Ipomoea batatas – Sweet Potato or When is a potato not a potato?

A crop of sweet potatoes grown at University of Reading

A crop of sweet potatoes grown at University of Reading

Many of you will be familiar with the knobbly, red-skinned, orange-fleshed, vegetable known as sweet potato but you might be surprised to learn that it is more closely related to the parasitic thread-like dodders than it is to our familiar white-fleshed potato.

Here you can see some of the sweet potato crop gathered in summer 2015.  There are two different varieties grown, one with orange flesh and one with yellow flesh.

Meet the family 

Ipomoea batatas is a member of the Convolvulaceae. Other plants you might know from this family are I. purpurea, or Morning Glory, which is commonly grown in British gardens as an annual climbing plant, and that bane of gardener’s lives, although it looks very pretty when growing in the wild, Field Bindweed, Convolvulus arvensis.

Ipomoea batatas

Ipomoea batatas growing round the pond in the tropical greenhouse
Photo by Irene Lucas

Ipomoeeae is one of 12 tribes in the Convolvulaceae family and Ipomoea is the largest genus in the tribe, having over 700 species (Heywood et al., 2007). These are mostly annual and perennial herbaceous climbers although there are a few erect shrubs found in the tropics (Lebot, 2009). I. batatas is a perennial herbaceous vine which is not known in a wild state. It is uncertain which species were its genetic parents.

The name Ipomoea means worm-like, from the greek ips (a worm) and homoios (resembling) (Coombes, 1994). Batatas simply means potato.

 What does I.batatas look like?

I. batatas shows the typical Convolvulaceae family characteristics of alternate leaves and funnel-shaped flowers with 5 stripes. Like other climbers in this family, I. batatas twines to the left without the help of tendrils or other climbing aids. Ipomoea batatas has thin stems which are light green to purple, either hairless or with soft short hairs, and which contain latex. When the nodes of trailing stems touch the soil, roots are produced.

Ipomoea batatas leaves

Here you can see how leaves vary in shape even on the same stem
Photo by Irene Lucas

Leaves arise every 2 to 20cm along stems, which results in a dense mass of foliage. They are very variable in shape, often on the same plant, as you can see from this photo of the plant growing in the tropical greenhouse. The bases of leaves are usually heart shaped and the blades egg shaped but the blades may or may not be lobed and the tips may be either pointed or rounded. The colour varies from light green to deep purple, sometimes with a purple stain at the base.  (Lebot, 2009). Leaf size is also very variable, ranging from 5 to 15cm long by 5 to 15 cm wide (Purseglove, 1974).

 

Ipomoea batatas flower

A flower in the tropical greenhouse
Note the 5 fused petals and the mid petal stripes
Photo by Alastair Culham

A side view of the flower showing its funnel shape
Photo by Alastair Culham

 

 

 

 

 

 

 

Flowers may occur singly or, as in the plant in the tropical greenhouse, in clusters.  They grow out of leaf axils and have stalks which vary in length from 3 to 15cm. The 5 sepals are deeply lobed and the 5 petals form a funnel shape tube, measuring between 3 to 6cm wide by  2 to 5cm long. The colour ranges from light pink to deep purple. The colour is usually deeper at the throat and paler at the margins  (Lebot, 2009). Flowers open and wilt within a day, lasting no more than a couple of hours if hot and sunny, but longer if cool and cloudy (Purseglove, 1974).

An Important Food

Ipomoea batatas tubers

Sweet potato tubers growing on the plant in the tropical greenhouse
Photo by Irene Lucas

The most widely used common name for I. batatas is sweet potato, indicating the important use of the root tubers as a food (Purseglove, 1974, Woolfe, 1992). While this name reflects the similarity in use of I. batatas and Solanum tuberosum, the food plant commonly known as the potato, they are not closely related, being in different families, although they are both in the order Solanales. I. batatas is the only member of the Convolvulaceae family that is a major crop plant, however some others are used locally and Ipomoea aquatica (water spinach) has quite wide use. In fact, it would be dangerous to eat many of its relatives as they are poisonous.

Sweet potato tuber
Photo by Irene Lucas

Sweet potatoes are the sixth most important food crop in the world, with 95% being grown in developing countries (cip.potato.org). Nutritionally, they are valuable as they contain carbohydrate, glucose (hence ‘sweet’ potato), and carotenoids, which are a rich source of vitamin A and give the tubers their yellow and orange colour   (www.botgard.ucla.edu). Some claim they are one of nature’s best sources of beta-carotene (World’s Healthiest Foods). The size of tubers is very variable. The one in the photo is about 15cm long. They are a very versatile foodstuff. As well as cooking the tubers as a vegetable, they can be turned into  flour, dried chips, juice, bread, noodles, candy, and pectin (cip.potato.org).

How do you grow sweet potatoes?

Small farmers in Africa

Many tropical crops are grown in layers to maximise the production in an area but sweet potatoes require too much light for this method. Smallholders tend not to spend much time or effort on looking after their sweet potato vines (Purseglove, 1974), but when grown commercially this is a labour intensive crop which requires much water and so tends to be grown year round in warm humid regions where labour costs are low (www.botgard.ucla.edu).

