The temperatures dipped below freezing in the early morning hours of Sunday morning May 9th with some thermometers seeing temperatures as low as 28 degrees Fahrenheit. With those early morning cold temperatures, we experienced a "radiation freeze".
What is a "radiation freeze"? Freezing temperatures are classified into two categories based upon the atmospheric conditions associated with the freeze. A radiation freeze occurs when a dry cold air mass settles in an area when there is little or no wind and an absence of cloud cover during the night. This is associated with the center of an atmospheric high pressure cell moving into an area. Under such conditions, it can be relatively warm during the day with the sun warming the soil and plants. At night, this heat is slowly released to the atmosphere. Because there is little or no wind, the air stratifies forming an inversion layer of warmer air about 30 to 50 feet above the ground. Cooler dense air is trapped beneath the warm air, and continues to cool. As it cools, it settles near the ground and drains to lower elevations within the area. The air temperature at 50 feet may be anywhere from 3o to 10o F warmer than the air near the ground. Because there is little air movement, and the fact that air, particularly dry air, is a poor conductor of heat, plant parts can be several degrees cooler than the surrounding air. Under these conditions, some plants, including grape vines, suffer tissue damage.
The second type of freeze is an advection freeze that occurs under windy conditions when a large, dry, cold air mass, several thousand feet thick, moves into an area. This is the type of freeze we experience when the wind is out of the north-west and an atmospheric high pressure cell is moving into the region. When this occurs, the air temperature is often colder than the plant temperature. Under such conditions frost protections systems are of little benefit.
Getting back to the early morning hours of May 9th, the wind was calm and the skies were clear making for a classic example of a radiation freeze. On Saturday, May 8th, the day was in the low 50's and pleasant when the sun was out. After sundown since there was no wind and mixing of the air, the temperatures fell to the dewpoint. Grape vines may have experienced foliage and yield loss since the dewpoint was near or below the critical temperature of fruit bud damage. Even a few clouds that night would have raised the temperature and reduced damage.
As with many questions relating to specific damage to grape vines, a standard Extension answer is "it depends". With the variability of site locations (elevation and slope) and age of some of the vines, the early Sunday morning frost was very site specific. Primary bud damage may have occurred and the need for the secondary or tertiary buds to produce a shoot will need to happen. This will delay bud emergence due to the primary bud damage and probably reduce production.
On the younger vines that were hit hard by the freeze, a "wait and see" approach is necessary to see if suckers from the base will rejuvenate the plant if the shoots above ground were severely damaged.
Producers may see specific plants damaged and surrounding vines showing limited damage. The "air drainage" of the cold air moving to the lowest elevations or being blocked by trees or structures will also influence the extent of damage to the vines.
How severe was the freeze damage to your specific vineyard, orchard, or garden site? Unfortunately, the answer remains "it depends".
You can stop in at our office if you have any more questions. The address is 209 Centennial, Suite A, Cherokee, Iowa. You could also call us during office hours 8-4:30 Monday thru Friday. The phone number is 225-6196.