How to Protect Plants from Freezing
Freezing is a powerful force in the garden. Change the temperature just a degree, 33°F to 32°, and many plants die. When the weather forecast includes a Freeze Warning, it is warning you that the cold temperatures will kill your plants. Knowing how to protect plants from freezing starts with understanding how cold damages plants and which ones are most vulnerable.
How Freezing Damages Plants
Why is 32° so special? Because water freezes at 32° and it expands when it freezes. Most liquids stay the same or shrink when frozen, water is unusual. The expansion as water turns to ice tears apart cells. Most plants die when their cells rupture. Only a few plant have some protection, either thicker cells or some kind of antifreeze that keeps the water fluid to lower temperatures.
Usually there’s a little leeway—not much damage at 31° degrees, substantial at 25°—not because 32° is not the critical trigger but because plants can be insulated by other plants, by being next to rock that re-radiates heat from the day, because the 31° at the airport is only 33° in a neighborhood where houses and cars leak heat, and other causes of local variation.
The Difference Between Frost and a Freeze
Frosts and freezes are different. A frost occurs when water in the air freezes and settles as tiny ice crystals over everything. The temperature can be as high as 36°. Frosts are much less damaging that freezes. Freezing occurs at 32° and below.
How to Protect Plants from Freezing
To protect plants from freezing, add a covering so the cold takes longer to reach them. Old sheets, blankets or tarps, lots of things work. Put them out before sunset, so any heat from the day is captured. If the freeze is combined with rain or snow, build a shelter on stakes or baskets so the cold water doesn’t reach the plants, which will undo your work. Make sure your covering reaches the ground so cold air doesn’t creep in below it. Water the soil deeply before the temperatures drop; wet soil insulates. Remember to remove whatever you covered the plants with in the morning once it warms above freezing.
A thick layer of mulch will give added protection against root damage.
For more help choosing and using covers, read our guide to protecting your garden from frost.
You may read about fruit orchards spraying their plants when frost is predicted. That is a complicated and tricky solution. The fine mist coats the twigs and then freezes. The ice insulates the twigs; even if the surrounding air becomes much colder they stay at 32°. Water has to be steadily applied all the hours below freezing, which means monitoring it for hours. Effective for protecting a peach or orange crop, this isn’t something most home-owners can do.
Which Plants Need Frost Protection
Generally, the best approach is not to plant frost-sensitives before your climate’s frost-free date, and to harvest or take indoors frost-sensitive plants when a freeze warning is given.
Which plants are frost-sensitive? Most plants. Surviving frost is rare in the plant kingdom; being badly injured or killed by it is usual. To be specific: among the vegetables, tomatoes, potatoes and sweet potatoes, eggplants, okra, most beans, corn, cucumbers, squash, and pumpkins are killed by moderate freezes. Herbs especially basil and rosemary and garden flowers like zinnias, nasturtiums, petunias, and impatiens are sensitive. Frost will kill the houseplants you set outside for the summer such as begonias, and succulents like jade plant and kalanchoe. Tropical plants like bananas, avocados, and citrus are very frost-sensitive. We grow a lot of tropical plants as annuals, hoping to harvest the food before winter. These plants, in a mild climate, grow on and on for many months. But a frost breaks their cells and they die. These will be seeds that the seed packet says “plant after the threat of frost has passed.”
Plants that are perennial in cold climates have to tolerate freezing. You don’t need to cover them. They go dormant in winter, the roots underground and the branches and twigs of shrubs and trees (but not the leaves) surviving until next spring. A few plants, mostly pines and their relatives, are evergreen, keeping their leaves despite very cold temperatures.
In between the plants that freezing immediately kills and the shrubs are the herbaceous plants of northern climates that come up in the early spring and can endure some cold. Broccoli, cabbage, mustards, carrots, beets, radishes, Swiss chard, peas, onions, lettuce and other greens, for example. We treat these as annuals. Hardy perennials will be killed aboveground by a hard freeze but return the next spring. These include strawberries, culinary sage, parsleys, thyme, garlic and chives, crocuses and daffodils, violets, and sweet alyssum. They will withstand a night with a low of 29° but many hours of below freezing temperatures destroy their shoots and leaves. Mulch them before winter sets in and watch from them in the spring.
For seasonal care, see our guide to preparing your garden for winter.
We cannot make freezing kinder to plants. What we can do is be aware of it and take appropriate action.
Explore more options in our guides to overwintering plants and crops and using cold frames in the garden.