3 Ways to Insulate Your Plants Against Spring Frosts

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.

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 flavor and texture of cool-season greens. Young seedlings can be protected from light frost with an old sheet or piece of row cover 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 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.

Cold Protection For Your Crops

1. Cover Plants with Plastic Jug Cloches

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

To cut a plastic jug 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 jug. Pop the jug 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 jug cloches with blankets or old towels to add several degrees of cold protection.

2. Use a Row Cover Tunnel

One of my favorite garden projects is to plant an array of spring veggies under a row cover tunnel. 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 row cover tunnel 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.

3. Insulate With a Warming Blanket

Old blankets or bedspreads can add ten degrees or more 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.