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 eat immediately, called new potatoes, are harvested sooner than potatoes that you plan to cure for storage over the winter.
Potatoes, like all root crops, are harvested most efficiently by digging them up. They can be bruised with rough handling so a little extra care is needed if you plan to store your spuds.
Here's how and when to harvest both kinds of potatoes.
When to Harvest Potatoes
Once you've planted your seed potatoes in cool but mostly frost-free weather (they can tolerate a very light frost), they'll need as many cool days as possible before harvesting. The flowers and foliage determine when to best harvest your crop. Harvest baby potatoes (new potatoes) two to three weeks after they've finished flowering, and harvest potatoes for storing (mature potatoes) two to three weeks after the plant's foliage has died back.
Harvesting New Potatoes
New potatoes are small, tender potatoes that are harvested and eaten right away. They do not store well. When the plants finish flowering, dig around the edges of the plant with a garden fork and lever up the bundle of potatoes to expose them. (You're less likely to cut the tubers if you use a garden fork instead of a shovel.) Typically, the potatoes are about 4 inches 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 of 38 to 40 degrees.
Harvesting Ripened Potatoes for Storage
To harvest large, mature potatoes for storage, let the plant continue to grow after blooming. Keep hilling up the soil or add mulch around the plants so the tubers aren't exposed to sunlight. Once the foliage has died back at the top dig up your tubers with a garden fork. The proper month for harvest will depend on the potato type and your particular growing region, but it is usually August or September. Don't worry if the plants have been killed by hard frost, as the first aboveground frost won't affect the tubers. However, keeping them in the cold ground too long can cause the potatoes to freeze so harvest soon after the foliage dies back.
Potatoes grown in containers may be ready for harvest somewhat earlier than in-ground potatoes, due to the fact that the soil tends to be somewhat warmer. But the signal for beginning the harvest is the same—wait until the foliage dies back.
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 won't rub off under thumb pressure. If you harvest them too early, they will not be fully ripe and should be regarded as "new" and eaten soon.
Mature potatoes can be washed and eaten immediately, but if you plan to store them, don't wash the potatoes; just let them sit out in a shaded location in 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 degrees. Stored potatoes should also be kept dry, so the refrigerator is not a good option. 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 curing and storage as this will turn the potatoes green.
Potato tubers and sprouts that are green in color contain solanine, which is highly toxic and can cause intestinal distress and even central nervous system problems if eaten in large quantities. Never eat the potato skins that are green in color or green eyes and shoots.
Keep Some Potatoes for Replanting
If desired, keep some 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. Let the potato pieces sit out for a few days, cut side up. This allows a protective skin to form on the exposed flesh and helps to prevent disease. Once the cut side has turned dark, plant the pieces with the eye or sprout pointing up. Each potato segment will produce an entire hill of potatoes in a few months.
Potato plant poisoning—green tubers and sprouts. Mount Sinai Health Library.