Sweet potatoes can be harvested whenever they reach a marketable size, but if you’re looking to maximize yields, they should be dug as late as possible in the fall, according to research done by Becky Sideman at UNH Extension. While vines can tolerate a light frost, the roots should come out before a hard freeze sets in, usually before mid-October. For more information on Becky’s research, including a list of varieties trialed in New Hampshire (albeit 10 years ago, now), see the full reports: Growing Sweet Potatoes in New Hampshire and Sweet potato early harvest study, 2014. Sweet potatoes require different storage conditions than other common New England root crops. Once harvest is completed, curing and storage considerations continue to be important.
Harvesting
Sweet potato roots will continue to grow until the leaves are killed by frost or until soil temperatures fall consistently below 65°F, whichever comes first. Time of harvest is often determined by digging up a few representative plants and assessing the percentage of roots in different size classes—the crop can be harvested whenever the majority of the roots are the desired size. If a hard frost occurs, the tops of the plants turn black. At that point, it is imperative to harvest as quickly as possible regardless of root size. Tuber chilling injury can occur if soil temperatures drop below 55°F. It is also important to avoid holding sweet potatoes in saturated, low-oxygen soil conditions prior to harvest, as this promotes rapid decay in storage. Take care when harvesting; unlike white potatoes, which form thickened, protective skins that bind tightly to the underlying tissue, sweet potatoes have thin skins that can be easily damaged by equipment or rough handling.
Curing
Any abrasions or wounds created at harvest can lead to rot in storage. Curing immediately after harvest is recommended when sweet potatoes will be held in storage for later sales. Curing minimizes damage and loss during storage by healing harvest wounds. During the curing process, a corky periderm layer is formed below damaged areas, which prevents invasion by pathogens and limits water loss. To cure sweet potatoes, keep roots at 82-86°F and high relative humidity (90-97% RH) for 4-7 days. Respiration rate is high during curing, so ventilation is important to remove CO2 and replenish O2. A greenhouse can provide good curing conditions.
A freshly harvested sweet potato is more starchy than it is sweet. During curing and storage, starches in the sweet potato are converted to sugars, improving flavor. The change in sugars is measurable within one week, but it is recommended to wait at least three weeks after harvest before consuming sweet potatoes to allow for more conversion of starches to sugars and maximum eating quality.
Storage
Sweet potatoes can maintain excellent quality for up to a year if proper storage conditions are achieved. The ideal storage conditions for sweet potato are the same as for winter squash; moderately warm (55-60°F) and 60-75% relative humidity. Like winter squash, sweet potato suffers chilling injury at temperatures below 55°F and injury increases with lower temperatures or longer periods of exposure. Signs of chilling injury include shriveled, sunken, dark areas on the tuber surface, and blackening of tubers when cut open. ‘Hardcore’ is a physiological disorder cause by chilling, in which areas of the tuber become hard—the condition is not apparent in fresh roots but appears after cooking. Because chilling injury is irreversible and makes tubers unmarketable, growers should take particular care to avoid field curing, or storage conditions that dip below 55°F.