Leaving potatoes in the ground over winter, often called in-ground overwintering, is an alternative to traditional cellar storage. This method relies on the insulating properties of the soil and a thick layer of mulch to protect the tubers from freezing temperatures. Success is highly dependent on preparing the potato bed correctly and understanding the local climate, as this is not a universally applicable method.
Determining If Your Climate Allows It
The feasibility of in-ground overwintering rests on maintaining a soil temperature above 28 to 30°F (-2 to -1°C). If the ground freezes solid to the depth of the tubers, the potatoes will freeze and become unpalatable. This occurs because freezing converts starch to sugar, resulting in a sickly sweet taste. Gardeners must consult localized climate data, such as USDA Hardiness Zones, to assess the risk of a deep, prolonged freeze.
Recognizing the difference between air and soil temperature is important, as the ground provides significant thermal insulation. Even when air temperatures drop below freezing, the soil several inches below the surface may remain warmer, especially if covered. However, regions with consistently deep freezing are generally unsuitable for this storage method. Assessing this risk determines whether mitigation efforts, such as heavy mulching, will prevent crop loss.
Essential Steps for Successful In-Ground Storage
Successful overwintering requires placing the tubers significantly lower than a typical summer harvest, below the freeze line. Trenching or hilling the potatoes 12 to 18 inches below the surface offers a considerable buffer against cold penetration. This deep placement leverages the earth’s natural warmth to keep the tubers stable during cold snaps.
Once the foliage has died back, applying a heavy layer of insulating mulch is the next crucial step. Materials like straw, chopped leaves, or hay should be spread 6 to 12 inches deep over the planting area. This organic layer traps heat within the soil and prevents the ground from freezing deeply. A well-drained site is also necessary, as saturated, cold soil conducts heat away rapidly and significantly increases the risk of rot.
Protecting Against Pests and Disease
Subterranean pests pose a threat to buried tubers throughout the winter months. Rodents like voles and mice are notorious for tunneling through the soil and gnawing on potatoes. To exclude these common pests, gardeners can line the bottom and sides of the planting area, particularly in raised beds, with half-inch galvanized metal hardware cloth.
Disease is another major risk, as dampness often proves more destructive than moderate cold. Fungal issues, such as late blight or dry rot, can affect tubers left in the ground, especially if the soil is heavy or poorly draining. Dry rot often enters through small wounds on the potato skin. Saturated soil encourages the spread of various bacterial and fungal pathogens. Selecting a well-aerated site and ensuring all tubers are undamaged before winter are the best preventative measures against microbial decay.
Harvesting and Usage After Winter
Overwintered potatoes should be retrieved as soon as the ground thaws completely in early spring. Harvesting before the plant begins its new growth cycle is important, as the tubers will quickly begin to sprout once soil temperatures rise. Digging too late risks a decline in the quality of the stored potatoes and reduces their viability for consumption.
Upon retrieval, carefully inspect the potatoes for any signs of damage or decay. Tubers that have frozen will exhibit a soft, watery texture once thawed and possess an undesirable sweet flavor, making them unsuitable for cooking. Undamaged, firm potatoes can be used for consumption or saved as “seed potatoes” for the next season’s planting. Overwintered potatoes tend to sprout more quickly than those stored in a cold cellar, so they should be used soon after harvesting.