Watching the Frost Grow
Fall’s first frosts form at first light
Most first frosts of the fall season occur within a few minutes before sunrise. At 7am on October 29, 2007, the lawn at Upakrik Farm was a lush green from the previous week’s rain. The blades of grass were laden with a heavy dew. The sky was clear and the air was quiet.
As I stood by the kitchen window at 7:15am, the outdoor temperature on the indoor-outdoor thermometer was 42 degrees with the sensor within inches of the outside brick wall, seven inches above the ground. These were ideal conditions to watch frost form on the lawn. By 7:18, I could see the frost form on the grass; by 7:25, the lawn was white. During that short time, temperatures dropped from 42 to 38 degrees This means that temperatures near the ground were between 32 and 36 degrees.
The coldest part of the day typically comes just before sunrise, especially when the air is quiet and the sky is clear.
I experienced this phenomenon many times in my research on winter-protection systems for container-grown ornamental plants.
Because of sod’s density, the leaf blades are insulated from the soil, allowing dew to freeze on the blades of grass. You will never see early frost accumulate on compacted exposed soil, on asphalt or on concrete because heat can escape, producing high temperatures.
However, if you were to cultivate or rototill the soil just a day or so prior to a frost, it would be possible for frost to form on the ridges of the soil.
Frost can occur at temperatures between 32 and 36 degrees because the dew is always contaminated by dust particles or microorganisms. Contaminated water has a higher freezing point than pure water and thus freezes at a higher temperature than pure water.
As the fall progresses, frosts will come earlier and earlier in the morning hours due to colder temperatures and the ground’s loss of heat.
The injury that results from early or late frost on non-cold-hardy plant tissues occurs when the ice melts. It takes heat to melt ice, and when the heat is drawn from the plant tissues, the tissues dehydrate and die.
Early frost is good for certain garden vegetables such as collards, kale, cabbage, broccoli and spinach. When these crops are exposed to a frost or freeze, they become sweeter. The cold temperatures of a frost trigger enzymes in the leaf tissues to convert starch to sugars. Placing collard greens in the freezer for an hour or so prior to cooking them will accomplish the same as waiting for the plants to feel the frost before harvesting.
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