Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
The seasons are changing with shorter winters, longer “summers”, more abrupt end to spring and warmer fall months. One of the more difficult problems to have to manage will be false springs. A false spring is an early warmup followed by later freezing periods causing damage to early developing crops.
In the past 15 years, the Eastern US has experienced five major false springs: 2007, 2012, 2017, 2020 and 2022. Add 2023 to this list with earliest blooming plants well ahead of schedule.
Historical records show that the bloom times for many plants are becoming earlier and earlier.
Some plums and apricots have started to bloom four weeks ahead of normal. Other fruits such as strawberries may be blooming ahead of schedule in plasticulture systems. These fruit crops are at great risk of losses due to freeze events. Other fruits such as pears, cherries, and blueberries may also flower early and be at risk. Apples and pears are later blooming but can also be damaged if they bloom early and freeze events occur in late April or early May.
Crops coming into bloom will be exposed to the potential of freezing temperatures throughout the rest of March and April.
Normally, the average date of the last frost in Delaware is somewhere between April 20-25. We still have seven 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. Open blooms are damaged at 32-34 °F. Closed buds have higher cold tolerance as do small fruit. For most fruits, critical temperature for losses after fruits have formed is 28-30 °F.
Some climate models predict that false springs will be the norm for the next several decades.
Japanese plum in bloom on February 27, 2023.
Apricot in bloom on February 25, 2023.
Fruit improvement programs may have to breed for later blooming. An example is with apricots which are early blooming. Rutgers University has been breeding later blooming apricots and has released “Sugar Pearls” as a first of the later flowering apricots for the Eastern US. In addition, fruit growers will likely have to spend more on frost and freeze protection.
Frost and freeze protection methods vary with fruits and the type of freeze expected. Advective freezes occur with freezing temperatures and high winds. Radiation (or radiant) freezes occur on cold, still nights. In this case freezing 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 for radiant freezes. Row covers in strawberries will protect against radiation freezes too. Other options are over the top sprinklers, ground sprinklers, and heaters.
Over the top sprinkling is commonly used for frost protection but it must be done properly. How this works is that as clear ice forms on plants, heat is released. For frost protection, overhead sprinkler systems are designed to deliver 0.1 to 0.2 acre-inches of water per hour. This method can be used for radiant freeze or frost protection when wind speeds are low and temperatures 24 °F or above. 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 be evaporative cooling of the plant. The latent heat of fusion (water freezing) will release heat (approximately 144 BTUs/lb. of water), while 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 must 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. In orchards, under-tree sprinklers can also be used to release heat.
Heaters that are placed throughout an orchard will add heat. Large numbers of small heaters are preferred (40 per acre). This is accomplished with fuel oil fired heaters, gas/propane heaters, or burn barrels using wood (check with regulatory agencies before using open burning in barrels). Heaters are much more efficient and less are required if they are used in conjunction with wind machines.
Frost protection fans above an orchard or vineyard mix the warmer air above the inversion layer with the colder air at ground level to protect against radiation frosts. These large fans can be permanently installed and will cover as much as 10 acres. Another type of fan is placed at ground level and pushes the cold air upward, again achieving mixing. Portable fans are also available.