Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Peaches, plums, and apricots have bloomed, several weeks ahead of normal. Strawberries have been blooming for weeks in plasticulture systems. These fruit crops are at great risk of losses due to freeze events. Other fruit may also flower early and be at risk. For example, pears are in bloom now and cherries and blueberries are starting to bloom.
Normally, the average date of the last frost in Delaware is somewhere between April 20-25. We still have four weeks of worry ahead for our fruiting crops.
For all these fruit crops the most susceptible stage of injury is when flowers have just opened. Closed buds have higher cold tolerance as do small fruit. For most fruits, critical temperature for losses after fruits have formed is 28° F.
Plasticulture strawberries blooming 3- 29-2012.
Frost and freeze protection methods vary with fruits and the type of freeze expected. Advective freezes occur with freezing temperatures and high winds. This is the most difficult to protect against. For strawberries, two layers of floating row covers may be the most effective strategy for advective freezes. Double covers have been shown to be more effective than single heavy covers in this case. Irrigation along with double covers can provide even more protection if done properly.
Radiational freezes occur on cold, still nights. In this case cold air is near the ground and warmer air is above. Wind machines and helicopters have been successfully used to stir the air and raise the temperatures in orchards in this case. Row covers in strawberries will protect against radiational freezes too.
Irrigation has also been successfully used for frost protection but it has to be done properly. How irrigation works is that as ice forms on plants heat is released. The key is to keep ice formation occurring through the night and continue through melt in the morning. Remember that initially, until ice starts forming, there will actually be evaporative cooling of the plant. The latent heat of fusion (water freezing) will release heat (approximately 144 BTUs/lb of water), whereas evaporative cooling will absorb heat from the plant (absorbing approximately 1,044 BTUs/lb of water) and lower plant temperatures. Therefore, irrigation must start well above critical temperatures. Also, the volume of water needed needs to be matched with the expected temperature drop and wind speed. In addition, uniformity of water application is critical. This is difficult to do in high wind situations.
This past week temperatures dropped below freezing in parts of Delaware on three nights, with some areas in the mid-twenties. NOAA has predicted an increased risk for lower than normal temperatures in the Mid-Atlantic region for the next 2 weeks.