Blueberry update: Surviving the freeze
By Mark Seitz
Pender County Extension Director
Field Crops / Commercial Horticulture Agent
As the NC Blueberry Festival looks to return in 2022 Mother Nature deprived area blueberry (and strawberry) growers of sleep over night. This is the second major cold front this month with one coming earlier in the month. Both events required farmers to stay up all night, or sleep in their trucks, to monitor the irrigations systems used to protect the blossoms.
While blueberry farmers use irrigation, most strawberry farmers use row covers to protect their plants. However, like people strong winds and cold air bring wind chill and any blooms that were touching the covers froze or the covers did not trap heat from the soils as well as they would in calm conditions. Freeze damage from the early March storm in both strawberries and blueberries is estimated at 5 to 10 percent where blooms were present. Thankfully, blossoms in many of the blueberry varieties had not yet opened at so it is assumed that the freeze damage was limited to early blooming varieties.
Today’s weather dropped temperatures in the region into the mid to upper 20’s. Thankfully the wind died down early Sunday evening, leading to calm, cold field conditions, which created almost ideal conditions for successful freeze protection. As the winds die off, the temperatures drop but those conditions give the overhead irrigation systems the ability to uniformly cover the bushes in a more uniform manner.
But doesn’t ice make things cold? Yes. Intuitively we think of ice making things cold. But when it comes to protecting blueberry blossoms, ice is good. The system is based on the science of thermodynamics. As the warm (45°F-50°F) water cools it forms ice. But before it does, there is an exchange of heat at the surface of the blossom as the water cools down to 32°F. That thermodynamic heat transfer keeps the surface of the blooms from freezing as long as the water continues to flow. If it stops, all the benefit of the cooling water goes away and the blooms freeze. So, farmers stay up all night to make sure their sprinklers do not freeze and pipes do not burst.
Similar impacts affected area wheat fields but we cannot frost protect wheat. In the early March freeze, a lot of the wheat seed heads had not emerged from the whorl (stem) of the plant. That gave them a measure of protection, thereby liming the frost/freeze damage. However, warm weather in the following two weeks has pushed the growth of much of the wheat forward so today’s (3/28) freeze could result in significant freeze injury. That damage typically does not appear for five to seven days after the injury so it is too early to predict what impact this freeze will have on wheat farms. Those assessments will be done later this week.
