This has been a spring for the record books, with high temperatures and early development of fruit plants. As many of us feared, temperatures are dropping and we are at risk for frost through early next week.  As fruit buds develop in the spring, they become increasingly less cold tolerant and more susceptible to cold temperatures.  See Critical Spring Temperatures for Tree Fruit Bud Development Stages by Michigan State University or see page 65 of the Midwest Tree Fruit Spray Guide for critical temperatures in fruit trees at which injury occurs.
 
Currently, most of the fruit trees across Iowa are at or beyond bloom.  The critical temperature for 10% injury for trees at this stage is 28 degrees F and 25 degrees F for 90% injury.  Critical temperatures for strawberries are 10 degree F at bud emergence, 22 to 27 degrees F at closed bud, and 30 degrees F at bloom. When frost injury does occur you will see a gradual darkening of injured tissue over a period of 24 to 48 hours.
 
There are two types of freeze events, advection and radiation.  An advection freeze occurs when a dry, cold air mass moves into a region and it remains relatively windy.   A radiation freeze occurs during clear, calm nights.  During a radiation freeze cold air settles at ground level while warm air (and with it heat) “radiates” or escapes into the upper atmosphere.  It is more difficult to protect plants from an advection freeze than a radiation freeze; however, there are options for protecting.  Protection methods include heat, air circulation, irrigation, and implementing cultural practices.
 
High tunnels provide little to no frost protection. Typically, the minimum outside temperature is the same as the minimum temperature inside the high tunnel. For protection in a high tunnel, be sure to cover sensitive plants with a row cover, circulate the air in the tunnel, and/or provide supplemental heat to the structure. If you choose to provide supplemental heat, be sure to properly ventilate emissions so that carbon dioxide, carbon monoxide, and ethylene do not buildup within the structure.
 
For more information on types of frost, critical temperatures, and control methods, see the frost handout by Dr. Paul Domoto, Dept. of Horticulture, ISU.
 
Other resources:

• The Midwest Strawberry Production Guide (pages 26 – 28) by the Ohio State University offers a thorough discussion on using irrigation as a frost control method.
• Midwest Wine Press offers an interesting discussion on using an airblast sprayer to circulate air in a vineyard.
• “Frost Protection in Vineyards” is an excellent 22 minute Online webinar narrated by Dr. Tom Zabadal of Michigan State University
• “Understanding & Preventing Freeze Damage in Vineyards”, a 108 page publication from the Institute for Continental Climate Viticulture & Enology (ICCVE) is another excellent resource frost.
• The Ontario Ministry of Agriculture, Food, and Rural Affairs includes critical temps for vegetables crops and a discussion on frost.

Check the ISU Extension and Outreach Iowa Produce website at www.iowaproduce.org for continuing updates.