Sweet Potatoes

• By Cleve Campbell
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• June 2016-Vol.2 No.6
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• 2 Comments

 History

The sweet potato (Ipomoea batatas) a member of the Convolvulaceae or morning glory family, is one of several native American plants discovered by Christopher Columbus and his shipmates on one of their American voyages, and Mr. Columbus and crew are given credit for transporting sweet potatoes back to Europe. However, the origin of sweet potatoes did not commence with Christopher Columbus’s discovery. The earliest cultivation records go way back to around 850 BC in Peru, and some archeological evidence suggests the cultivation of sweet potatoes may have begun around 2500-1850 BC. By the time Christopher Columbus arrived in the new world in the late 15^th century, sweet potatoes were well established in the Americas.

Benefits and Uses

This South American and Central American tropical crop is remarkably nutritious and versatile; often referred to as one of the super foods, it is high in calcium, potassium, vitamins A and C, iron, and thiamine. It is low in sodium and is a good source of fiber and other important vitamins and minerals. A complex carbohydrate food source, it provides beta-carotene which may be a factor in reducing the risk of certain cancers. Sweet potatoes contain no fat and are a healthier alternative to white potatoes, which have a high glycemic index , meaning that starch from white potatoes are quickly metabolized, leading to a rapid increase in blood sugar. Sweet potato starches are metabolized at a slower rate. They can be eaten raw, boiled, baked, used in soups, desserts, breads, or stir-fries- and don’t forget that southern classic, homemade sweet potato fries!

A little known fact about sweet potatoes is that its leaves are edible, so they can be added to fresh garden salads, steamed like spinach or kale, or used in stir-fries. And the list goes on and on. In the early 1900’s George Washington Carver developed over 100 products using sweet potatoes —  ranging from flour to writing ink. And in recent years one North Carolina entrepreneurial farmer developed a creative use for sweet potatoes by processing them into an adult beverage.

Temperature Requirements

Unlike potatoes, which like cool weather, sweet potatoes enjoy hot weather and can be grown in the home garden with little difficulty. One of the keys to growing sweet potatoes is patience.  Do not plant too early; they need hot weather. The soil temperature should be above 65º F before planting this crop, and the plants need sun — lots of it — about 10 hours, in fact.

Sweet Potatoes in Virginia

Our state has a long history of growing sweet potatoes, as they were first cultivated in Virginia in 1648, possibly even earlier.  Did you know that George Washington was a sweet potato farmer before he became a general and the first the first U.S. president?  So if you want to add a little Virginia tradition to your vegetable garden this year, there’s still time. According to the Virginia Cooperative Extension Publication No. 426-331 we can plant sweet potatoes in our area up to the last week in June.

Varieties

Examples of Sweet Potato Varieties
Photo: North Carolina Sweet Potato Commission

There are hundreds of varieties of sweet potatoes, in colors ranging from white, orange, and red to purple. Virginia Cooperative Extension Publication No.426-480 recommends the following varieties for our area: Centennial, Jewel and Barker.

Soil and Fertilizer Requirements

Historically, sweet potatoes have been a poor soil crop that produces a decent harvest, however; well-drained sandy or clay loam soil yields sweet potatoes with better root shapes. Heavy or clay soils often yield rough or irregular root shapes. Sweet potatoes tolerate acid soils and perform best with soil pH of 6.0 to 6.5. It is best to base fertilizer applications on the results of a soil test. If a soil test has not been taken, the recommendation is to apply 3 pounds of 5-10-10 fertilizer per 100 square feet or the equivalent amount of organic fertilizer. After the vines begin to run, in about 3-4 weeks, sidedress with 4 pounds of 5-10-10 fertilizer per 100 feet of row. Be careful with the amount of fertilizer applied as over-fertilized sweet potato plants produce excessive amounts of foliage, resulting in smaller-sized potatoes.

