It’s not really autumn in the garden until the weather service starts issuing frost advisories. That’s when we have to kick it into gear and get those final chores done. That's often easier to do than covering all the tender plants you've put out in spring when a late frost threatens. Thankfully, frost warnings are only predictions. There is still hope the frost will hold off, but there is also a chance it could be worse than a frost.
It could be a freeze. What’s the difference?
Before we tackle frost versus freeze, let’s start with what an “advisory” is. Advisories are issued for a time frame of at least 3 hours up to about a day of weather when there is a good likelihood of excessive cold temperatures. They break down as follows:
Frost Advisory - This is when the temperature is expected to fall to 36 degrees F. Down to about 32 degrees F.
Freeze Warning - Issued when there is at least an 80% chance that the temperature will hit 32 degrees F. or lower.
Hard Freeze - When the temperature falls below 28 degrees F.
All Frosts are Not Created Equal
Light Frosts - As air temperatures cool, the ground also begins giving off heat. This is called radiational cooling. Generally, the clearer the sky, the more heat is given off. For a frost to kill plants, the earth has to lose enough heat so that the 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 your plants are killed by frost but the lower portions remain green. We tend to refer to this as a light frost.
Frosts tend to be short-term events that occur overnight or in the early hours of the morning.
Hardy plants may come through unscathed, but tender plants will probably suffer some damage and begin their season-ending decline. It’s one of the reasons the USDA designates their hardiness zones by the first and last expected frost dates.
Hard Freeze - Freezes are usually caused by what is called advective cooling. Advection is the transference of some atmospheric condition, like heat, humidity, or cold, by the movement of an air mass. Think arctic blast; when one of these blows in, all the annual plants and tender perennials tend to call it a day
Freezes can be quick or they can linger. If they are cold enough, even a short freeze can do a lot of damage. Even hardy perennials will be killed back to ground level by a long, hard freeze. This is when the ground crunches when you walk on it
It’s interesting to note that frost and freeze advisories are only issued during growing seasons, both spring and fall. If the plants are dormant, it’s just called cold weather.
Protecting Plants from a Frost
Cover Them - There’s nothing we can do to stop frosts and freezes from happening, but we can reduce the damage they do to our 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 at hand that will cover large plants or borders.
If the air cannot settle on your plants, they should not be hurt.
Be sure to remove the coverings once the temperatures have warmed again the next day, but keep them close at hand. You can be sure there will be more advisories to come.
Water Them - It may sound counterintuitive, but water can insulate. Wet soil can hold 4 times more heat than dry soil, and water sprayed directly on plants will form a layer of ice that insulates the plant underneath it. This only works for a frost; don’t push your luck and try this during a hard freeze.
It is important to keep your plants, including trees and shrubs, well watered until the ground has frozen. They may be dormant, but their roots are still active and need water. Once the ground has frozen hard, it’s time to protect marginally hardy plants by putting down a winter mulch to keep the soil frozen.
You don’t want repeated freezing and thawing to push the plant’s crown out of the ground.