3 Ways to Insulate Your Plants Against Spring Frosts

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Upturned pots covering plants for an extra layer of protection inside a tunnel cloche

Most people check the weather forecast each day to decide what to wear, but gardeners have bigger concerns. Will the precipitation come as rain or snow? Will it get cold enough to hurt the lettuce? To some extent you will need to guess your way through turbulent spring weather, but cloches and covers can keep plants safe while adding a month or more to the front end of the growing season. Here is a quick guide to reducing damage from spring frosts and freezes in the vegetable garden.

Cold-tolerant greens like kale shrug off frosts

4 Types of Frost

First, a review of the language of frost, which is always a matter of degree. Humidity and air movement play important roles, but in general there are four levels of frost:

Light or scattered frost develops when temperatures range from 32°F to 38°F (0°C to 3°C). Light frost rarely damages plants and may enhance the flavour and texture of cool-season greens. Young seedlings can be protected from light frost with an old sheet or horticultural fleece spread over the ground.

A heavy frost or light freeze occurs at temperatures between 29°F and 32°F (just under -2°C to 0°C). Cool-season greens are rarely damaged, but potatoes may be nipped back to the ground. Covers and cloches do a good job of protecting plants from light freezes.

A hard frost can mean the end for unprotected crops

A damaging freeze or killing frost involves temperatures from 25°F to 28°F (just under -4°C to -2°C) for more than four hours. Temperatures this low will kill tomatoes to the ground, and may damage fruit tree blossoms and young fruit. The blossoms of spring-blooming shrubs turn to brown.

A hard or severe freeze comes with temperatures below 24°F (-4°C) for more than four hours. Under these conditions, fruit trees that have already bloomed are severely impacted, and unprotected seedlings sustain serious damage. Doubling up on protection, so that seedlings have both cloches and cloth covers, can make a huge difference.

Cloches made from plastic bottles are ideal for use with cabbage family crops and widely spaced lettuce

Cold Protection For Your Crops

1. Cover Plants with Plastic Bottle Cloches

For widely spaced plants such as kale and broccoli, cloches cut from large plastic bottles are game-changers. I have used purchased plastic cloches, and they were not as dependable as plastic milk bottle cloches, which can be secured in the ground with a long stick pushed through the handle. Each spring I “borrow” some bottles from the recycling centre, use them for a couple of months, and then return them to the waste stream.

To cut a plastic milk bottle cloche, start by using a sharp knife to make a V-shaped cut in the top of the handle, big enough for a slender stick. Then use heavy scissors to cut off the bottom of the bottle. Pop the bottle over a plant, push a stick down through the handle, and you are in business. I generally discard the tops after the weather starts to warm so the cloches can vent freely. Should the weather turn very nasty, you can cover plastic bottle cloches with blankets or old towels to add several degrees of cold protection.

In addition to taming spring weather, tunnel cloches hide vegetables from rabbits and deer

2. Use a Tunnel Cloche

One of my favourite garden projects is to plant an array of spring veggies under a tunnel cloche. I love how the broccoli, cabbage, and other cool-season plants perk along out of sight. I secure the long edges of the tunnel with heavy boards, and when I open the tunnel for weeding, it seems like a miracle!

Yet sometimes there is trouble in paradise, for example heavy wet snow that will flatten a tunnel cloche in no time, or a sudden plunge in temperatures. Rather than risk disaster, I place small flowerpots or boxes over the seedlings inside the tunnel until the storm passes. Installing covers during the day traps a few degrees of heat.

Save old blankets to use for temporary protection of early plantings in the vegetable garden

3. Insulate With a Warming Blanket

Old blankets or bedspreads can add several degrees of cold protection. Use hoops or stakes to hold blankets aloft, particularly in wet weather. Plants don’t mind being deprived of light for a couple of days when it means snug protection from a hard freeze.

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Show Comments


"If cold is going to stay or if you want to start really early use a 6 mil clear plastic over your hoops and some old christmas lights(the kind that heat up). Hang them from the hoops and you will then have a mini greenhouse! I have had lettuces, chard and kale well into November here in zone 4-5 in the US"
cindy Rains on Friday 17 March 2023
"Thanks for the tip, Cindy! I have used old string lights in my chicken coop, too."
Barbara Pleasant on Saturday 18 March 2023

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