All About Sweet Potatoes

By Jim Wilson

Take an old cold tater and wait."

It was country singer Little Jimmy Dickens who sang this paean to sweetpotatoes, and I can tell you from experience that there is nothing quite like a leftover baked sweetpotato to divert a hungry child. Goodness! they're good -- baked, fried, or made into pies topped with marshmallows, eaten warm from the oven or as leftovers. And they're good for you, full of complex carbohydrates, vitamins and trendy goodies like beta carotene and fiber.

At one time sweetpotatoes were considered strictly a southern vegetable, but early-maturing varieties, black plastic mulches and floating row covers are making good crops possible almost anywhere.

Sweetpotato Basics

I may lose some friends by divulging these facts about sweetpotatoes. First, they are not yams, and second, the vines form roots, not tubers. I've told gardeners these facts and have had them look at me as if I've said there's no Santa Claus. But it's the gospel truth.

The confusion began with a gem of an advertising program more than a half-century ago. Commercial growers in Louisiana wanted to give their sweetpotatoes a unique product advantage so they would outsell those grown elsewhere. So, they decided to market them as "Louisiana Yams," and the name stuck -- like beggar lice to corduroy trousers.

The problem is that the two plants are nowhere near alike. True yams, members of the genus Dioscorea, are native to the tropics around the world and can't be grown without protection north of USDA Hardiness Zone 9. The roots of some yams appear to have bark for skin; their flesh is white or purplish, bland rather than sweet, and some species grow to enormous size. Sweetpotatoes, Ipomoea batatas, are native to the American tropics.

Perhaps you've noticed that I spell sweetpotato as one word whereas most gardeners make it two. Vegetable specialists prefer "sweetpotato," to distinguish the crop from any old potato that happens to taste sweet.

How to Grow Sweetpotatoes

I hope that you will try growing sweetpotatoes. One of life's supreme delights is unearthing at least three or four good-sized sweetpotatoes per "hill" at digging time. That's equivalent to 15 pounds of keeper roots per 10 feet of row. If you haven't grown sweetpotatoes before, the vines lie flat on the ground and spread four feet across on dwarf cultivars and up to 10 feet across on standard cultivars.

On fertile soils, you needn't add fertilizer, but on rather poor or fast-draining sandy soils, you should work in enough to feed the crop for the entire season. My favorite sweetpotato fertilizer is cottonseed meal, worked into the soil at the rate of about five pounds per 100 square feet. Add compost to sweetpotato beds sparingly; too much and the roots will develop a skin condition called "scurf." The preferred soil pH is 5.8 to 6.2; add lime if your soil is more acid than these parameters, but don't try to bring alkaline soil down to a pH of 6.2 by adding sulfur or alum. Instead, work a two-inch layer of peat moss into the soil. Its naturally low pH will help buffer the excess calcium and magnesium ions.

If nematodes cause problems where you live, sow a dense crop of French marigolds, then turn them under once they're full size. That will repel the microscopic worms for two years. You could also leave the soil bare and weed-free for a season, but that wouldn't look as nice.

I recommend that you buy rooted "slips" of named cultivars from mail-order catalogs or, in warm climates, from garden centers. But, if you wish to grow a few slips, here's how. In early spring, stand a root in a glass jar half-full of water and place it in a sunny, warm spot. Shoots will sprout and form fibrous roots. After danger of frost is past, pull the slips apart, preserving as many roots as possible.

Or, 10 weeks prior to the frost-free date for your area, lay the root on its side in a large pot filled halfway with sand, cover t root with two inches of sand and moisten it, and set the pot where temperatures average 75? to 85? F. When the root begins to sprout, move the pot to a sunny windowsill or beneath fluorescent lights. Feed the sprouts lightly; there are no plant nutrients in sand.

A single root should give you at least a dozen rooted slips. Place slips about 14 inches apart in rows 2 1/2 to four feet apart. Set slips deep enough to cover three-fourths of the stem and water them promptly after transplanting to the garden.

Don't let your vines suffer for moisture during the growing season; they need as much as sweet corn. But reduce water come August -- heavy watering late in the season may cause developing roots to crack.

Sweetpotatoes grow so easily and rapidly in zones 7 and south that a second crop can be started from tip cuttings taken from the first crop. We ridge up heavy soils for planting sweetpotatoes, but on the preferred sandy or sandy loam soil, we prepare broad beds raised four to six inches. Floating row covers aren't needed, except for protection against insects early in the season. The challenge with sweetpotato culture where summers are long and warm is to keep roots conveniently small. On over-fertilized southern and western soils, sweetpotatoes can grow as large as a loaf of French bread.

Short-season gardeners can't produce sweetpotatoes as abundantly as gardeners in the South and warm West. The sweetpotato is, after all, a tropical vegetable, accustomed to long, warm summers. Nevertheless, many gardeners have proved that with a frost-free growing season of three months, they can produce keeper" roots of sweetpotatoes.

One of my former associates on "The Victory Garden" program of public television, Kip Anderson, manages its suburban location in Lexington, Massachusetts. It's on the cusp of hardiness zones 5b and 6a. Kip has grown several crops of Vardaman sweetpotatoes for their richly colored orange flesh and compact growth habit, which keeps vines from trailing into the aisles.

