Coauthored by Dr. Shawn P. Conley (15%) and Dr. Jim Specht (85%)
Last nights cold temperature has led to an influx of questions regarding the potential impact on either the wheat crop or emerged soybean seedlings. In short there is nothing to worry about in either crop. In the case of wheat, which is still in the jointing growth stage, cold temperature would need to reach 24 degrees F or less for 2 plus hours before injury occurred (Please see Recent Cold Temperatures will have Little Impact on WI Winter Wheat Crop for more details).
In the case of soybean I was fortunate enough to be copied on this email from Dr. James Specht (please see below) which saved me a few hours of library time today so thank you Jim.
First of all 34F will not impact above-ground tissue. Second, tissue freezing does not even take place at 32F because cell cytoplasm has solutes in it – like a modest anti-freeze, which depresses freezing point of the tissue a degree or two less than 32F - thus air temps surrounding the tissue have to get to below 31 or 30F before tissue freezing can occur. Third, the soil surface is typically warmer than the air temperature (particularly when the soil is wet) and does not give up heat acquired during a sunny day as fast as the air does after sunset. In actuality, the interface between soil surface temp and the air temp near that soil surface will be closer to the soil temp than to the air temp which most peopled measure on thermometers viewable at their height (not at ground level). Biophysically, control of the soil temp over the air temp this is called the “boundary layer effect”). So don’t trust air temperatures read on thermometers unless you know what the air temperature near the soil surface was (put a thermometer on the soil surface tonight where the cotyledons are and check it just before dawn (when the soil surface temp reaches its nadir for a 24-hour temperature cycle) and send out an e-mail blog to your producer colleagues early the next day. Fourth, the cotyledons are a huge mass of tissue that are about 95% water. That big amount of water-filled tissue is hard to freeze unless the exposure to temps of 30F at the soil-air interface is many, many hours. Cotyledons will freeze faster (in fewer hours) but only if the soil surface temps get well below 30F (say 25F). The only concern I would have is when cotyledons are no longer closed and protecting the young stem tip. However, if that is in fact frozen off, the nodes to which the cotyledons are attached will regenerate TWO main stem tips. Not an ideal way to start the growing season, but better than having to replant (0.5 bu/ac loss per each day that soybeans are NOT in the ground on May 1).