted, native potato’s development seems likely to help improve nutrition and income and reduce food crises at the household level. The most immediate actions should be those oriented toward small-farmer needs. But seen in broader context, programs covering the entire chain of production, from the plant’s basic scientific underpinnings through to its production and utilization on a large scale, and even government policy, are needed.
Although the full extent of their adaptability has not been tested, the plants seem likely to prove useful in hunger-fighting interventions
throughout much of the continent. They can grow in harsh climates to which conventional staples are poorly adapted. They appear to host few economically serious pests and diseases. And the foods they yield fit well into all or most traditional cuisines, including imported ones.
There is also a need to set up a native-potato rescue program. Its goals would be to publicize the species and its potentials, to advance fundamental knowledge of the crop, and to inspire consumers and companies to use more of it. In this regard, the fate of this food could depend on a dash of marketing savvy aimed at transforming its image as being somehow inferior. Women could take an active role in shaping these efforts. That would help to ensure that they achieve additional income instead of just additional work.
The following are offered as some possibilities for specific action.
Purify the Planting Stock As noted, many of the native potatoes that farmers now plant probably suffer from chronic afflictions of viruses and bacteria, which get passed on down the generations ad infinitum. The development of tissue-culture technology in recent decades provides a powerful tool for cleaning out such hitchhiking freeloaders. In the cases of other root and tuber crops this technology has produced almost miraculous leaps in plant health and productivity. Now a government research organization in South Africa has applied it to the native potato. The Biotechnology Division of ARC-Roodeplaat has developed a program of both meristem culture and thermotherapy that eliminates viruses from the tissues.11 ARC-Roodeplaat is already providing virus-free planting material within South Africa.
This important advance could bring far-reaching and rapid improvements continent wide. Other African nations should take note. Through contracts, collaboration, or separate efforts, they could supply their own farmers with native potato planting materials uninhibited by chronic disease. In fact, a network of pure-stock providers covering sub-Saharan Africa could be the single advance that sparks a rapid renaissance in this very old and much neglected indigenous food plant.12
In addition to providing virus-free germplasm, ARC-Roodeplaat maintains an in-vitro genebank containing South Africa’s selected and unselected materials. For other nations this too is worth copying or collaborating. Keeping tubers viable in long-term storage is difficult if not impossible; keeping the tissues in test tubes is easy by comparison.
Survey Part of the problem with these poorly documented crops is to know just what exists. It is therefore important to collect and evaluate the
germplasm. Also, a search for the wild progenitors would go far toward establishing the crop’s genetic identity and exact place(s) of origin within Africa.
To preserve this dwindling resource’s genetic diversity for future generations should be a priority. Collections should be made especially in isolated areas. The agronomic traits of the different germplasm should be characterized and important qualities noted. Disease resistance and other standard qualities are of course important. But it is the tuber qualities that are paramount: size, shape, color, flesh texture, cooking qualities, and above all taste.
Women know most about how best to produce and process this crop and their knowledge should be also collected in the dozen or more countries that employ the crop most.
Taxonomy Someone (or preferably several persons independently) should gather representative tubers from different plant types found across Africa, grow them out, and conduct cross-pollination, DNA, and other identity tests. This will determine just how many species make up the crop that in this chapter we collect under the name native potato. Cross-pollination between the species (perhaps supported by modern techniques such as embryo rescue) might perhaps lead to more robust and bigger tubered varieties due to hybrid vigor and sexual sterility. Polyploid induction, which has been successful in enlarging ginger rhizomes, is also a possibility.
Physiology Here is a plant with a possible big future and almost no scientific past. It presents plant physiologists with a clean slate waiting to be filled in. Issues to resolve in the laboratory include:
Processes of tuberization;
Processes of pollination; both species seem to have major sterility problems, and reportedly do not produce seed.
Rates of growth;
Tolerances to soil type and soil nutrients;
Limits of temperature and altitude; and
Propagation The whole issue of propagating these crops deserves investigation. In principal, the very best means of propagating the crop for small farmers would be via stem cuttings. Those little pieces of stem are easy to collect, easy to store and transport, and easy to put in the ground. In addition, they are available in huge quantity from parts of the plant that are not edible. Although it has been reported that stem cuttings root only reluctantly, recent research found that they root fairly easily under mistbed
conditions.13 This discovery by itself makes the crop more attractive to prospective growers.
