Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu
As we move closer the spring season, it is important to consider critical temperatures for strawberry buds and blossoms and freeze protection, especially where floating row covers are being used to obtain earlier production.
While plants are in a dormant state or when buds are not yet active in strawberries, the buds can tolerate temperatures down to 10ºF. As buds become active but before flowers open, the buds can survive down to temperatures of 22-27ºF (depends on just how close they are to opening).
Removing row covers during warmer winter periods can help to delay bud activity and reduce susceptibility to freezes. Replace row covers in times when freezes are expected. Highest yield potentials are usually obtained by uncovering and covering in the late winter and spring based on expected temperatures when compared to the practice of keeping row covers on continuously into the flowering or early fruiting stage (another issue is that pollinating insects are restricted with covers resulting in smaller fruit size if left on too long).
As flowers open, strawberries can only tolerate drops in temperature down to 30ºF. Small green fruit can stand temperatures down to 28ºF.
During flowering and fruiting be prepared to freeze protect using row covers and in very cold conditions, sprinklers. There are usually several days where temperatures drop into the low 20s during the spring and freeze protection is critical. Removing row covers too early with no provision to replace them during freeze events will often result in severe yield losses.
There are two types of freeze events that you need to be concerned with. Radiation freezes are where heat from air is lost to a black, cloudless sky with little or no wind conditions. We can be very successful at protecting strawberries in these conditions with row covers and/or sprinklers. Advection freezes are where cold air is moving into the area with high wind. This cold air blowing across the crop with significant wind speed is much harder to deal with and where some losses are likely, even with freeze protection measures.
For row covers, the heavier, the more protection against freeze. However, the heavier the cover, the less light, and the more growth is restricted. A balance is what is desired. Generally, covers in the 0.9-1.25 ounce/sq. yd. range give adequate freeze protection without restricting light too much.
Sprinklers can also be used to freeze protect, but they must be used correctly. The idea is to slowly build up ice over the period when temperatures are below freezing over the plant or row covers. As ice is formed, some heat is released to the plant surface and to the surrounding air, due to the heat of fusion. To do this, sprinklers must be turned on before temperatures are at 34-35ºF. Use low volume sprinklers and apply irrigation throughout the night, building ice all night. Continue into the morning until ice has melted. An application rate of 0.15 inch per hour with no wind will provide protection to 22ºF. At colder temperatures or higher wind speeds more water will be needed.
It is recommended that for protection against frost above freezing, use sprinklers or row covers alone, for freezing temperatures in the mid to high 20s use sprinklers or row covers alone, for temperatures in the low 20s or below, use both sprinklers and row covers. A combination of row covers and sprinklers has been shown to protect well below 20ºF.