Sweet potatoes from left to right (2 of each kind): Hayman White, Puerto Rican, Nancy Hall, Jersey Red and Jersey Yellow. (Deb Lindsey/FOR THE WASHINGTON POST)
Gardening columnist

Keen gardeners and enterprising local farmers have another month of harvesting oddball veggies such as parsnips, leeks and Brussels sprouts, but for Kenny Harris, the last great popular crop of the year is the sweet potato.

This tender root crop swells just as the first frosts arrive. It is the perfect dish for the Thanksgiving table, valued as much today as in early America for its storage qualities, culinary versatility and ability to warm our hearts when the chill sets in.

But for Harris, who farms in three counties across two climate zones in southern New Jersey, the quest for the perfect, old-time sweet potato is fraught with risk and anxiety.

He took me the other day to his main sweet potato field, 10 acres under intensive cultivation on the outskirts of Cape May. His crop had survived the flooding of Hurricane Sandy, but it was later blackened by a nip of frost. One more heavy frost and five months of work would be for naught as the leafy blackness would descend into the tubers below. His foreman and crews were busy lifting and grading the clumps of elongated tubers. This is an anxious time for Harris, even factoring in that he’s a hands-on guy who sweats the details and is constantly on the cellphone directing operations at his four farm markets. It is not until his fieldworkers unearth the roots that any of them can know whether a season that began in April has paid off, whether the potatoes are productive, unblemished, and spared from diseases and pests.

The pickers might get two or three choice potatoes from each clump, leaving behind as many as half a dozen or more that are undersized, diseased or split.

Harris, 52, is willing to endure the dicey dance with frost and the low yields because these are not your usual sweet potatoes. The type commonly found in supermarkets is plump, orange, cloyingly sweet, and bred for ease of growing and shipping. It is productive, disease-free and early to market, a dream for any grower. But for sweet potato purists, it is simply not as interesting or steeped in regional lore.

The sweet potato is a classic example of why home vegetable gardening is so important. It is an enterprise that connects you to delicious and heirloom varieties missing in big agriculture. Not to mention freshness.

But Harris’s devotion to them has a heroic quality. He grows a lot of different crops, from peaches to, yes, winter Brussels sprouts. But for him, the sweet potato is about honoring the heritage of his native region. He grows the standard supermarket variety to get a jump on the season, but he has brought back heirloom varieties that decades ago defined the richness and diversity of the sweet potato in the Mid-Atlantic region.

“Kenny is from the old school,” said his friend Barry Perlow, a landscape designer with Meadows Farms in Northern Virginia. “He was raised by old time south Jersey farmers. He’s a perfectionist.” Perlow used to manage Harris’s farm market and garden center in Bridgeton, N.J.

Sweet potatoes fall into two basic types, the sweeter varieties popular in the South, known as yams, and the drier, mealier types called Jersey sweets. Botanically, they are all varieties of a New World species of the morning glory family. The true yam, by contrast, is from an unrelated tropical plant that produces huge starchy tubers.

His “yam” varieties are Hayman white, with cream-colored skin and flesh; Puerto Rican (also called Porto Rico), with orange skin and flesh; and Nancy Hall, with creamy yellow skin and flesh. His Jersey heirlooms are Jersey red and Jersey yellow.

With some soil preparation, all of them could be grown in the Washington area, which has about the same growing season and hardiness zone as south Jersey.

Unlike true potatoes, which are started from whole or sliced tubers, the sweet potato is propagated from the shoots that erupt from the seed tuber once it is coaxed into growth in the spring. The gardener must plant the rootless “slip” right away in the prepared garden bed: Make a hole, stick the shoot in about halfway and space them 18 to 24 inches apart. You must keep the soil wet for a week or two until the slips grow roots. Instead of tugging them from the tuber, Harris cuts the shoots at soil level, which reduces the risk of a slip carrying a disease in the tuber.

A bed dedicated to sweet potatoes should be sunny, free-draining and sandy, which is why you need to amend the soil. Heavy clay soil just won’t do. Don’t add lime or high nitrogen fertilizers, but feed them with potash. The vines should be watered regularly; uneven moisture causes the potatoes to crack. The vines grow to three feet or more and need some space, along with good soil preparation. Joe Brunetti, a Smithsonian horticulturist who grows them at the Victory Garden at the American History Museum spaces them two feet apart. “I use a very loose loam. They do okay, a little knobbly,” he said. “If I were growing them to perfection I would absolutely add a lot more sand.” This, in turn, means watering them diligently to get them established and during dry spells.

Rodents can be a problem, particularly voles, and if you have rats in your urban plot, this might not be the crop for you. Harris’s crop is not bothered by critters, probably because they can’t tunnel into soil that is so sandy. Home gardeners do have an advantage over Harris: They can use tubers that are small and with splits that wouldn’t sell in the grocery store but are just as tasty.

Harris’s rejects are sold cheaply to people who like to feed deer, animal lovers and hunters alike. The variety Nancy Hall, in particular, has grudging tubers. They are thin and fingerlike, and sometimes a vine will produce no tubers at all. But to Harris, this variety is the ultimate sweet potato in flavor and texture. “Half the reason we are growing Nancy Hall is so we have them to eat,” he said, holding a clump and adding that he “couldn’t go through life” without eating the variety.

He rescued three of the varieties from a grower who was retiring about a decade ago. He bought the farmer’s equipment, seed potatoes and two seasons of growing advice, but not before the old-timer came to Harris’s farm in Salem County to see whether he had the right conditions. Harris later acquired the warmer zone farmland in Cape May, about a 45-minute ride to the east.

Sweet potatoes will keep well into the spring if they are handled and stored properly. Cuts and bruises from digging or rough treatment will cause them to rot. Dig them carefully and never wash them until you are ready to cook with them. And they must be cured by lying out in a warm room for a week or more. Harris keeps them at 80 degrees for two weeks before storing them in a cool room. Temperatures must stay above 55 degrees to prevent spoiling — they are not for the traditional cool root cellar.

Curing not only preserves them, it improves the flavor in some varieties. “I won’t sell Puerto Rican or Nancy Hall unless they have cured,” Harris said. “Their flavor doesn't really peak until Christmastime.”

Growing in 2013

Sources of slips for late spring planting:

Marlboro Farm Market , Bridgeton, N.J. Five heirloom varieties of yams and Jersey sweets. No shipping and limited availability. Call in the spring for details: 856-451-3138.

Sand Hill Preservation Center , Calamus, Iowa. Large selection of sweet potato varieties. The 2013 sweet potato catalogue is available next month. 563-246-2299

Southern Exposure Seed Exchange , Mineral, Va. Good selection of varieties. 540-894-9480.

You can also start slips from organically grown tubers from the grocery store. In early to mid-April, set the tuber, vine end up, in a glass of water and grow in a bright location indoors. Harvest the slips for planting outside around Memorial Day.

Mouthwatering sweet potato recipes

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Gardening archiveRead past columns by Higgins, including those on tulips and boxwood blight.