Radiation frosts occur when skies are clear, winds are calm, and humidity is low. As heat is radiated to the sky the existing air mass cools and shoots, leaves, and nutlets radiate (lose heat) to the cold night sky. Exposed tissues are the most likely to be damaged during a frost night. Shoots, leaves, and nutlets on the top sides of limbs may be injured while tissues underneath a limb or protected by dense foliage may escape injury.
Walnut flowers and small nutlets can be frozen when exposed to temperatures of 30 F for only 30 minutes. Shoots, flowers, or small nutlets can be cut and examined to determine if the tissues appear watersoaked or blackened to varying degrees. In contrast, healthy tissues will appear moist and bright green inside when examined.
The probabilities of frosts are greater with early leafing varieties like Serr and Ashley. As later leafing varieties such as Howard and Chandler begin growth the probability of frost continually declines. During the April 20, 2008, radiation frost, the earliest leafing varieties had the least injury since they had more leaf tissue providing protection when the frost occurred. Remain vigilant. To gauge the frost potential, check the dew point temperature before retiring for the night. When the dew point is above 45 F, frost is rarely a problem.
Dr. Rick Snyder, Cooperative Extension biometeorologist makes the following observations on frost protection:
• For weather forecasts, go to the National Weather Service Forecast Office Web site at http://www.wrh.noaa.gov/sto, click on “Forecast Weather Stations” on left, set the interval in hours and the duration in days, and enter your zip code or location. These forecasts are very helpful for predicting autumn frosts, winter freeze and spring frost events.
• If the soil is dry, it should be wetted at least three to five days ahead of a freeze. If there is water on the soil surface before a freeze, it will make the soil surface colder because of evaporative cooling. Water conducts and stores more heat than air spaces, so wetting the soil three to five days prior to a frost night will fill the air spaces and the soil will store more heat. You don't want water on the surface during the frost night unless you keep re-wetting it with sprinklers or with continuous running furrows.
• Under tree sprinklers should be turned on before the wet-bulb temperature falls to the critical damage temperature and turned off the next morning after the wet-bulb temperature goes above the damage temperature. To be really safe, turn off the sprinklers when the wet-bulb temperature exceeds 32 F. For example, from Dr. Snyder’s Web site http://biomet.ucdavis.edu, click on “Frost Protection” and then “When to turn the sprinklers on or off for frost protection”. Table 1, in that Web article indicates that with a wet-bulb