Harvesting and Storing Potatoes
Curtis E. Swift, PhD, Area Extension Agent (Horticulture)
Colorado State University Cooperative Extension
Tri River Area Mesa, Delta, Montrose and Ouray Counties
The keeping quality of white ("Irish") potatoes (Solanum tuberosum L.) increases greatly if tubers are harvested after vines are killed by the first fall frost. Potatoes intended for long-term storage should not be harvested until the vines have been dead for at least ten to fourteen days. This permits the proper thickening of potato skins, which increases the length of time potatoes can be stored. Potatoes harvested too early easily `skin' during the harvesting and handling period and do not store well.
Potatoes are best dug when the soil temperature is above 45 degrees Fahrenheit and the soil is not wet. Digging when the soil is cold and wet often results in splits and cracks. Potatoes harvested from cold wet soils are also more susceptible to bruising, more difficult to cure and more prone to breakdown during storage.
Potatoes should never be allowed to freeze in the ground, but since many of the soils in the Tri River Area do not freeze until mid-December, gardeners in this area have sufficient time between the first frost and the ground freezing to allow for the harvest of these tubers. If the soil is frosty or the air temperature is near or below freezing, it would be best to begin harvesting later in the morning or afternoon when temperatures have risen. Soil temperatures would be warmer from 11 a.m. to 11 p.m. Commercial growers often harvest into the evening hours to take advantage of this warmer soil temperature.
Bruising can occur when the tubers are being dug or during any of the operations involving moving the tubers. Internal black spots often develop within six to eight hours after bruising or may not appear until a day later. Consequently, care should be taken to avoid dropping the tubers during any of the steps from harvest through storage.
Despite precautions taken to prevent injury to the skin of a potato, some damage is likely and a curing process is necessary for any wounds to heal. This process is facilitated by a curing process.
The Healing Process
A cut in a potato tuber undergoes two steps when it heals. Suberization is the first of these steps and involves the development of a waxy, fatty compound called suberin which is produced by cells just below the cut surface. This seals the wound preventing water loss and invasion by pathogens. This process occurs one to three days after wounding and is typically complete within four to seven days.
The formation of a specialized tissue called wound periderm is the second step in this healing process and results in the development of a permanent, protective layer of cells that replaces the `skin' that was destroyed by the wound. This corky layer is a final protective coating which prevents infection and water loss. The development of this wound periderm begins shortly after suberization and is complete within one to two weeks. Ideally, potatoes should be cured for this entire period, but many home gardeners will move their tubers into permanent storage after the four to seven day suberization period.
When potatoes are first dug they should be placed in piles and allowed to go through a sweating (curing) period. During this period a temperature of 70 degrees Fahrenheit hastens the healing of bruises and cuts helping to prevent rot. If the crop is relatively free of wounds, a curing temperature of 57 to 60 degrees Fahrenheit can be safely maintained. Potatoes can be left in the field during this curing process and covered with burlap or some other material to prevent sun scald. Potatoes can also be cured in a warm room. Tubers to be stored should be clean, firm and free from disease. Rains during the curing process will inhibit this healing process and piles left outside should be protected from fall rains by piling the tubers under a makeshift shed roof. The piles should not be covered with a tarp, however, as air movement through the pile is necessary to promote healing and a tarp will cut off this necessary air supply.
Storage
Potato tubers should be stored in sacks, bins or crates or in piles in areas that are free of disease organisms. Washing storage containers and areas down with a disinfectant several weeks prior to storage will help reduce storage rot problems.
Immediately following the sweating period, tubers should be stored in a humid area ( 90 to 95 % relative humidity) at a temperature near 40 degrees Fahrenheit if the tubers are for eating or to be used for seed potatoes. Processing tubers used for french fries are typically stored at 45 degrees Fahrenheit; tubers for potato chips are stored at temperatures of 50 to 55 degrees Fahrenheit. If stored at temperatures below 36 degrees Fahrenheit, potatoes may become sweet. If potatoes become sweet, exposing them to ordinary room temperature for a few days tends to restore natural flavor.
Potatoes held in storage for too long or at the improper temperature or humidity may break dormancy. The tubers will start to shrivel and sprout. Tubers in this condition should be used as quickly as possible to prevent breakdown and decay.
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Placed on the Internet September 11, 1996 Last updated: 11/11/2002 18:32:10
Comments on this page should be addressed to Dr. Curtis E. Swift, Area Extension Agent, Horticulture
Colorado State Cooperative Extension
2775 US Hwy 50, Grand Junction, CO. 81503
voice: 970-244-1834
fax: 970-244-1700