Related and unrelated plants: Sweet potatoes, morning glories and yams

sweet potato, Ipomoea batatas

   The sweet potato, Ipomoea batatas, is a morning glory, because it is in the genus Ipomoea and lots of its close relatives are called morning glories. It looks a lot like the weedy morning glories that annoy gardeners in the U.S. South. It has pale flowers with purple centers that open early in the morning and close by midday. It is a vine, with, originally, more-or-less heart-shaped leaves. 

sweet potatoes, cooked

    Sweet potatoes are native to the Americas, where they have been cultivated for millennia. They were probably one of the earliest domesticated plants. Sweet potatoes have been found at sites in Peru dated to 8,000 - 10,000 BC. They were in widespread cultivation in both Mexico and Peru by 2500 BC. 

     Today they are grown around the world. 

Sweet potatoes with ornamental red leaves

    


     
  Columbus encountered sweet potatoes and took them to Europe. 


    The sweet potatoes Columbus found were starchy ones, but soon after, Spanish explorers were introduced to sweeter varieties. They were quickly accepted by many cultures around the world. The problem in Europe was that, being from the American tropics, sweet potatoes need a long growing season and cannot survive frost. They grow tubers in Italy, Spain and Greece but not in England or Germany. Tropical Africa and Asia found them easy to grow.

    Recently, varieties of sweet potato with colorful leaves are being used as ground covers. You never know where you’ll meet a sweet potato these days. 

Ipomoea batatas sweet potato, traditional garden, Hawaii 

     Sweet potatoes are an important part of the traditions of Polynesia, including Hawaii. Hawaii is the most distant set of islands in the world, 2,400 miles from the nearest continent, 1,313 miles from the nearest island (Midway). Consequently every Hawaiian plant has an interesting story, because it came a very long way. Polynesians settled in Hawaii between 300 and 740 AD, bringing sweet potatoes with them. In Hawaii and elsewhere across the islands of the Pacific--Oceania--there is clear archaeological evidence of sweet potatoes well before European contact (Magellan, 1520s). 

beach in Hawaii

    
    That posed a problem to botanists. Sweet potatoes aren’t coastal morning glories that might float to Hawaii. In fact, they’ve been in cultivation so long that they don’t make very many good seeds, so scenarios of the seeds sticking to something and riding off to a Pacific island are highly improbable as well.  

     And yet, not only are they found at archaeological sites from 1200 AD and earlier all across the Pacific, but the names in Oceania and the South America are similar. The word for sweet potato in northwestern South America, from Quechua the language of the Inca, is kumara, cumar or cumal. Polynesians called their sweet potatoes kuumala, and related terms (kumara in Maori, ‘uala in Hawaiian). Transferring the name requires human contact. 

sweet potato leaves

    Botanists and ethnographers have wrestled with this problem since the 1940s. 

    Another line of evidence supporting contact between people in South American and Oceania, probably before 500 AD, appeared this February (2013). Researchers from France headed by Caroline Roullier collected sweet potatoes across the Pacific and in Central and South America and compared their DNA.They also compared DNA from the oldest sweet potatoes they could find, herbarium specimens, dried and pressed plants collected between 1600 and 1900. They knew that repeated movement of sweet potatoes over the last 300 years had confused the situation. Nevertheless they added two substantial points to the argument for exchange long before European contact. First, sweet potatoes from the eastern Pacific (Hawaii, Pitcairn, French Polynesia) were genetically similar to sweet potatoes in northern South America, where the names matched. Sweet potatoes from Mexico or the Caribbean, where sweet potatoes were called camotil or camote, were not so similar to eastern Pacific sweet potatoes. Secondly, the preserved sweet potatoes, collected by the earliest European explorers especially Captain Cook’s first voyage, 1569, were kumara-type sweet potatoes, not camote. The authors conclude that sweet potatoes, domesticated in the Americas, have been brought to the islands of the Pacific repeatedly, and that the earliest was hundreds of years before European sailing ships arrived. 

the Pacific from Hawaii

      Open ocean navigation has become a major interest in Hawaii in the years since I lived there. There are fascinating displays there now about the ways Polynesian navigators guided ships far from land. And increasingly people are recreating those voyages. Pretty clearly they could--and did--sail to Ecuador or Colombia and back. 

