All garden potatoes are harvested late in the growing season, but exactly when to harvest them depends on whether they are being harvested to eat immediately (these are known as new potatoes) or if they are being cured for storage over the winter.
Potatoes can be a mystery when it comes to trying to figure out when it's time to harvest. All of the important development seems to happen underground.
So how can you tell when it's time to harvest?
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 the bundle of potatoes up to expose them. (You're less likely to cut the tubers if you use a garden fork than you are if you use a shovel.) Usually, the potatoes will be at a depth of 4" to 6" down.
If you are careful, the smaller potatoes can be left in place and gently replanted to allow them to continue growing.
Though normally eaten right away, new potatoes can be stored for several months, though they won't keep as long as fully ripened and cured potatoes. Keep them in a dark location at a temperature 38F to 40F.
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 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 above-ground frost won't affect the tubers. But do not allow the tubers themselves to freeze by remaining 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 off, but instead 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 (38F to 40F.) 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 you want, you can 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 sprouting from the "eyes". When planting time times, cut large potatoes into 2 oz. segments, each with 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.