For those who planted sweet potatoes in their garden this year and have yet to harvest them, the time is drawing near. Sweet potato is a warm-season vegetable that cannot tolerate cold temperatures. Normally, this nutritious, under-used root crop is harvested after the first light frost of the fall season which, at our latitude, frequently occurs in late October.
Sweet potato (Ipomea batatas) is a member of the Convolvulaceae,or morning glory, family. It is thought to be native to tropical South America where it has been used as a food source by early civilizations such as the Incas and Mayas more than 5000 years ago. Columbus likely encountered sweet potato in his early voyages to the West Indies but it was not until his fourth voyage (to Yucatan and Honduras) that he recorded its discovery in his journals. He is credited with its introduction to the New World (Spain) in about 1500 and a number of different types were cultivated there by the mid-1600s.
Sweet potato was grown in what is now Virginia as early as 1648. From there it was taken both north and south. The southern migration of the plant was much more successful than the northern, again, because of the plant’s need for warm temperatures. Native Americans were known to have grown sweet potato extensively by the 1700s and soon thereafter it became a popular staple of the South. Even today, sweet potato is much more popular in the south than the north as a food.
Although sweet potato skin color varies fairly extensively among cultivars, flesh color is either white or dark orange. The white-fleshed types usually are drier in consistency and originally were favored in northern areas of our country. Orange-fleshed types, favored in the South, typically have moist flesh and often (erroneously) are referred to as “yams”. Yams (Dioscorea species) are perennial herbaceous vines cultivated in tropical regions of the world for their starchy tubers. Yams are still mainly a curiosity in the United States.
As mentioned previously, sweet potatoes usually are harvested after the first frost of the fall season. The latter is likely to do considerable damage the leaves. However, the roots remain unharmed, but should be dug as soon as possible following the frost. Sweet potato roots have thin skins and bruise easily. Therefore, care should be taken when harvesting them. Using a potato fork, gently lift the roots from the soil. Any soil that remains on the roots after they have been dug should be removed. Also, it is a good idea to let the freshly-dug roots dry atop the soil in the sun for several hours before proceeding further.
Sweet potato roots that are accidentally cut during the harvest process should be separated from those that are sound, since they will not store well. Roots exude a milky like sap when damaged. However, this does not “seal” the wound and protect the root from invasion by organisms that cause decay.
Following harvest, sweet potatoes should be cured in a warm, humid environment for about 10 days. This practice produces higher sugar content and better color in the flesh while allowing minor wounds to heal. A temperature of 80 to 85 degrees F is ideal; a loose covering of plastic will help to maintain a high relative humidity around the roots. Failure to maintain humid conditions around the roots or curing at overly warm temperatures will lead to desiccation and the loss of quality.
Following curing, sound sweet potatoes have a storage life from between six to ten months when stored at temperatures of between 55 and 65 degrees F in a humid environment. The latter can be facilitated by placing roots in a plastic bag with holes cut in. This will allow for good air circulation and gas exchange.
Do not store sweet potatoes in locations where temperatures drop below 50 degrees F. Refrigerators and outdoor pits are suitable for storing a number of different vegetables. However, sweet potato is not one of them. Overly cool storage temperatures reduce storage life and promote the onset of decay.
One of the most common problems encountered in sweet potato storage is a disease known as scurf. The latter is caused by Monilochaetes infuscans, a soil-borne fungus. The causal pathogen normally attacks the skin of the sweet potato root producing symptoms of dark (nearly black) irregularly shaped lesions. These lesions become more severe under humid storage conditions. While scurf does not make the root inedible, it does shorten its storage life and render the sweet potato unsaleable.
As is the case with most plant diseases, prevention is the best cure. Once introduced to the soil, the scurf pathogen has the ability to remain for many years. There is no chemical control or known resistant sweet potato varieties. Therefore, avoid planting sweet potato slips from roots afflicted with scurf. If scurf does appear, then rotate the planting site to an area as far from the contaminated area as possible. Avoid using tools or equipment in both areas of the garden without thorough disinfection.
Sweet potato is a nutritious vegetable that probably deserves more attention than it gets in the diet of the average American. In addition to being rich in starches (for energy), it contains complex carbohydrates, dietary fiber, protein, beta carotene (pro vitamin A), vitamin C and vitamin B6. Sweet potato is relatively low in calories (a 100 gram serving is reported to contain about 115 calories) and also is an important source of minerals such as calcium, magnesium and potassium.
For those who did not grow sweet potatoes this year, October is a good time to secure an adequate supply of freshly-dug roots for storage. Local farmers’ markets or produce auctions are excellent places to shop.
REVISED: September 29, 2015