Like many gardeners, when Autumn comes around I obsessively check my weather app for signs of frost or a hard freeze. I put a lot of work into growing a late summer and fall garden, and know that my future harvest depends on protecting plants from frost.
Luckily, it’s easy to protect plants from frost if you understand which vegetables need protecting, and have tools at the ready. Let’s dive in!
What is a Frost?
A frost is a thin layer of ice that forms when water vapor changes from a gas to a solid. It occurs when temperatures go below the freezing point. A light frost happens between 28-32°F, but it’s not until the temperatures dip below 28°F that many garden plants face damage.
Frost injury occurs in plants when water in their cells turns into ice crystals, stopping the movement of fluids and damaging the plant’s tissue. The tissue appears water-soaked and soggy, becoming darker in color and limp over time.
In addition to garden vegetables, new perennials, bushes, or trees planted outside can be vulnerable to frost damage, so you’ll want to pay attention to them as well.
What Plants Need to Be Protected from Frost?
Frost does not affect all plants equally, so it’s valuable for gardeners to understand the cold tolerance of your plantings.
Tender, or cold-sensitive, plants are easily damaged or killed by frost unless you offer protection. All summer, or warm-season plants like tomatoes, cucumbers, basil, peppers, corn, and green beans fall into this category.
Citrus trees, many annual flowers, and common houseplants are also tender, as are young seedlings and sprouts, even of perennial plants or cold-hardy plants.
Hardy plants handle frost and freezing temperatures well. Some annual vegetable plants are cold-hardy, such as cabbage, kale, broccoli, cauliflower, carrots, radishes, beets, and others.
In cold conditions, some plants convert the starches in their cells to sucrose (sugar) to act as a natural “anti-freeze.” Therefore, you will find that many of your favorite cold-hardy vegetables actually taste better after a light frost! (Try a frost-sweetened carrot – you’ll never go back!)
The cold tolerance of cold-hardy plants varies significantly, from plant to plant and variety to variety. For instance, 19F temperatures last week lightly damaged my “Lacinato” kale, but did not affect my “Vates” kale at all. Brussel sprouts are still thriving without any cover, while most of my cabbages are surviving only because of a row cover.
For a wonderful resource on cold tolerance in vegetables, I recommend reading this article by Sustainable Market Farming. They’ve been keeping records of winter kill temperatures for years, so when I forget to keep my own records, I remind myself which veggies to worry about using their lists!
Know Your Frost Dates!
If you don’t already know your average first/last frost dates, use an online (or other) tool to look up your average first frost date in the fall and your average last frost in the spring, and mark them on your calendar.
Average first/last frost dates are typically based on 30 years of historical temperature data for your location, but your actual first/late frost dates may vary.
For instance, my published first frost date is roughly September 18, but this year we did not get a frost until mid-October. My average last frost date is May 24, but most local gardeners will not plant tomatoes and other tender plants until Memorial Day weekend.
As your average first frost date approaches, pay attention to your daily forecast. Keeping an eye on the weather is an essential task for gardeners, and certain environmental conditions increase the chance of frost.
Cloudy nights insulate the earth from swings in temperature that might cause a sudden frost, but clear skies at night allow heat to escape into the atmosphere. Another factor that increases the likelihood of a frost is calm conditions without any wind. A lack of air movement means that warmer air currents cannot move over the ground and protect it from frost.
Pick Appropriate Plants for Your Planting Zone
An important step that you can take to protect plants from frost is to pick appropriate plants for your growing zone. For example, I know that in my zone 4 climate, warm-loving annuals like okra, sweet potatoes, or eggplants require extra love to thrive in my short gardening season. Cold-hardy plants like Brussel sprouts, carrots, and spinach are real stars, tasting even more sweet when the cool Autumn temperatures arrive.
I select seeds that are appropriate for short season growing and favor cold-tolerant vegetables with names like “Winter King.” And regretfully, I know that I should not plant warm-loving fruit trees like citrus and figs in the ground.
How to Protect Plants from Frost
Now that we’ve covered the basics, let’s talk about some practical ways that you can protect plants from frost.
Bring Potted Plants Inside
When you see a frost on the forecast, move your potted plants and hanging baskets inside in the evening. Container-grown plants are prone to frost damage because they lack the earth insulation that in-ground plants benefit from.
When moving potted plants inside, avoid a sudden change in temperature. Ideally, move your container plants to a garage, shed, or basement.
You may need to do this frost-avoidance dance for a few weeks, but you will have the pleasure of enjoying your container plants a bit longer!
If you have space indoors, certain plants will do just fine in a sunny window. I have a potted rosemary plant sitting next to my desk right now; come summer, it will go back outdoors onto my sunny porch.
