Iowa’s climate, with its warm growing season, allows gardeners the flexibility to cultivate both early-maturing and late-maturing potato varieties. This means the harvest window can extend from mid-summer for tender young tubers well into the fall for robust storage crops. Properly timing the harvest and following specific post-harvest steps are necessary to ensure the best yield and longevity for the potatoes.
Determining Maturity: Signs the Potatoes Are Ready
The decision of when to harvest potatoes depends entirely on the intended use, distinguishing between immature “new potatoes” and fully mature “storage potatoes.” New potatoes have delicate, thin skins and are best enjoyed immediately after harvest, not stored long-term. These can be gently dug up once the plants begin flowering, typically about 6 to 8 weeks after planting, when the tubers are about 1 inch in diameter.
For potatoes intended for long-term storage, allowing the tubers to fully mature underground is necessary. The most reliable indicator of this maturity is the natural yellowing and subsequent die-back of the potato plant’s vines and foliage. This process signals that the plant has stopped channeling energy into growth. Once the vines have completely died back, it is beneficial to wait approximately two weeks before digging.
This waiting period allows the potato skin, or periderm, to toughen and “set” against the underlying flesh. When the skin is fully set, it adheres tightly to the tuber and resists rubbing off easily. A well-set skin protects the potato from moisture loss and potential disease entry points during storage. Harvesting before this hardening process is complete results in easily damaged tubers with a significantly reduced shelf life.
Harvesting Techniques and Preventing Damage
The physical act of harvesting must be executed gently, as any nick, scrape, or bruise on the potato skin can become an entry point for pathogens, leading to rot in storage. Selecting a warm, dry day for the harvest is beneficial, as wet or muddy soil increases the likelihood of skin abrasions and makes handling more difficult. Dry conditions also allow the potatoes to air-dry slightly on the soil surface immediately after they are lifted.
Gardeners should use a spading fork, rather than a shovel, to lift the potatoes, as the rounded tines are less likely to slice through the tubers. Begin digging a sufficient distance away from the main plant stem, perhaps 12 to 18 inches, to account for the spread of the underground tubers. Gently insert the fork and loosen the soil, then carefully lift the entire plant and its cluster of potatoes.
It is important to recover all potatoes from the soil, as any remaining tubers can harbor pests or diseases that affect future crops. Once lifted, gently brush off the majority of the loose soil, but avoid any washing or aggressive scrubbing of the freshly dug potatoes. Washing the tubers before curing can introduce moisture into microscopic wounds and decrease their storage potential. Any potatoes that are visibly damaged, cut, or bruised during the digging process should be set aside for immediate consumption rather than long-term storage.
Post-Harvest Curing and Storage Preparation
Immediately following the harvest, the potatoes require a two-week curing process to prepare them for successful long-term storage. Curing is a period of controlled healing that allows minor scrapes to seal over through a process called suberization, which further thickens the skin. This temporary environment should maintain a temperature between 50 and 60 degrees Fahrenheit with a high relative humidity, ideally between 85 and 95 percent.
The high humidity during curing minimizes moisture loss while the moderate temperature encourages the formation of a protective layer under the skin. Potatoes should be placed in a dark, well-ventilated location, such as a garage or cool basement, to cure. Exposure to light must be avoided during this stage, and throughout storage, because light causes the tubers to turn green and develop bitter-tasting glycoalkaloids.
After the two-week curing period, the potatoes are ready for long-term storage, which requires different environmental conditions than curing. The ideal long-term storage temperature is cooler, between 38 and 45 degrees Fahrenheit, which helps suppress sprouting and respiration. Humidity should remain high, near 90 percent, to prevent the tubers from shriveling and losing water content. Storing potatoes in a dark, slightly humid, cool location, like a cold cellar or a dedicated second refrigerator set to the proper temperature, prevents sprouting and maintains quality for several months.