Kip uses a variant of "ridging up" for improved drainage and additional warmth. His loamy soil has been modified with organic matter and has so much residual fertility from previous crops that he doesn't add nutrients to grow sweetpotatoes. He shovels soil into high ridges, levels the tops and smooths the flanks with a rake, and spreads black plastic as a mulch. Then he seals in the edges of the plastic with soil, and cuts small Xs through the plastic for setting out slips. Kip staggers two rows of plants down each ridge so that they are 12 inches apart. The close spacing increases yields. Another former "Victory Garden" personality, host Roger Swain, grows the Georgia Jet cultivar at his New Hampshire farm.

The black plastic keeps the soil several degrees warmer than non-mulched areas, suppresses weeds and sheds the occasional torrential rain. Cold rains can lower soil temperatures, slowing the conversion of nutrients to forms that can be absorbed by plant roots, and leach away nutrients.

Toward the end of the growing season, Kip spreads floating row covers over the vines. In the North, sweetpotato roots don't begin to fill out and gain weight rapidly until early September. Row covers ward off early frosts and add a critical two or three weeks to the growing season. He delays harvesting until after a killing frost and shears off the blackened vines before the roots pick up an off flavor from the decaying vegetation.

Kip says that about a third of his harvest is good-sized roots, a half- to one pound in weight. Another third is rather small roots -- plump, but not large enough for a single root to make a serving. The remaining roots are too slender to keep. Kip digs his sweetpotatoes carefully, handles them gently and cures them in a warm area for 10 days.

The only additional trick I would use to further hasten growth would be to make the Xs in the plastic a bit larger and cover the newly transplanted slips with bottomless milk jugs sunk into the soil. Jugs serve as mini-greenhouses to concentrate heat and ward off cold winds. Remove the jugs after two or three weeks.

Sweetpotatoes also grow well in the upper Midwest. Gurney Seed and Nursery Company, in Yankton, South Dakota, reports, "We know of customers in North Dakota and Minnesota who have grown them quite successfully, often with a mulch of black plastic to warm the soil. Most wait to harvest until after a killing frost, which gives the roots time to grow to baking size."

Don't rush harvest where the season is long; a few extra days can produce a more intense color, indicating a higher carotene content. In southern climes, dig sweetpotatoes before night temperatures drop to 50? F in the fall, 100 to 130 days after transplanting.

To harvest, remove any plastic mulch and recycle it. Locate the "hills" or crowns of the plants and tentatively push a spading fork into the soil about a foot away. If you encounter resistance, stop and move out another 6 inches. Pry up until you see the roots, and use a combination of shaking the crown and prying to pull the root mass out of the soil. Don't let the roots lie in the sun; take them straight away into a warm room for curing.

At a temperature of 85° to 95° F, for about 10 days, the roots will lose enough moisture to minimize spoilage. Move them to a cooler, more humid area for long-term storage; 60° to 65° F is ideal, 55° F is too cool. Lay the roots on ventilated racks, not touching each other, or wrap them individually in newspapers and lay them on a shelf. Then, when winter weather turns your outlook gray, pull out two or three roots and make a sweetpotato pie. Live it up; add a topping of marshmallows. But close your windows while baking, or neighbors will flock to your kitchen like possums to ripe persimmons!

Which Sweetpotato Should You Plant?

I know that memory plays tricks, but I remember that sweetpotatoes tasted better when I was a boy in Mississippi. I checked with Dr. T. P. Hernandez, a Louisiana sweetpotato breeder who retired a dozen years ago. He, too, recalls the extremely sweet, soft-fleshed, moist, reddish purple sweetpotatoes that were sold as "Louisiana Yams." They were the cultivar called Unit One Porto Rican, but commercial growers in Louisiana had to stop growing them because of their susceptibility to a number of plant diseases, Fusarium wilt in particular. I am willing to concede that perhaps more of the starches in Porto Rican converted to sugars in baking, but I can't split a baked sweetpotato today without a pang of nostalgia for the good old days, yam scam notwithstanding. Dry, yellow sweetpotatoes make me long for the true Porto Rican roots.

Most of the newer varieties are vining types. 'Beauregard'(early, free of strings even if stressed and resistant to soil rot) and 'Jewel' (good flavor, resistant to stem rot and internal cork diseases, preferred for sandy soils where seasons are long) account for 90% of commercial plantings.

It's a different story for home gardeners where bush types make better use of limited space. Two popular bush types are 'Vardaman' and 'Bush Porto Rico'. Some seed catalogs list these as maturing in 100 and 110 days respectively. They can do it in a warm summer, but in zone 5 they need floating row covers to concentrate heat and to hustle them along early and late in the season (temperatures can rise too high during midsummer for row covers to remain over plants).

Dr. Melvin T. Hall, a sweetpotato breeder at the University of Georgia's Coastal Plain Experiment Station in Tifton, confirms that 'Georgia Jet' (aka 'Jet'), a soft-fleshed vining type, is ready for harvest in 90 days or less from Georgia to New York. 'Centennial', a blocky orange-fleshed variety with copper skin, also matures in 90 days. Sumor is a white-fleshed 90-day variety. Many of the colorful heirloom varieties, such as 'Bermuda Pink', 'Kore Purple', 'Frazier White' and others are only available through the Seed Savers Exchange, a network of seed-saving gardeners based in Decorah, Iowa.

Now if by some misfortune you live in California or Arizona, you have a problem. Out-of-state mail-order nurseries cannot ship sweetpotato slips to you. It's the best excuse to start your own slips from market sweetpotatoes sprouted at home. The only nursery I know in the West that grows and ships sweetpotato slips is Exotica Rare Fruit Nursery, P.O. Box 160, Vista, CA 92085 (catalog, $2).

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