Sweet potatoes need average temperatures of at least 24°C with 4 to 6 months frost free and 50cm or more rain in the growing season.  While the plants prefer high light intensities, they also need a short day length (Woolfe, 1992) for tuber formation.

There are a large number of cultivars of sweet potato which vary in features including; the colours of both tuber skin and flesh, the size and shape of tubers, the texture of the cooked flesh, the depth of rooting, and resistance to disease. This genetic diversity gives scope for crop improvement and the choice of cultivar for different locations and tastes (Woolfe, 1992).

Sweet potatoes are propagated from vegetative stem cuttings that root readily, unlike conventional potatoes that are propagated by tubers.

Where did sweet potatoes originate and where do they grow now?

Remnants of sweet potato roots have been found in Peru dating from 8000 BC, but it is not clear whether these were wild or domesticated plants. However it is known that they were cultivated in the Casma valley in Peru as long ago as 2000BC  (Woolfe, 1992) and domesticated at least 5000 years ago in Central America (Austin, 1977). For centuries, sweet potatoes have been an important crop for Pacific islanders and the Maoris of New Zealand  (www.botgard.ucla.edu). Mystery surrounds how sweet potatoes reached the Pacific islands as plants rarely produce fruits or seeds, so must be propagated from stem cuttings or root tubers. Various theories have been suggested but all have their critics. Post Columbus, the spread of sweet potato accelerated.

Map showing where sweet potatoes originated and their early spread
Irene Lucas
Based on Hobhouse, et al. 2004,

Sweet potatoes are now commonly grown in over 100 tropical countries, from 40°N to 32°S (Purseglove, 1974). China is the largest producer, from where large quantities are exported to Japan (www.fao.org Pages 127-128). Two forms are grown in the United States where, confusingly, the drier, yellow tubers are known as sweet potatoes and the watery, orange tubers are known as yams (www.botgard.ucla.edu). The true yams belong to the monocot genus Dioscorea. Thankfully this confusion is not common in the rest of the world.

What else can you use I.batatas for?

I. batatas has several other uses. Both the leaves and tubers are used as fodder and in Asia about half the crop is used for animal feed whereas in Africa it is grown mainly for human consumption (www.fao.org  Pages 127-128).  One bonus of feeding animals on sweet potato vines is that they may produce less methane gas than when fed on alternatives (cip.potato.org).

I. batatas is also used as a source of industrial alcohol, sugar and starches and research is  being carried out into the use of I. batatas as biofuel. Dyes for cloth can be made by mixing the juices of sweet potatoes and limes when, depending on the proportions of each juice, the dye colour ranges from pink through red to black. Interest is also being shown in using the anthocyanin pigments that are found in the purple varieties for food colourings and  in the cosmetics industry (cip.potato.org).

A purple-leaved variety of I. batatas growing in a pot with Coleus and Lantana

 

Gardeners grow selections of I. batatas for their attractive leaves, rather than their tubers, with dark leaved forms being particularly popular.

Does I.batatas have any other common names?

There are many other, local, common names for I. batatas.  These include batata, camote (Spain), kamote (Borneo), goguma (Korea), man thet dhaeng (Thailand), ubi jalar (Indonesia), ubi keladi (Malaya), shakarkand (India), Satsuma imo (Japan), boniato (Caribbean) and kumara ( New Zealand Maori) (www.cipotato.org Annex – Section 3, page 42).

References:

Austin D.F. (1977) Hybrid polyploids in Ipomoea section batatas.  Journal of Heredity 68; 259-260.

Coombes, A,J., 1984.  A-Z of plant names.  London: Chancellor Press

Heywood, V. H., Brummitt, R.K., Culham, A., Seberg, O., 2007. Flowering plant families of the world.  Richmond: Royal Botanic Gardens, Kew

Hobhouse, H., Knapp, S., and Lowndes, M. Seeds of Trade. 2004, http://www.nhm.ac.uk/nature-online/life/plants-fungi/seeds-of-trade/.

Lebot, V. , 2009. Tropical root and tuber crops: cassava, sweet potato, yams and aroids.  Wallingford: CABI

Purseglove, J.W., 1974. Tropical crops. Dicotyledons. London: Longman

Scott, Best, Rosegrant, and Bokanga (2000). “Roots and tubers in the global food system: A vision statement to the year 2020”. International Potato Center, and others. ISBN 92-9060-203-1. Annex – Section 3, page 42

Woolfe, J.A., 1992, “Sweetpotato: an untapped food resource”. Cambridge, UK: Cambridge University Press and the International Potato Center (CIP)

http://www.botgard.ucla.edu/html/botanytextbooks/economicbotany/index.html

http://cipotato.org/sweetpotato/processing-uses

http://cipotato.org/cipotato/publications/pdf/005448.pdf

http://www.fao.org/docrep/013/i1500e/i1500e18b.pdf pages 127-128

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