Spring Frost – The Grim Reaper

by Gary Mount

One of the very great risks in growing fruit in New Jersey is that freezing temperatures around bloom time, known as spring frosts, can kill buds or blossoms and devastate the crop. On spring nights, freezing air can move into the fields or on clear, windless nights, objects near the ground, such as fruit buds can radiate heat right up to the sky and become even colder than the surrounding air. When that happens, there can be frost even though the air temperature is above freezing.

This freezing and killing of fruit buds is not good. Dead buds mean no fruit. Without a consistent production of fruit, well, there goes the farm. But farmers have been trying to control frost since ancient times when grape growers burned brush in their vineyards. Modern farmers also try heating. Sometimes they use fans to mix the lower level cold air with the warmer air from above. And sometimes they try irrigation, which is what I am going to try this year on my strawberries.

Irrigation is one of the most effective methods available to help a farmer survive a frost. But it’s not that the water being applied is 55 degrees and warms the plant. Actually, when the water first hits the plant and starts to evaporate, the plants temperature cools a few degrees. Farmers start irrigating a few degrees above freezing to avoid causing worse damage than that which they are trying to control.

Frost protection by irrigation works from a scientific principle: as water freezes it liberates heat. If this seems confusing, think of how ice melts when heat is added. The reverse of the process, making ice, gives off heat. As long as enough water is applied continuously, ice keeps forming and the temperature of the strawberry bud does not go below freezing. Ice may build up, but the freezing process stabilizes the temperature even when the surrounding air temperature is well below freezing.

This process releases considerable heat. A gallon of heating oil burned in a heater releases about 144,000 BTUs of heat into the field or orchard. That same gallon used in a diesel-powered pump could spray about 14,000 gallons of water on the field. If all this water froze into ice it would release over 16,000,000 BTUs of heat — 120 times more!

Planning and constructing an irrigation system for frost protection has been interesting. First, a water supply and reliable pump is needed. My new well and pumping system will be adequate to do the job. Then there is the layout of sprinklers and piping in the strawberry patch. Fortunately, a lot of written information is available. My frost protection folder is now about four inches thick! And, finally, knowing when to turn the irrigation on is important. Over-irrigation is expensive and can damage the plants. Waiting too long might just waste the whole effort.

I have consulted with some of my fellow farmers about this. Some of them get very little sleep during strawberry bloom time. They put a sleeping bag in their pickup truck, park out by the field, and wake up every half hour or so to check the temperature. There is no room for error. In extreme cases, temperatures can drop as much as 9 degrees in as little as 15 minutes!

The trouble with these methods is that I like my sleep. The prospect of being awake night after night, followed by working all day does not appeal to me. Fortunately, I have a device that warns me if the temperature falls in my greenhouse. This machine dials my phone and announces, “The temperature is low.” I have a spare device that I will use for the strawberries. When I hear the message, it means I had better get out there and start the irrigation.

This spring, if you drive by early some morning and see the irrigation running on the strawberries, you’ll know why. And when you come back in May to pick some strawberries for dinner, you’ll know that just maybe those berries came from buds that were safely protected under the ice this spring.