|Newly harvested home garden sweet potatoes PHOTO/Amy W.|
However, soil temperature isn't the only factor that can affect the "quality and storage life" of sweet potatoes; correct post-harvest curing and storage can vastly improve the sugar content and keeping qualities of home garden sweet potatoes.
Clemson University's Extension Sweet potato publication explains, "Sweet potatoes should be cured to heal wounds and to convert some of the starch in the roots to sugar. The optimal conditions for curing are to expose the roots to 85 °F and 90-percent humidity for one week. Few home gardeners can supply these conditions, so place the sweet potatoes in the warmest room in the house, usually the kitchen, for 14 days. No curing will occur at temperatures below 70 °F."
The online article "Growing sweet potatoes in the Sacramento area," by the Master Gardeners of Sacramento County, California, also notes that newly dug sweet potatoes are more starchy than sweet, and that the curing process that improves keeping quality also aids in the conversion of some of that starch into sugar.
For curing, these gardeners suggest using a slightly longer exposure of 10-14 days at 85 degrees F and 90% humidity. Most of us do not have access to the once-ubiquitous sweet potato curing barns (see this 2009 Master's Thesis -- Sweet Potato Curing Barns: An Agricultural Landmark) that were designed to provide these ideal conditions, but the Sacramento article includes ideas for how to create those conditions at home.
For storage after curing, the Sacramento Master Gardeners offer this additional information: "Once the roots are cured, they can then be stored in a dry, dark, well-ventilated place at 55° to 60°F for several months. Sweet potato roots are very sensitive to chilling injury at temperatures below 50°F, so do not store them at lower temperatures or quality will deteriorate. Symptoms of chilling injury include fungal decay, internal pulp browning, and root shriveling."