The standard method of propagating P. esculentus involves planting portions of the tubers, but the effect of planting small pieces of tuber on final yields has yet to be determined.
In the case of S. rotundifolius, the large and medium sized tubers are eaten, and the small ones used to establish the new crop. This is a process possibly leading to the preferential selection of plants producing small tubers, a feature also needing careful investigation.
Genetics In this clonal crop seed production is rarely considered or necessary, and the plant’s genetics are barely known. They should now be investigated. Efficient plant-breeding strategies can likely be devised. They will not, however, be easy…given the sterility mentioned above. If crosspollination can be achieved, varietal improvement seems likely to produce rapid advances because plant breeders can select elite types out of open-pollinated populations and then clone them for farmers.
Targets for improvement include:
Large tuber size;
Smooth and regular-shaped tubers;
Fast (and perhaps slow) maturing plants;
Handling Clearly, there is a need for improved harvesting, cleaning, and processing. Techniques that lower labor requirements and/or enhance value are especially required. Methods that work on other root crops will provide invaluable guidance here.
Nutrition Nutritional studies would be more than helpful, especially those that can clarify the optimum dietary mix with other foods. A notable need is to evaluate the amino acids making up the protein.
In addition, there are odd and unconfirmed reports of the young shoots and leaves being used as boiled vegetables. The literature is unclear on how widespread and safe is this practice; some mints have toxins. The broad survey of the crop’s users (mentioned above) will shed light on this, but there is a need to check the possible risk and possible reward of eating the young shoots as potherbs.
Food Technology Here again, virtually everything remains to be discovered. Food technologists could have a field day pioneering the better
handling and processing of an unexplored tuber with the potential to help needy people. In addition, consumption could be increased in urban areas as well as rural, providing that the tuber-based products are made available in convenient forms, both fresh and processed. Required are such things as:
Extended storage life (temperature and humidity);
Techniques to reliably transport tubers considerable distances;
Processing and storage at the home and village levels; and
Commercial processing (flour, chips, or fries for instance).
An interesting possibility is that the foliage might prove useful as a flavoring or fragrance. It is now a waste product, but the fact that it comes from a relative of basil, peppermint, and the rest suggests that it might have a future in many things from soups to soaps.
Horticultural Development The most fundamental field research involves developing an understanding of just how best to produce the crops in quantity. This includes:
Mass production of quality planting materials;
Optimum plant density;
Production under shade;
Production in poor soils;
Cultivation under excessively wet conditions, and also excessively dry;
Production systems that include legumes and rotations;
Year-round production in the hot, humid tropics;
Reproductive pruning (removing the flowers and tops of the plants to force bigger tubers); and
Minimum fertilizer requirements.
Intercontinental Collaboration As mentioned earlier, it is noteworthy that S. rotundifolius is also grown in South and Southeast Asia. Possibly, Asia’s great expertise on vegetable crops might be harnessed to provide Africa with new and developed germplasm as well as new insights into handling the crop. The reverse is also possible: African specialists can provide Asia with valuable germplasm and insights of their own. Also, it may be possible to obtain expertise in tuber-crop development from South America (notably at the International Potato Center), as well as properly introducing an African tuber crop to the lands across the Atlantic.
