By Bruce Steelman, UT Extension Agent for Cannon County
Temperatures are predicted to drop below freezing during the next few days. Most fruit crops are in are either in bloom, close to bloom, or at petal fall. The critical temperatures at which damage would be expected to occur on various crops are:
Strawberries: popcorn bud (buds are swollen & beginning to open) – 28°F, full bloom – 30°F, after bloom and fruit developing – 28°F
Apples & pears: pink bud and beyond – 28°F for 30- minutes or longer would be expected to result in about 10% bud kill
25°F for 30 minutes or longer would be expected to result in about 90% bud kill
Peach and other stone fruits: full bloom – 10% kill at 27°F for 30 minutes or longer & 90% kill at 24°F
petal fall and beyond – 10% kill at 28°F and 90% kill at 25°F
Grape: bud burst and beyond – expect damage to begin at 30°F
Blueberry: bloom – 27°F (rabbiteye may experience damage at 30°F), fruit damage will start at 28°F
Blackberry: tight bud – 22°F, popcorn stage – 26°F, bloom – 30°F
Weather conditions leading up to the cold events and during them will impact the actual amount of damage received. A mild breeze may actually lessen the potential for damage as it could disrupt the cold air that often is found next to plant tissues and result in a slight increase in temperature there.
What can be done to protect buds?
Plasticulture strawberry growers should be using row covers for both nights where low temperatures are predicted. To maximize soil heating during the day, delay putting on the row covers until late afternoon if possible. Covers should be removed the next morning once temperatures have started to rise to facilitate soil heating during the day, but replaced in late afternoon in anticipation of low temperatures that night as well.
Options for active frost control techniques in other fruit crops are limited. Some predictions call for windy conditions, making the use of wind machines and possibly irrigation not feasible.
For homeowners, covering bushes and small trees with sheets or blankets (not plastic) can slow down the loss of heat radiated from the soil under the plants. However, windy conditions could make this difficult to do and lessen the effectiveness of a cover. If plants to be protected are close to a power source, running an electric cord and putting an incandescent light bulb in the canopy of the plant may supplement the benefit of a cover by the addition of more heat.
Irrigating over the canopy of a plant to form an ice layer is only effective if continuous water application is possible until active melting of the ice begins the following day. Building an ice layer on the plant and then shutting down the irrigation system will result in more damage than if the irrigation system was never used. Evaporative cooling will drive temperatures of the ice and whatever the ice is on below that of the surrounding air causing more damage than doing nothing.
The full impact of a frost will not be evident for a while. Growers should not make hasty judgements in regards to the amount of crop loss until adequate time has passed to adequately assess the crop status. Care, including sprays, should be maintained until a realistic evaluation of the crop status can be made. With several fruit crops, a “full crop” be set on just a small percentage of fruit buds that may be present on the plant.