Yard and Garden: Potatoes

AMES, Iowa – Iowa State University Extension and Outreach horticulturists share information about the potato (Solanum tuberosum), one of the most important vegetable crops in the world. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

Why are my potatoes knobby?

Fluctuations in soil moisture levels during tuber development may cause knobby potatoes. Watering on a regular basis (about once a week) during dry periods will help prevent this problem. 

Why are some of my potato tubers green? 

Potato tubers are actually enlarged underground stems. When potato tubers are exposed to light (either in the garden or storage), their skin turns green due to the formation of chlorophyll. The chlorophyll itself is not a problem. However, higher levels of glycoalkaloids also develop in the green tissue. Green tubers have a bitter taste when eaten. They may also cause an upset stomach and more serious health problems.  

Tubers with small green areas can be safely eaten if the green portions are cut off and discarded. It would be best to discard potatoes that are largely green.

When growing potatoes in the garden, hill soil around the base of the potato plants to prevent the tubers from being exposed to light. After harvesting, store potatoes in a dark location. 

When should I harvest potatoes?  

Potatoes grown for storage should be harvested after the vines have died and the crop is mature. To check crop maturity, dig up one or two hills after the plants have died. If the skins on the tubers are thin and rub off easily, the crop is not fully mature. Allow the crop to mature for several more days before harvesting the potatoes. When harvesting potatoes, avoid bruising, skinning or cutting the tubers. Damaged potatoes should be used as soon as possible. 

How should I store potatoes? 

After harvesting the potatoes, cure the tubers at a temperature of 50 to 60 degrees Fahrenheit and high relative humidity (85 to 90 percent) for two weeks. The curing period allows minor cuts and bruises to heal. Thickening of the skin also occurs during the curing process.  

Once cured, store potatoes at a temperature of 40 F and relative humidity of 90 to 95 percent. Store the crop in a dark location as potatoes turn green when exposed to light. If storage temperatures are above 50 F, the tubers may begin to sprout in two or three months. When stored below 40 F, potatoes develop a sugary, sweet taste. Sugary potatoes can be restored to their natural flavor by placing them at room temperature for a few days prior to use. Do not store potatoes with apples or other fruit. Ripening fruit give off ethylene gas, which promotes sprouting of tubers.