Sweet Potatoes Buried Treasure

by John C. Bouwkamp.

John C. Bouwkamp is an Associate Professor of Horticulture at the University of Maryland, College Park.

Since most of us will never savor the excitement of digging for buried treasure, digging sweet potatoes in your garden may be the next best thing. The day-by-day progress of your crop of tomatoes and beans is readily apparent but the reward for your efforts with sweet potatoes must await the day of harvest. Variety names such as Jewel, Goldrush, Nugget, Nemagold, Gem, and Maryland Golden allude to the feeling of buried treasure by their developers.

The sweet potato, like many of our vegetable crops, originated in the Tropics. The exact area of origin is subject to debate but was probably in tropical America or somewhere in the tropical South Pacific islands. Two of the ancient civilizations of tropical America, the Mayan and the Peruvian, grew and cultivated sweet potatoes, although maize was their staple crop.

Primitive cultures of the South Pacific islands give a central role to the sweet potato in their celebrations, suggesting that sweet potatoes were cultivated for food in two widely separated parts of the world from ancient times. Whether the crop originated in the New World and was transported to the Polynesian islands or vice versa remains open to speculation.

Although Columbus noted the use of sweet potatoes by West Indian natives on his fourth voyage, there is no record of pre-Columbian cultivation of sweet potatoes by Indians in the continental United States. Sweet potatoes were grown in Virginia as early as 1648, most likely from roots obtained in the West Indies.

A frequent source of confusion is use of the terms sweet potato and yam. The true yam (Dioscorea sp.) is of African or Chinese origin and belongs to a different plant family than the sweet potato. It is only rarely grown in the United States.

The Blacks, when first brought to this country, mistook the sweet potato for a type of yam since the sweet potato was unknown to them and the two crops grew and were used in a similar manner. Their word nyami was shortened to yam and the sweet potato became known as a yam in many parts of the United States, particularly in the South.

The terminology was further confused since the dry-fleshed varieties grown in the Middle States and North (known as sweet potatoes) were noticeably different from the moist-fleshed varieties grown in the Deep South (known as yams). Thus many people came to believe sweet potatoes and yams were different vegetables.

At present, the same varieties are grown throughout the United States and a sweet potato by any other name is still the tasty, nutritious vegetable many people associate with holidays and special meals.

Almost everyone knows that sweet potatoes are a delicious addition to any meal, but not many know they also are very nutritious. An average sized boiled sweet potato (2-inch diameter, 5 inches long) will provide over half the recommended daily allowance (RDA) of Vitamin C and more than twice the RDA of Vitamin A for an adult male, 23 to 55 years old and weighing 154 pounds.

In addition, this root will provide nearly 5% of the protein, 6% of the calcium, 9% of the phosphorous, 11% of the iron, 10% of the thiamin, 5% of the niacin and nearly 6% of the riboflavin required by an adult male.

All these vitamins and minerals are provided with about 170 calories (6.4- of the RDA). Empty calories? Not sweet potatoes!

Growing Requirements

What will you need to know about growing sweet potatoes?

First, sweet potatoes are a tropical crop and need 4 to 5 frost-free months for growing. They thrive in hot weather. Probably little or no growth occurs when soil or air temperatures are below 60 F. Growth appears to be optimum when soil temperatures are near 70 and air temperatures near 85 .

Second, since the root is the part you will harvest, the soil should be loose and friable (crumbly) allowing for unimpeded root enlargement and easy digging.

Meet these two requirements, and you should be a success as a grower.

Choose a well-drained site in the full sun. The pH should be in the range of 5.5 to 6.5. If your garden is moderately fertile, apply 3 1/2 pounds Of 5-10-20 (or a similar analysis) of fertilizer per 100 feet of row before ridge construction or transplanting. If your garden is very sandy and/or infertile, 5 pounds per 100 feet would be more appropriate. If it's very rich and fertile, 2 to 2 1/2 pounds should suffice.

In many parts of the country, sweet potatoes are grown on ridges 8 to 15 inches high. These are especially important on heavy or poorly drained soils. If the soil remains water logged for several days, the roots may rot. Ridges are a good idea if this is likely to occur.

If your garden is a fine sandy loam and well drained, probably no ridge is necessary.

Should you wish to grow on ridges, construct them before transplanting. Rows should be 3 to 4 feet apart.

Plan to obtain disease-free plants, also called sprouts, from a reputable dealer. Your choice of varieties differs widely in various parts of the United States, and not all varieties will be available at any location. Your local Extension Office would be your best choice for help in choosing a variety.

You may also wish to produce your own sprouts after you have some experience with growing sweet potatoes. Set plants 2 to 3 inches deep, and 12 to 16 inches apart in the rows, one plant per hill. Don't transplant until soil temperature reaches 60 F. A good rule of thumb is to plant sweet potatoes 1 to 2 weeks after frost danger is past.

After transplanting you may wish to water the plants with 1/2 cup or more of water or 1/2 cup of a starter solution. Starter solution can be made by mixing 1/2 ounce of a soluble, high phosphorus fertilizer (15-30-15 or similar analysis) per gallon of water. Be careful not to overdo the starter solution or you may damage the plants.

Hoe and cultivate sweet potatoes frequently (once per week) after the vines have begun to run. This serves two purposes to control the weeds and to keep the vines from rooting at the nodes. If these nodes are allowed to root, some storage roots may develop and the main storage roots will not develop as quickly.

Three to four weeks after transplanting, make a second application of fertilizer at the same rate as before transplanting. Place the fertilizer near the rows and work it into the soil.

After 6 to 8 weeks the vines will become too large to cultivate and you must weed by hand.

Sweet potatoes at harvest.