Planting

Sweet potato Slips

Sweet potatoes are propagated vegetatively rather than from seeds and are typically started from transplants, which are called “slips.”  Slips are baby plants that are sprouted from a mature sweet potato. While you can grow sweet potato slips yourself, it is always a good ideal to start out with certified disease-free plants or vine cuttings from a reputable garden supplier.  If you have soil with a high clay content it is recommended to plant the slips on a ridge or “pseudo raised bed.”  Till the soil and build a pseudo raised bed (high ridges), about 10 inches wide and

Raised bed for sweet potatoes

10-12 inches high with flat-topped ridges, spaced about 4 feet apart. Then plant the transplants with 8 to 12 inches between plants at a depth of 3 to 4 inches.

Sweet potatoes require at least 1 inch of water per week to grow well. Watering is especially important during the period right after transplanting — which is the establishment and root-development period. In general, weeds can be controlled by mulching between the row s with straw, clean (weed free) grass clippings or leaves. Once the vines begin to run, they usually will shade out weeds.

Harvesting

Sweet potatoes are usually ready to harvest in 90-120 days, depending on the variety. The tops usually begin to turn yellow as it gets close to harvest time. In any event, they should be harvested before frost.  Once the soil cools to around 50 degrees F, the quality and storage life of the sweet potato is reduced.

One word about digging sweet potatoes —  you cannot be too gentle! Carefully loosen the soil with a digging fork and gently pull the potatoes from the ground by their crowns. The skin on sweet potatoes is very thin, and extreme care should be taken during digging and handling to avoid skinning and bruising the roots. Even a very small wound can become infected with decay organisms. I have a gardening friend that lines the container with rags to avoid bruising or scratching the potatoes.  Also,  avoid direct exposure to sunlight for more than 30 minutes — that much sun may cause sunscald, which renders the sweet potatoes more susceptible to rot during storage.

Curing

Freshly dug sweet potatoes are considered to be “green” — not the color green, but uncured — and are not very sweet and moist.  In this uncured state, sweet potatoes have a shortened storage life. After the roots are dug, the curing process should begin within an hour or two. The curing process is required to bring out their sweetness, improve their nutritional value, and increase their storage life. During the curing process, starches are changed to sugar and a protective skin (periderm) develops, which thickens over wounds and creates a barrier to pathogens. To cure, place the sweet potatoes in a warm, humid room for 5 to 10 days at 80-85ºF and a high relative humidity — about 80 to 90%. These exact conditions may be hard to establish around the home, so select a location that comes close to these conditions. A general rule is that the lower the temperature, the longer the curing time; for example, a curing temperature of 65-75ºF may extend the curing period to 14-21 days.

Storage

After the sweet potatoes are cured, move them to a dark location where a temperature of about 60-65ºF can be maintained. Sweet potatoes are subject to chilling injury, so they should not be refrigerated. Properly cured and stored sweet potatoes can last for up to 10 to 12 months under ideal conditions.

Is it a sweet potato or yam?

Sweet Potato (Left) vs.  Yam (Right) Photo: North Carolina Sweet Potato Commission

 What do sweet potatoes and yams have in common? If you are a botanist, absolutely nothing! They are completely unrelated. Sweet potato (Ipomoea batatas) is part of  the Convolvulaceae family, and, like it cousin the morning glory, is a dicot (two embryonic seed leaves), and native to South and Central America. Whereas, the yam is in the Dioscoreaceae family, along with onions, lilies and grasses,  is a monocot (one embryonic seed leaf) and is native to West Africa. Sweet potatoes have a smooth thin skin and the yam has a rough scaly skin. The list of differences goes on and on.

“Candied Yams”

So how did this confusion come about? Well, it may have started several hundred years ago with the West African word nyami, referring to the starchy, edible root of the Dioscoreaceae family.  The word nyami became “yam” in America — and was used by some people to refer to sweet potatoes, which do have some similarities to the nyami found in West Africa.

To add to the confusion, in 1937 the state of Louisiana coined the term “yam” as part of a marketing campaign to differentiate Louisiana-grown sweet potatoes from other sweet potatoes grown on the East coast. In 1937, the Louisiana sweet potato became a yam!

Today the terms sweet potato and yam are generally used interchangeably; the USDA requires that sweet potatoes labeled as “yams” always be accompanied by words “sweet potato” to differentiate them from true yams.

