Healthy sweet-potato vines should produce a bushel of sweet potatoes from a row 25 to 30 feet long, with an average yield of 320 bushels per acre. The first tubers should be ready for harvest in late August, and the harvest usually continues until early November.
Most of the increase in tuber size occurs during the last three or four weeks before harvest. Potatoes that remain in the soil continue to grow and increase in size until the weather cools. Surprisingly, one problem new growers sometimes have is that they fail to harvest before the potatoes become too large for market preferences.
During the actual harvest, it is important to make every effort to minimize injuries to the tender skin on the roots. Undamaged potatoes will sell better and have a much longer storage life. Automatic harvesters are sometimes used, but they cause excessive skin injuries, so a majority of the sweet potato fields are ploughed and then the tubers are harvested by hand.
One harvesting key is to not allow freshly harvested sweet potatoes to be exposed to the sun for more than an hour. Growers often shade the harvested boxes of potatoes with cut vines while they remain out in the fields. At the late end of the season, growers are careful to harvest before frost kills the vines, because if the crop remains in the field after a frost, the roots may begin to decay.
Newly harvested sweet potatoes are not very sweet. They require one or two months of storage and curing before they will develop the sweet, moist taste customers expect. Freshly harvested sweet potatoes can, however, be candied or made into pies, and many growers sell part of their crop in this uncured green state.
Sweet potatoes are best cured by storing them in a humid, dark and warm (80 to 90 degrees F) room for a week or so before being moved to temperature-controlled (ideally, 60 degrees F) long-term storage. If the temperature in the storage area is too cold, the tubers will develop a hard center, but if the temperature is too hot, the tubers may shrivel and sprout.
After the roots are dug, they should be cured to heal the cuts and trigger development of the sugar-creating enzymes, the LSU AgCenter horticulturist explains. Cure by storing in a warm, humid room for five to 10 days. A temperature of 80 F to 85 F and a relative humidity of 80 percent to 90 percent are ideal. These exact conditions will be hard to establish around the home, so select a room or building that comes close to these conditions.
After curing, store roots at 55 F to 60 F for six to eight weeks. This storage further develops the sugars and maltose sugar-creating enzyme. This enzyme will really kick in while baking at 350 F to 375 F to develop the sweet, syrupy sugars that Louisiana yams are famous for.
Stored cured roots may last several months or more. The length of time sweet potatoes can be held in storage without sacrificing quality will depend on the environment they are stored in. The conditions above are “ideal,” but sweet potatoes are held under a variety of environmental conditions, and quality and longevity in storage will vary accordingly.
Exposure to low storage temperatures for several days will cause the sweet potatoes to develop a hard center and reduce their eating quality. When the roots are stored at high temperatures for a long time, they begin to sprout, shrivel and become dry, stringy and pithy. Sweet potato roots, held over for use as seed potatoes for the next spring, should be dusted with 2 to 4 ounces of 5 percent Imidan per bushel to help control the sweet potato weevil.