Use a Thick Layer of Mulch
A thick layer of mulch on your plants is akin to wearing a warm woolen sweater when the temperatures dip. Spreading a 3-6” layer of mulch over your in-ground garden protects the soil and the plants’ roots from temperature changes.
You can mulch with any of the following:
- Straw/Mulch hay
- Wood Chips
- Leaf Mold
- Grass Clippings
- Shredded Leaves
Grow in a High Tunnel or Greenhouse
In an ideal world, those of us growing in cold climates would have access to a high tunnel or heated greenhouse to grow food year round, but that is not always possible or affordable.
Still, if you have the space and money to invest in a high tunnel, they are wonderful additions to your garden. We purchased a 20 x 50 foot high tunnel for our Oregon homestead and grew year-round in it. Winter months brought crisp carrot and kale harvests, while summers gave tomatoes and peppers the extra boost of warmth they needed to produce abundant crops.
This year I’m going to be making 7 ft gothic tunnels over my beds, using a friend’s design and equipment list. By his calculations, this will give me covered growing space for under $3 a linear foot.
Cover Your Plants with a Blanket
One of the best ways to protect plants from frost is to cover your plants with a frost blanket or a row cover. While you don’t need to purchase row cover – blankets, bed sheets, towels, and drop cloths all work well – I like to purchase row cover and use it for both frost protection and insect protection. I use Agribon AG-19, which gives me about 4 degrees F frost protection and allows 85% light transmission, so it’s appropriate to use as an insect cover.
I use thin wire poles from Johnny’s Seeds to create hoops over my garden beds, then drape row cover over to create a low tunnel. I use rocks or sandbags to weigh down the cover all around the bed and at the ends.
If you’re facing an even heavier frost, then add a layer of plastic over the blanket. This adds a layer of insulation for the heat from the soil.
I haven’t tried this myself, but I have heard of using mylar thermal emergency blankets to trap the heat, and reflect heat into the soil. (Just make sure you remove the blanket before the temperature increases the next day, or you might fry your plants!!)
Place Hot Water Jugs Under Row Cover
Here’s another interesting way to protect plants from frost: Fill plastic milk jugs with hot water and set the jugs around your plants as night falls. Then, cover the beds with row cover or a blanket, using rocks or sandbags to keep the edges of the cover down.
The hot water jugs will slowly release its heat, creating a microclimate of warmth.
Make Individual Cloches for Your Plants
A cloche is a bell-shaped cover that gardeners make from plastic or glass that keeps your plants cozy as the temperatures dip low. Many people use milk jugs with the bottoms cut off, but you can buy reusable plastic or glass garden cloches.
If you get creative, a lot of things around your house can become DIY garden cloches.
For example, a large Rubbermaid tote can cover more than one plant in your garden. In addition, an upside-down bucket, an upside-down flower pot, juice containers, and plastic bowls are some great options for homemade garden cloches.
String Christmas Lights Around Your Plants
Although I have yet to try this, I have seen other growers string Christmas lights around plants or fruit trees before placing a row cover or blanket. The lights (choose non-LED ones) emit a small amount of heat that may be just the boost your plants need to be protected from an impending frost. This may be particularly useful during an unexpected spring cold snap.
Wrap Fruit Trees
If you fruit trees, young ones between one to four years old may be more sensitive to frost injury.
If trees have buds or blossoms in the spring, a hard frost might stunt the growth, leading to a reduced harvest throughout your growing season. If you have citrus trees, then you need to be particularly careful about low temperatures. Anything below 29°F might cause severe to permanent damage to your trees.
Using towels, blankets, cardboard, rags, pipe insulation, burlap, or felted tree protector wraps, start at the base of the trunk and wrap around the trunk, making sure to overlap the layers every few inches. Then, use twine or weatherproof tape to secure the wrap.
What to Do With Frost Damaged Plants
Frost-damaged plants have leaves and branches that turn black or brown. When the weather warms up and the danger of any frost passes, then, it’s time to prune away the damaged foliage.
If a spring frost “touched” the tops of your plants, a bit of pruning is all you may need to set the plant back on the right path! This is especially true of something like basil – a plant that is tender, but greatly benefits from pruning!
Sometimes, a plant looks wilty and dead but will keep growing and producing! We had a 13 degree F night last night, and my Brussel sprouts look pretty sad. But I bet when it warms back up to the thirties this week, we will find some perfectly good sprouts under the droopy leaves!
I’m a huge fan of growing food well into the fall and winter. But successful fall gardening demands careful attention to frost and protection of your crops.
For more information about how to plan and plant a fall garden, including a list of cold-hardy vegetables and varieties, check out my book, Homestead Honey’s Guide to Fall Gardening.
If you have any questions about protecting your garden from frost, or extending your growing season, please leave them in the comments!
And as always, if you find this article valuable, please share, or subscribe!
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