So what’s the difference? It may just depend on who you ask. If you ask a botanist the general response would be along the lines of: “They are totally different; they have nothing in common; if is grown in this county it is a sweet potato, not a yam.”   And indeed, the nyami is not as moist and sweet as a sweet potato.  On the other hand, if you asked a marketing person, you might get a response something like, “ All sweet potatoes are yams, but not all yams are sweet potatoes.” With that being said, it certainly clears up the confusion!

The sweet potato is truly a versatile vegetable; it is one of the few vegetables that can be featured as a vegetable or as a dessert. It is fairly easy to grow, with few pest or disease problems in our area —  with one exception:  deer love the vine. If you have a problem with dear in your garden, you’ll need to protect this wonderful vegetable.

Thanks for joining us in The Garden Shed.  We hope you drop by again next month.

Sources:

“A Sweet Potato History,” Library of Congress website,  https://blogs.loc.gov/inside_adams/2010/11/a-sweet-potato-history/

The Worlds Healthiest Foods,” What’s New and Beneficial about Sweet Potatoes”, http://www.whfoods.com/genpage.php?tname=foodspice&dbid=64

N.C. State University, Perspectives on Line, “Research reveal Sweet Potatoes are weapon against diabetes,” https://www.cals.ncsu.edu/agcomm/magazine/winter07/diabetes.html

“Yes, you can eat sweet potato leaves,” The Minnesota Project (, June 26,2012) ,https://minnesotaproject.wordpress.com/2012/06/26/yes-you-can-eat-sweet-potato-leaves/

“Carver Sweet Potato Products,” Tuskegee University, http://www.tuskegee.edu/about_us/legacy_of_fame/george_w_carver/carver_sweet_potato_products.aspx

“Farmer turns leftover sweet potatoes into award-winning vodka,”   Red & White For Life, (N.C. State Alumni Association, Oct. 29, 2013)  https://www.alumniblog.ncsu.edu/2013/10/29/farmer-turns-leftover-sweet-potatoes-into-award-winning-vodka/

“Sweet Potato,” Clemson Univ. Extension, Publication No 1322, ,http://www.clemson.edu/extension/hgic/plants/vegetables/crops/hgic1322.html

“Vegetable Planting Guide and Recommended Planting Dates,” Virginia Cooperative Extension Publication No. 426-331, http://pubs.ext.vt.edu/426/426-331/426-331.html

“Vegetables Recommended for Virginia, ”Virginia Cooperative Extension Publication No. 426-480, , https://pubs.ext.vt.edu/426/426-480/426-480.html

“Sweet Potato Louisiana’s Most Popular Vegetable,” http://www.lsuagcenter.com/portals/communications/publications/agmag/archive/2012/spring/sweet-potato-louisianas-most-popular-vegetable

“Home Garden Sweet Potatoes,” University of Ga. Coop. Extension, Publication C1014, http://extension.uga.edu/publications/detail.cfm?number=C1014

Purdue University Consumer Horticulture, “Dig those Sweet Potatoes”, https://hort.purdue.edu/ext/sweetpotatoes.htmlhttps://hort.purdue.edu/ext/sweetpotatoes.html

“How to Grow Sweet Potatoes,” North Carolina Sweet Potato Commission, ,http://www.ncsweetpotatoes.com/sweet-potatoes-101/how-to-grow-sweet-potatoes/

“Curing and Storing,” North Carolina Sweet Potato Commission, ,http://www.ncsweetpotatoes.com/sweet-potato-industry/growing-sweet-potatoes-in-north-carolina/curing-and-storing/

“Curing and Storing Sweet Potatoes,” LSU AgCenter, http://www.lsuagcenter.com/topics/crops/sweet_potatoes/lsu-agcenter-horticulturist-discusses-curing-and-storing-sweet-potatoes

N.C. Cooperative Extension, “ What is the Difference Between a Sweet Potato and a Yam?” ncstate.edu

“Sweet Potatoes,”Clemson Extension, Publication HGIC 1322    http://www.clemson.edu/extension/hgic/plants/vegetables/crops/hgic1322.html