Whether the goal is to harvest tender, immature “new potatoes,” or to harvest fully mature potatoes for storage and use over the fall and winter, it’s helpful to follow some basic guidelines on how and when to harvest potatoes.
All potato varieties can be harvested as new potatoes — dug up before the plant reaches maturity, while its tubers are still small. By the time that the plants have begun to flower, most of them will have developed at least some immature tubers ready for harvest. At this stage the tubers have thin skins and less dry matter within. They are small, so they can be cooked and served whole. But the thin skins that make them so succulent and delicious also reduce their ability to store well. The thin skins allow easier evaporation of the interior moisture, so they should be consumed shortly after harvest. New potatoes should be harvested and handled carefully in order to reduce bruising and damage to the skins, both of which can cause decay.
New potatoes can be harvested in spring and early summer, but this tends to sacrifice the parent plant so that it will not produce mature storage potatoes later in the season. If the plant is lifted with great care, some of the immature tubers can be removed as new potatoes, and the plant can be re-potted in new soil. This causes some stress to the plant, and is not generally recommended. Usually a row (or container) is sacrificed for new potatoes, and the left rest to mature to full size.
Always harvest potatoes with gentle care. Use a fork to gradually loosen the soil around each plant. Potato Grow Bags and other containers are useful, as they can be dumped, soil and all, into a wheelbarrow or over a tarp to sift through the soil and harvest each tuber by hand.
Storage potatoes are harvested once the plant is completely mature at the end of its growing season. At this time, the foliage begins to yellow and dry, normally from the lower leaves progressing upward. Some late potato varieties may still be green and bushy by the time early and mid-season plants have completely withered. For the best storage potential, mature tubers should not be harvested for at least two weeks after the foliage above ground has died. This waiting period allows the skins of the tubers to thicken, which is key to long term storage. Thick, unbroken skins (just as in winter squash and onions) reduce the loss of moisture from within.
If frost is expected within two weeks while plants are still green and vigorous, many growers defoliate the tops in order to trigger the skin setting process. A weed trimmer can be used to shred the leaves and stems of the plants so that death is gradual rather than sudden. If the plants die suddenly (including death to hard frost), the tubers may be discoloured. It is simpler to just select the appropriate variety for a given growing region in order to avoid artificial defoliation.
Again, all potatoes should be dug with care to avoid piercing the skins or bruising the tubers. In garden beds, it’s a good idea to remove soil methodically, and feel around for each of the tubers as they are uncovered. Keep dug potatoes out of direct sunlight, and preferably out of extreme heat or cold. The ideal range for harvesting storage potatoes is 13-18°C (55-65°F). If dug spuds are exposed to sunlight, the risk of soft rot and sun scald are increased. Just keep them under the cover of burlap sacks or tarps until they can be moved into long term storage.
Optimum storage conditions are in a dark location at 4-7°C (40-45°F), with 90% humidity. This is easy to achieve in a cold cellar, but can be managed by simply storing the tubers in paper sacks or burlap sacks in a garage or shed. Check stored potatoes regularly and thoroughly in order to remove any that are starting to turn.