For many regions, it doesn't truly feel like autumn in the garden until the weather service starts issuing frost advisories. That’s when you have to kick it into gear and get those final chores done to prepare your garden for winter. While a frost advisory is usually not a major concern for garden plants, there's always a chance that an anticipated frost could become a freeze. So it's important to know the difference between a frost and a freeze—and what each can do to your garden.
Frost and Freeze Advisories
Before a frost or freeze, weather services will usually issue an "advisory." Advisories typically are put out for a time frame of anywhere between a few hours to a whole day when there is a good chance of unusually cold temperatures. The advisories generally fall into the following categories:
- Frost advisory: This occurs when the temperature is expected to fall to a range of 36 degrees Fahrenheit down to about 32 degrees Fahrenheit.
- Freeze warning: A warning is usually issued when there is at least an 80 percent chance that the temperature will hit 32 degrees Fahrenheit or lower.
- Hard freeze: This takes place when the temperature falls below 28 degrees Fahrenheit.
Frost and freeze advisories are only issued during the growing season—both in spring and fall. During the winter while plants are dormant, there's no need for an advisory.
As air temperatures cool, the ground begins to give off heat. This is called radiational cooling. Generally, the clearer the sky, the more heat the ground gives off. For a frost to kill plants, the earth has to lose enough heat that a freezing temperature occurs at ground level. If the ground is still warm, it is possible for the frost to hover a bit above ground level. That’s what happens when the tops of plants are killed by frost, but the lower portions remain green. This is generally referred to as a light frost.
Frosts tend to be short-term events that occur overnight or in the early hours of the morning. Hardy plants can come through unscathed, but tender plants will often suffer some damage and begin their season-ending decline. It’s one of the reasons the U.S. Department of Agriculture designates its hardiness zones by the first and last expected frost dates.
Freezes are usually the result of advective cooling. Advection is the transference of some atmospheric condition, such as heat, humidity, or cold, by the movement of an air mass. Think of an arctic blast: When one of these blows in, all the annual plants and tender perennials tend to die off for the winter.
Freezes can be quick, or they can linger. If it's cold enough, even a short freeze can do a lot of damage. Hardy perennials can be killed back to ground level by a long, hard freeze. This is the type of freeze that causes the ground to crunch when you walk on it
Protecting Plants From Frosts and Freezes
There’s nothing we can do to stop frosts and freezes from happening, but we can reduce the damage they do to our plants. Try these techniques to protect your plants during frosts and freezes.
Cover Your Plants
When an advisory is issued for your area, pull out the row covers or grab some sheets, blankets, pots, baskets, or whatever you have that will cover your plants. If the cold air can't settle on the plants, they shouldn't be hurt. Be sure to remove the covers once the temperature has warmed the next day, but keep them close. You can be sure there will be more advisories to come.
Water Your Plants
It's important to keep your plants, including trees and shrubs, well watered until the ground has frozen, as their roots are still active and need water. And there's an extra benefit to watering. It might sound counterintuitive, but water can insulate a plant from the cold. Wet soil can hold more heat than dry soil. Plus, water sprayed directly on a plant will form a thin layer of ice that will insulate the plant underneath it. However, watering only works for frost protection, so don’t push your luck and try this during a hard freeze.
Put Down Winter Mulch
Once the ground has frozen hard, it’s time to protect perennial plants that you want to come back the next year by putting down winter mulch to keep the soil frozen. You don’t want repeated freezing and thawing due to sunlight or changes in temperature to harm the plant. A brief warming can trigger the plant to come out of dormancy and send up tender growth that normal winter temperatures will quickly kill, weakening the entire plant. For mulch, most insulating materials will do, such as straw, pine needles, or shredded leaves. Remove the mulch from the soil come springtime.