Using Sprinklers to Minimize Frost Damage - Knowledgebase Question

Aliquippa, PA
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Question by krisdom1
April 27, 2001
Is it possible to prevent frost damage on flowering shrubs by hosing down the shrubs early the next morning after the frost--before the sun hits the plants? I'm referring to shrubs like azaleas, hydrangeas, etc.

Answer from NGA
April 27, 2001
Plants that are adapted to northern climates can often take a light frost. However, flowers--especially open flowers--can be damaged by a late hard freeze.

Plant tissue can be protected from freezing by sprinkling water on the plants because heat is released as water on the plant freezes. This heat will pass into whatever is in contact with the freezing water e.g. a fruit bud.

Experimentation has shown that while the ice accretion on the bud remains wet the temperature of the bud remains at approximately the freezing point, but the bud temperature will drop quickly once the water is completely frozen. Commercial sprinkler systems are designed to ensure that there is always a layer of water on the surface of the ice accretion so that latent heat is continually released.

I believe the sprinkling must take place BEFORE the flower freezes, not after the fact. I got the following info from a Michigan State University?s Fruit Crop Advisory Team Alert on protecting strawberries from frost damage:

Sprinklers can protect flowers down to 20?F on a still night. Use a nozzle size and spacing that delivers 0.1 inches of water per hour. Larger nozzles delivering more water may be needed if there is a wind. For example, a rate of 0.1 inches per hour may protect flowers to 20?F with no wind, but up to 0.6 inches per hour may be needed at the same temperature with a 5-10 mph wind. During bloom, sprinklers should be turned on when the temperature drops to 34?F, and continued until the ice is melted.

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