When potato plants reach the end of their growing cycle, the foliage turns yellow and eventually dies back completely. This signals that the tubers have stopped enlarging and are nearing maturity. Immediate harvesting is not recommended for those intending long-term storage. The waiting period following vine death significantly influences the quality and shelf life of the harvested potatoes.
The Essential Curing Process
The time potatoes spend in the ground after the vines have died initiates a physiological process known as “skin set” or curing. This period allows the potato’s outer layer to toughen and mature, which is necessary for long-term storage. The skin, which is thin and easily damaged immediately after the plant dies, begins to thicken and adhere firmly to the underlying flesh.
During this curing, the potato synthesizes suberin, a waxy substance that fortifies the skin. This fortified skin acts as a protective barrier, reducing the risk of abrasions and bruising during harvest and handling. The suberization process also seals minor cuts or scrapes the tubers may have sustained.
Sealing these small wounds prevents the entry of decay-causing pathogens and reduces moisture loss from the tuber. Potatoes harvested before this process is complete, often called “new potatoes,” have delicate skin that easily sloughs off. These immature potatoes must be consumed quickly as they are ill-equipped for extended storage.
Determining the Optimal Harvest Window
The guideline for maximizing tuber quality is to wait approximately two to three weeks after the potato foliage has completely withered. This timeframe permits the skin to fully develop its protective characteristics. However, environmental conditions can shorten or lengthen this ideal waiting period.
Soil moisture is one of the most influential factors governing the harvest timeline. If the soil becomes waterlogged, the tubers risk developing rot. In excessively wet conditions, the waiting period should be significantly shortened, or the potatoes should be harvested immediately to prevent disease.
Growers often reduce or stop watering entirely a few weeks before harvest to allow the soil to dry out. Dry soil minimizes the risk of rot, makes digging easier, and reduces the chance of damaging the tubers. Conversely, if the soil is extremely hot, tubers may need to be lifted sooner, as prolonged exposure to high temperatures compromises their quality.
A small test harvest is recommended around the two-week mark to determine if the skin has properly set. Gently dig up one or two potatoes and attempt to rub the skin with your thumb. If the skin resists rubbing off or peeling, the curing process is complete, and the crop is ready for harvest. If the skin easily rubs away, the tubers require more time underground, and you should wait an additional week before testing again.
Risks of Delayed Harvest
Leaving potatoes in the ground past the optimal window introduces several negative consequences for the crop.
Pest and Disease Exposure
Prolonged delays increase the tubers’ exposure to various soil-dwelling pests. Wireworms, for example, can bore small holes into the potatoes, compromising the protective skin and making the tubers unsuitable for long-term preservation. Burrowing animals, such as voles, may also target the crop.
If the soil stays consistently moist, the extended period increases the likelihood of soil-borne diseases. These conditions can cause soft rot or physiological disorders like enlarged lenticels, which are small pores on the potato surface that swell when saturated.
Greening and Sprouting
Another concern is the potential for the potatoes to turn green. This occurs if the soil around the tubers cracks or erodes, exposing them to sunlight. The green coloring indicates the buildup of solanine, a bitter compound that is toxic in large quantities, rendering affected potatoes inedible.
In regions with mild winters, leaving potatoes in the ground too long can cause them to break dormancy and begin to sprout prematurely. Sprouting depletes their energy stores and diminishes their quality.