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How to Protect Plants from Frost
As we get further into September, the chance of frost becomes more and more likely. In fact, the average first frost date for our zone (5a) falls between September 21 – September 30. That’s not far away! (If you’re looking for an exact date for your town or region, use this calculator.)
In spring, there can still be light frosts even after Memorial Day. That means we only get about three months of garden-growing time, making proper frost protection even more important.
Knowing how to cover and care for your plants when frost threatens usually gives a few extra weeks of garden enjoyment. Read on to learn what you can do.
Science of Frost
Frost usually occurs on clear nights when there aren’t clouds. Without clouds keeping warm air close to the earth, cold air settles near your plants. Rather than becoming dew, the water in the air forms a layer of ice crystals on unprotected surfaces.
The frost that forms can turn water within the plant’s cells into ice crystals, which stops its fluids from moving freely. This dries out its leaves and stems, causing them to shrivel and turn black.
A frost can technically occur anytime the surface air temperature reaches 36° or below. Since surface air temperature is measured several feet off the ground, the temperature by your plants will be even lower. When the surface air temperature falls below 32°, it is known as a freeze. A severe freeze (24° or below) will kill most plants.
Most weather sites will only put up a frost warning when there is a 50% or greater chance of frost. It’s always better to be prepared, and assume it will frost when the temperature falls below a certain range. After all, it’s easy to uncover, but it’s not so easy to restore your dying plants!
Since many of us live in rural areas far from a local weather station, it’s best to plan ahead. If the weather for your nearest town or a similar geographic location shows nighttime temperatures of 40° or lower without rain or clouds, plan to cover your plants.
Know Your Plants
Before getting into how to cover your plants, it’s helpful to know what plants you need to worry about.
Tropical and heat-loving plants naturally have a low tolerance for frost, and even cold in general. Peppers, tomatoes, begonias, impatiens, basil, beans, summer squash, and cucumbers are just a few plants that quickly shrivel and die if they get “frosted.” You’ll notice that many slow or stop their growth before an actual freeze comes, just because they don’t like temperatures under 50°.
Others can stand a little chilly weather and a light frost, but anything colder will permanently damage them. These include lettuce, cauliflower, peas, and many root vegetables like carrots and beets. (Geraniums fall into this category as well – here’s how to overwinter them.)
Most plants will die in a hard frost, but broccoli, spinach, onions, and other plants that do well in cool weather will likely pull through.
When in doubt, consider the sowing/transplanting instructions for your plant. Does it say two weeks before the last frost date? After the danger of frost has passed? After the soil temperature reaches 70°? This will give you a good idea of how cold-tolerant the plant is. The later in spring a plant needs to be sowed, the sooner you’ll need to cover it to keep it alive.
How to Protect Plants from Frost
The method you use for protecting a plant will depend on its size and mobility. First, take note of whether your plant is in a container or in the ground. Then consider how much space it takes up, and whether it can be moved.
How to Protect Plants from Frost in Containers
It’s best to bring small planters and hanging baskets inside. Since they are easy to move, you can bring them indoors the evening before a frost and put them back outside the next day when the air warms up.
Regardless of how light the frost is, it’s better to bring potted plants inside rather than cover them. If you keep the temperature in your house very warm, try to put the potted plants in a garage or shed where the temperature difference won’t be quite as shocking.
Protecting Garden Plants from Frost
The most common, and often fastest, method for a garden or other area with many plants in one space is to cover them with a large sheet or lightweight blanket that drapes all the way to the ground. If you haven’t saved old sheets and blankets for this purpose, you can purchase special frost blankets. For even more warmth on chillier nights, cover them with a space blanket. Put stakes around the area before covering them, so that their leaves and stems won’t be damaged by the weight of the blanket.
For small, individual plants, it’s best to cover them with a hollow object, such as a bucket. You can also purchase reusable plastic or glass garden cloches, or even make your own. Anything that will cover them without resting on their leaves and stems will do.
Another option for root vegetables and bulbs is to add mulch. While we traditionally think of wood chips, straw and leaves can also be used as mulch to keep cold (and, as a bonus, weeds) away. Cover with 3 – 6 inches of material to keep the ground warm.
Other Tips for Frost Protection
It’s important to note that you’re not just keeping the frost from “landing” or forming on the plants when you cover them. You’re making the air near the plants warmer so the water won’t freeze inside them. That’s why it’s essential to have the cover extend all the way to the ground, without any openings for air to escape. Don’t forget to use stones or other anchors if it’s windy.
It’s also important to cover the plants in the early evening or late afternoon before the warmth from the ground escapes. The residual heat from the soil will warm the plant and the surrounding air, keeping the plants free from damage. Of course, in the morning, you’ll need to remove all covers to avoid overheating your plants or bringing them out of dormancy.
Now that you’ve learned how to frost protect your plants, I hope you can get several more weeks of enjoyment from your garden! After all, winter will come soon enough. Let’s enjoy the last of summer’s bounty while we can!