Potatoes are unique because their edible parts, the tubers, develop entirely underground, making it difficult to judge their readiness for harvest. Home gardeners often consider leaving mature tubers in the soil for extended periods as a form of natural storage. While the soil offers a stable environment, this practice has a limited window of safety. Understanding the plant’s maturity signals and environmental risks is important for maximizing yield and ensuring long-term quality.
Determining the Optimal Harvest Time
The most reliable sign that potato tubers have reached maturity is the natural dieback of the foliage, known as senescence. This process begins when the above-ground vines turn yellow, wilt, and eventually turn brown and completely die off. The plant stops feeding the tubers at this point, redirecting its energy away from vine growth. It is helpful to stop watering the plants a couple of weeks before this dieback begins, which helps prepare the skins for harvest. It is important to wait until the vines are fully dead before harvesting, which typically takes about two to three weeks after the initial yellowing.
This waiting period allows the potato’s skin to “set,” or toughen up, a process known as suberization. A mature potato’s skin should remain firmly attached to the tuber when rubbed with a finger. If the skin easily rubs off, the potato is immature and will not store well, making it susceptible to bruising and decay. Harvesting at this moment of optimal skin set is a significant factor for successful long-term storage.
Risks of Delayed Harvesting
Leaving mature potatoes in the ground past the window of optimal readiness significantly increases the risk of damage and spoilage. A major concern is the increased exposure to soil-borne pathogens and pests like wireworms or voles, which can bore into the tubers and introduce decay organisms. The longer the tubers remain in the soil, the greater the likelihood of infection from common diseases such as scab or late blight, especially in persistently wet soil conditions.
Another substantial risk is secondary growth, a physiological disorder triggered by environmental stress, often a period of renewed moisture following a dry spell. This stress can cause the mature tubers to break dormancy and begin to sprout or form small, secondary tubers directly on the original potato. This process, sometimes called tuber chaining, consumes the potato’s stored starches, reducing its nutritional quality and overall storage life.
The onset of secondary growth creates misshapen tubers and heat sprouts, significantly affecting the edibility and overall quality of the crop. Even if the new growth is pruned, the original potato is considered physiologically older and will have a shorter lifespan once placed into storage. Tubers that are exposed to sunlight because of shifting soil or animal activity can also turn green, a sign that the toxic compound solanine has developed. Though small green spots can be trimmed, extensive greening renders the potato inedible.
The most immediate threat in many climates is damage from freezing temperatures late in the season. While a light frost may not harm the tubers deep in the soil, a hard freeze can penetrate the ground and cause internal discoloration, turning the potato flesh gray or black. Frozen potatoes will turn to mush upon thawing and must be harvested before the soil temperature drops below freezing.
Preparing Potatoes for Long-Term Storage
Potatoes intended for long-term preservation must undergo a specific post-harvest treatment called curing. This process is necessary to heal any minor cuts, scrapes, or bruises incurred during the digging process. Curing allows the damaged skin cells to form a protective layer, or periderm, which greatly reduces moisture loss and prevents the entry of decay organisms.
The ideal environment for curing involves holding the tubers at a temperature between 50°F and 60°F with a very high relative humidity, typically between 85% and 95%. This period should last approximately ten to fourteen days, depending on the maturity of the skins and the extent of any harvest damage. Proper curing is the most important step after harvest to ensure a long storage life and maintain quality.
Following the curing phase, the potatoes are ready for long-term storage in a cool, dark location. Temperatures should be lowered further to between 40°F and 45°F to effectively inhibit sprouting and minimize the rate of respiration. Storing at temperatures below 40°F can cause starches to convert to sugars, resulting in an undesirable sweet flavor. Maintaining high humidity in the storage area is also important to prevent the tubers from shriveling due to excessive water loss.