Depending on the sweet potato variety, they need about 100 to 140 days to mature. It won't hurt to leave the mature potatoes in the ground for an extended time, provided you dig them up before the soil temperature drops below 55 degrees Fahrenheit. Sweet potatoes rot if they are left in the ground too long. When digging up the potatoes, handle them with care as they easily bruise.
Don't wash sweet potatoes after digging them up. If the dirt on the potato is moist, allow the potato to dry, then gently shake off any surface dirt. Plan to wash the potatoes right before you prepare them to eat. Set sweet potatoes in a roomy basket after digging them up so there is sufficient airflow.
Curing helps change the starch in the sweet potato to sugar, thus giving the potato a sweeter flavor. Cure the sweet potatoes immediately after harvest. Set the sweet potatoes in a dark area, with temperatures between 85 and 95 degrees Fahrenheit, and 85 to 90 percent humidity. Keep them in this area for about a week. If sweet potatoes are stored without first going through a curing process, they won't have the same sweet flavor. Before storing the potatoes, discard any that show signs of rot or decay.
Sweet potatoes keep for about three months when correctly stored. After curing the sweet potatoes, move them to a cooler location at about 60 degrees Fahrenheit, with high humidity. If the storage temperatures drop below 50 degrees, the sweet potatoes can begin to rot; or this reason, don't store sweet potatoes in the refrigerator. Store the potatoes in such a manner that they have plenty of air circulation.