For getting good crop sweet potatoes should be harvested before frost kills the vines or soon thereafter. To give highest yields it need in 130-170 days from planting are needed. But “baby bakers” or smaller roots can be harvested up to a month earlier.
Roots continue to grow until frost affects the vines. However, an extremely hard frost can cause damage to roots near the surface.

Try to prevent chilling injury of roots when soil temperatures drop to 50°F or lower, as this can result in internal decay in storage. Direct sunlight for over an hour can sunburn the roots, so dig only those that can be picked up immediately.

The greatest danger from delayed digging of sweet potatoes harvest is in the effect wet soil has on the roots. Excessive moisture can prevent digging injuries from healing properly allowing decay of the roots.
Disease control continues through harvesting and storing of sweet potatoes.
Even a small wound can easily become infected with decay organisms. Take care during digging and handling to avoid skinning and bruising the roots.

If possible, line containers with rags or other soft material to avoid scratching the roots. Put healthy roots directly into clean storage containers at harvest. Although large amounts of soil clinging to roots during storage are not desirable, freshly-dug sweet potatoes are easily damaged during the washing process. Allow sweet potato roots to dry and cure before removing excess soil.

A temperature of about 4ºC (39ºF) is best. A corner potato storage room in a basement can be quite satisfactory.
For best store of potatoes a special storage area should be built if you regular grow potatoes.
Coolest part of the house is best where storage room should be built in the. A corner of the basement will work best.
The walls, ceiling and the door between a warm basement and the storage area should be well insulated.

Do not insulate exterior concrete walls.
If it is practical, include an opening to the outside. This duct provides for air circulation, allowing cool fresh air to replace the warm air of the storage room.
Use pallets to keep potatoes off damp floors, provide circulation and avoid potential storage rots.
Do not store potatoes with apples or pears as the ethylene fruit produces will cause the tubers to sprout.

Properly dried and stored potatoes should keep well for three to six months and keep all usefull vitamins and mineral.