Main Botanical Names Plectranthus esculentus N.E. Br. and Solenostemon rotundifolius (Poir.) J.K. Morton
Synonyms Coleus esculentus, Coleus dazo, Coleus dysentericus, Coleus parviflorus, Coleus tuberosus, Plectranthus rotundifolius, Plectranthus tuberosus, Plectranthus floribundus among others
Family Labiatae (Lamiaceae) Mint Family
Afrikaans: Wilde aartappel
Burkina Faso: fabourama
English: Livingstone potato, wild potato, country potato, Hausa potato, Madagascar potato, coleus potato, Sudan potato, scrambled eggs, Zulu round potato (S. rotundifolius), elongated native potato, Swedish begonia
French: Madagascar potato
Ghana: fra-fra potato
Nigeria: saluga, tumuku, Rizga (Hausa)
Venda: Mutada, Matheta
Zulu: Umbondive, Ibonda, Ugilo, Ulucanqu, Uluhlaza, Isisqwili, Ushizane, Umhlati, ulujilo, Imbondwe, uJwangu, uShizan, uJilo, uJikwe, uHlazaluti, iZambhane
India: koorka, koorkan, kizhangu
Indonesia: ketang, kentang dwaja
Malaysia: kembili, ubi kembili
Sri Lanka: innala, ratala
Malawi: buye, nyumbu, njowe, cezani
Shona: Shezha, Tsenga, Tensa, Tsenza
Tswana: Makwele e Sechuana
Sotho: Tapole emahlo (wild), Tapole-ea-mahlo
The plants are perennial herbs with prostrate or ascending habits. Plectranthus esculentus grows about 1 m high; Solenostemon rotundifolius is more prostrate and is generally under 30 cm tall. Each has a distinctive fragrant or pungent odor, due to volatile oils contained in glands or sacs in the leaves and other parts.
The stem, which is succulent and square in cross-section, is covered in white hair. The prostrate, lateral, trailing branches root at the nodes.
The leaves form in opposite pairs or whorls at intervals along the stems. They are hairy, oval, and aromatic. They can be up to 6 cm long and have toothed margins. Some plants have a central purple marking on the lamina.
S. rotundifolius flowers before the stems leaf out. In P. esculentus flowers are found in early spring on leafless stems. In both species they are borne on elongated, terminal racemes. Each flower is small (about 1.5 cm long) and bilaterally symmetrical with united petals, and a four-lobed ovary that produces four one-seeded nutlets. In color, they are variously reported as being violet, deep red, or yellow. Seed-set seems rare, and pollinators are unknown (though bees probably pollinated their ancestors).
The shallow, fibrous root system produces tubers that are dark brown or black in color and form in clusters around the base of the stem. Those from S. rotundifolius are round to ovoid and generally egglike. Those from P. esculentus are roughly cylindrical, up to 2 cm in diameter and 5 to 10 cm long (some as long as 25 cm have been found in south Africa).
Possibly of Ethiopian or, more likely, later multiple origins, these and related species are now widely distributed in Africa.
Within Africa Cultivated in West, Central and southern Africa. Also reported in East Africa.
Beyond Africa South and Southeast Asia (India, Sri Lanka, Malaysia, Indonesia).
Basically, there are no formal varieties. However, the West African agricultural literature refers to:
variety nigra, with small tubers and black skin, widely grown in Mali and the Upper Niger region;
variety robra, small, red-gray or red-yellow tubers; and
variety alba, a white-tuber variety which is also cultivated in the Upper Niger region.
Like most else about this crop, the environmental requirements are uncertain.
Rainfall For P. esculentus, acceptable yields have (as already noted) been obtained with as little as 450mm. For S. rotundifolius, however, the minimum annual rainfall requirement is said to be approximately 1,000 mm. The plant is clearly adapted to relatively high rainfall, but it produces optimum yields only in areas where the precipitation is distributed throughout the growing period.
Altitude Limits are unreported.
Low Temperature A freeze is likely to be fatal to both crops, but in trial plots in South Africa P. esculentus has survived short periods of -3°C and S. rotundifolius -5°C.14
High Temperature Unknown. Temperature tolerance is a feature of many root and tuber crops although very high soil temperatures are generally damaging.
Soil Like most root crops, this one responds best in deep, well-drained sites that are well prepared before planting so that the underground portion can swell to its full size with minimal restriction. Well-drained sandy loams are preferable to clays since the crop is sensitive to waterlogging.
Daylength Sensitivity Recent work shows the South African variety is daylength sensitive with a critical photoperiod of 12.5-13 hours.15
Several southern African wild Plectranthus species are also edible, with one at least regarded as a delicacy in the winter season. These extremely obscure roots have never been domesticated and probably don’t warrant a formal attempt. Nonetheless, in the context of native potato research, they could perhaps provide valuable insight into such things as genetic and physiological effects.