Onion bulbs freeze at 30 to 32 F.; however, they can withstand temperatures as low as 23 F without damage as long as they are not moved before they thaw. Less injury occurs when thawed at 39 to 41 F than at higher temperatures. Hardenburg et al. state that onions only slight frozen may recover with little perceptible injury if allowed to thaw slowly and without handling.

Freeze damage appears as water-soaked, gray to yellow areas when the bulb is cut. An entire scale all the way around the bulb may show injury, while adjacent inner and outer scales may or may not show injury. The skin often is loose on the concave side where scales and tissue is affected due to separation of the tissue. The surface tissue may develop a granular, rough texture.

Following is the weather data for late September as reported by the Colorado AgMet web site.

Delta weather site (3 miles W of Delta):

• September 24 - low of 29.9F; high of 77.6F
• September 25 - low of 25.9F; high of 55.9
• September 26 - low of 28.6F; high of 77.9

Olathe weather site (3 miles NE of Olathe)

• September 25 - low of 28.7; high of 68.5

At some sites, growers reported temperatures as low as 19F, well below the minimum temperature reported in the literature which onion bulbs can tolerate. Following freezing, onion bulbs must be thawed slowly. The high temperatures following freezing conditions resulted in the development of dead tissue and softening of bulbs. Secondary organisms have invaded some of these bulbs. Consequently, growers are blaming the problem on bacterial infection instead of freeze damage. According to Dr. Howard Schwartz, CSU Extension Plant Pathology Specialist, bacterial problems were not a concern this year.

Onions in storage should be dried down as soon as possible by pushing as much air through the pile as possible. This air should not be heated or secondary bacterial organisms may spread throughout the storage facility.

Hardenburg, R.E. Watada, A.E., and Wang, C.Y. 1986. The Commercial Storage of Fruits, Vegetables , and Florist and Nursery Stocks. USDA, ARS Handbook #66. Washington, DC
Salunkhe, D.K., and Desai, B.B. 1984. Postharvest Biotechnology of Vegetables: Volume II. CRC Press, Inc., Boca Raton, FL.
Schwartz, H.F., and Mohan, S.K. 1995. Compendium of Onion and Garlic Diseases. APS Press, St. Paul, MN.
Schwartz, H.F., Westra, P., and Cranshaw, W. 1990. Colorado Onion Integrated Pest Management. Bulletin 547A, Colorado State University, Cooperative Extension, Fort Collins, CO.

Onions left in field due to freeze damage

Onions showing damage: note separation of scales which resulted as this onion was dried down in storage

Onions showing damage: note water-soaked tissue