If you live in a climate with cold seasons, you’ve probably seen frost. Delicate plants are vulnerable to being damaged or killed when nighttime temperatures fall to the frost point, which is at or near freezing. So what exactly is frost and what can we do about it?
First, let's look at the basics. Air contains moisture in varying levels. Dew is the glistening condensation that forms just above the ground and settles on plants and grass. Dew occurs when air near the ground is cooler and more moist than the higher layers of air. Fog is also condensation from the ground air being cooler than the layers above it. Fog is always accompanied by dew, although dew can happen without fog.
Frost, sometimes referred to as hoar frost, is water vapor (or moisture in the air) that has frozen without first becoming a liquid. On clear, cool nights, when heat radiates from the ground and the temperatures fall to freezing or below at the ground surface, frost crystals form and coat low-lying cold surfaces. It can be a barely-visible coating or a thick white layer resembling a dusting of snow.
If the outdoor temperatures are lower than indoor temps, and the humidity level is higher inside the structure, frost can form inside windows, creating fascinating visible crystal structures.
When frost forms on plants, it coats all surfaces, including leaves, stems and fruit. Some plants are frost tolerant and are not sensitive to the icy coating. However, if plants aren’t capable of standing up to the freezing air temperatures or layer of frost, their internal cells are damaged, and leaves and fruit can be killed.
If a frost warning is issued and plants need to be protected, there are several ways to maintain higher temperatures at the ground level. A physical cover, such as cloth or even a layer of paper, can be placed directly on top of sturdy plants, but for young or delicate plants, posts or stakes should be used to support the sheet to keep it just above the plant tops. The covering must be removed in the early morning so the sun's heat doesn’t scorch the plants. Anything else that will hold in heat, such as a water-filled protector designed to stand up around plants, or a large container placed over the plants, will help keep frost from finding an unwelcome landing spot.
For protecting large fields or orchards, farmers have sprayed their trees and crops with water to keep surfaces from freezing. It might sound odd, but a thick layer of water won't freeze as readily as humidity in the air will. Also, wet soil holds more heat than dry soil, so watering your garden just before a frost can prevent plant damage. The air temperatures above moist soil are slightly warmer than above dry soil and can protect plants, as well. When temps fall considerably below the freezing level, it might be necessary to keep a steady flow of water on plant surfaces, to keep them protected from the cold air.