   The sweet potato’s distribution alerts us to long distance travel across the Pacific by humans, something we might otherwise know nothing about.  

    
    
   In moving all around the world, sweet potatoes encountered and became confused with yams.

    Sweet potatoes are members of a big genus of vines, Ipomoea (See previous blog: morning glories). Some, but not many, other morning glories also have root-tubers. And, just because a morning glory has a root tuber doesn’t make it edible. The bush morning glory’s tuber is solidly woody and you need a saw to cut it open. Ipomoea pandurata, Indian potato of the US Southeast, is noted in many books as edible but morning glory expert Dan Austin argued persuasively that that is the result of historical authors confusing it with other plants. Native Americans used it as a purgative and did not eat it. I don't believe the tuber of any Ipomoea species except Ipomoea batatas, sweet potato, is eaten by anyone.

    Another big genus of vines, Dioscorea, is also found across the tropics and has edible tubers. These plants are classified in the pantropical family Dioscoreaceae, not closely related to sweet potatoes. While Ipomoea batatas is native to the Americas, there are species of Dioscorea with edible tubers in Central and South America, Africa and tropical Asia. In all three areas, Dioscorea species have been in cultivation since at least 3000 BC. Yam is the common name for plants in the genus Dioscorea. 

yam, Dioscorea, in a garden in China

   Columbus was familiar with yams and when he first saw a sweet potato, he called it a yam. Yams, Dioscorea, have been important foods in Africa for millennia and were deeply rooted in the culture, ritual and religion of several areas. Undoubtedly slaves brought to the Americas ate sweet potatoes the way they had eaten yams in Africa. 

    For millennia, the edible tubers of sweet potatoes and yams have had a variety of sizes and colors. A yam can look a lot like a sweet potato, or vice versa. 

yam Dioscorea,  in a garden in China

     Wikipedia currently states that to reduce confusion, U.S. markets must mark sweet potatoes as sweet potatoes, even if they also call them yams. I have never seen yams (Dioscorea) for sale in a U.S. mainland grocery store. All the tubers called yams I've seen for sale have been sweet potatoes. That's why I keep sticking the scientific name in after the common name here. Yam is a common name for two different plants. 

     In southern China in 2009, I finally saw yams (Dioscorea) growing. And in 2010, to my surprise, I found Chinese yam plant, Dioscorea polystachya, for sale from an herb supplier in the U.S. Of course I bought it and put it in my garden. It twined up the support and then up the sunflower but did not flower.  It surprised me by coming up again last year. I'll be watching for it this year. The veins of the Dioscorea leaf are distinctive, but otherwise it was "any small vine." The confusion with Ipomoea is understandable. 

sweet potatoes a garden in China

For more details:
Austin. D. F. 2011. Indian potato (Ipomoea pandurata, Convolvulaceae)--a record of confusion. Economic Botany. 65 (4):48-421.

Roullier, C., L. Benoit D. B. McKey and V. Lebot. 2013. Historical collections reveal patterns of diffusion of sweet potato in Oceania obscured by modern plant movements and recombination. PNAS (Proceedings of the National Academy of Sciences, USA) 110 (6): 2205-2210.

References I consulted:

Bohac, J. R., P. D. Dukes and D. F. Austin. 1995. “Sweet potato, Ipomoea batatas (Convolvulaceae). pp. 57- 62 In J. Smartt and N. W. Simmonds. 1995. The evolution of crop plants. Longman Press, London.

Hahn, S. K. “Yams, Dioscorea spp. (Dioscoreaceae). pp. 112-120 In J. Smartt and N. W. Simmonds. 1995. The evolution of crop plants. Longman Press, London.

Harkins, J. G. and J. Francisco-Ortega. 1993. The early history of the potato in Europe. Euphytica 70: 1-7.

The Plant List http://www.theplantlist.org/ (checking scientific names)

USDA plants website http://plants.usda.gov/java/  (checking plant distributions and common names)

sweet potatoes!

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Kathy Keeler

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