All garden potatoes are harvested late in the growing season, but exactly when to harvest depends on how you will use them. Potatoes that you will harvest and eat immediately, called new potatoes, are harvested sooner than potatoes that you plan to cure for storage over the winter.
Harvesting New Potatoes
New potatoes are small, tender potatoes that are harvested and eaten right away. They do not store well.
Harvest new potatoes when the plants are finished flowering by digging around the edges of the plant with a garden fork and levering up the bundle of potatoes to expose them. (You're less likely to cut the tubers if you use a garden fork than a shovel.) Typically, the potatoes are about 4 to 6 inches deep in the soil. If you are careful, smaller potatoes can be left in place and gently replanted to allow them to continue growing.
While they are normally eaten right away, new potatoes can be stored for several months, but they won't keep as long as fully ripened and cured potatoes. Store new potatoes in a dark location at a temperature 38 to 40 F.
Harvesting Ripened Potatoes for Storage
To harvest large potatoes for storing, let the plant continue growing after it is done blooming. Keep hilling up the soil or add mulch around the plants so that the tubers aren't exposed to sunlight. Once the foliage has died back at the top, dig up your tubers with a garden fork.
Don't worry if the plants have been killed by hard frost, as the first aboveground frost won't affect the tubers. However, do not allow the tubers themselves to freeze by keeping them in the cold ground too long. When the foliage is dead, harvest quickly.
Check the potatoes for ripeness by rubbing the skins with your thumb.
If they are fully ripe and suitable for long-term storage, the skins will rub off under thumb pressure. If they are not fully ripe, the potatoes should be regarded as "new" and eaten soon.
Don't wash the storing potatoes; just let them sit out in a single layer for a couple of weeks to fully cure. Then, brush off any dry soil, and store in a dark, cool place at 38 to 40 F. Discard any potatoes that have damaged skins (or eat them right away). Damaged potatoes won't keep as long in storage. Potatoes that have been fully cured and ripened in the ground may keep for several months. Avoid exposing them to light during storage, as this will turn the potatoes green.
Keep Some Potatoes for Replanting
If desired, keep some potatoes as "seeds" for replanting potatoes in the spring. Three to four weeks before planting time, bring your seed potatoes out into a warm, sunny area and cover them with moist burlap or moistened paper towels. Soon, the eyes will begin to grow green shoots. When planting time comes, cut large potatoes into 2-ounce segments so that each segment contains a sprout. These form the seeds for planting into your garden hills. Each potato segment will produce an entire hill of potatoes in a few months.