Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Tree fruits are ahead of schedule on bloom in March this year and as a result there has been a greater risk of freeze damage. Plasticulture strawberries are also in bloom and subject to freeze injury. We had temperatures in the mid 20s (F) last week and on March 30 temperatures dropped as low as 25°F in parts of Delmarva.
Plums and apricots are past full bloom, peaches are in full bloom, and cherries are in pre-bloom stage. Research has shown that when stone fruits are in the First Pink Stage (flower petals coming out of bud but not open), the temperatures required to cause 10% and 90% kill at this bud development stage were 25°F and 15°F, respectively. At First Bloom, the temperatures required to cause 10% and 90% kill were 26°F and 21°F, respectively. At Full Bloom Stage the temperatures required to cause 10% and 90% kill were 27°F and 24°F, respectively and at Post Bloom Stage the temperatures required to cause 10% and 90% kill were 28°F and 25°F, respectively.
For strawberries the critical temperature during bloom at the blossom level is 28°F. Below 28°F, there is a progressively higher risk of flower damage, and below 26°F most blooms will be damaged or killed. Flowers that are not open and just emerging from the crown can tolerate temperatures down to 22°F and once fruit has formed temperatures down to 26°F can be tolerated for short periods of time. Flower acclimation is also important. Plants with flowers exposed to several cold days before a frost will be more tolerant than those exposed to warm days before a frost. In addition, not all flowers in a field will have equal risk of damage. Flowers under leaves or near the soil will often be warmer than those higher on the plant or those more exposed.
Most Delmarva growers are using floating row covers for frost and freeze protection in platiculture strawberries. Row covers reduce temperature loss by cold winds, radiational cooling, convection, and evaporative cooling. With row covers, temperatures drop more slowly under the cover and therefore the nighttime temperatures will be higher than in uncovered areas. This will usually give you 2-8 degrees of protection depending on the thickness of the row cover and weather conditions. Monitoring temperatures under the row covers can verify this. In freezing, windy conditions where outside temperatures are expected below 23°F, additional protection may be needed. This can be done by double covering the strawberries or by sprinkler irrigation over the row covers during the night.