If you want to grow vegetables in winter or early spring, you’ll need to learn how to protect your plants from frost and freezing temperatures. Even in the more southern locations, frosts and freezes do occur so it’s best to be prepared.
Some vegetables, called warm-season vegetables, should only be planted when the risk of frost is gone. Others called cold-season vegetables will do fine with frost and freezes with some protection depending on their hardiness. These different vegetable types are listed in this article on growing winter vegetables.
Fortunately, there are many ways to protect your cold-season vegetable plants from frost, but first here’s a little explanation on what a frost and freeze are.
The Difference Between a Frost and a Freeze
You may be surprised to learn that a frost and a freeze are not necessarily the same thing and do not necessarily happen at the same time. It has to do with the amount of moisture in the air and the surface air temperature.
Firstly, you can see a frost. It’s those lovely mornings when the sky is blue, the air is fresh and the ground is covered with a pretty white layer of icicles. You walk on the ground and you may hear or feel a slight crunch. Or your car is covered with a thin layer of ice crystals, which a few swipes of the wipers will clear away. It’s freezing cold, but not necessarily below freezing point.
A freeze, on the other hand, you don’t necessarily see. It’s when the temperature in the air is below the freezing point. And, it really is freezing. There may or may not also be a frost.
Frost and Freeze Conditions
To be more precise, a frost can occur when the surface air temperature falls below 36 degrees Fahrenheit, whereas a freeze occurs when the surface air temperature reaches 32 degrees Fahrenheit or lower. In either case, whether a frost happens or not depends on the moisture in the air, and it won’t happen if it’s windy or cloudy or raining.
For growing vegetables, there’s one more distinction to mention and that’s when a hard freeze occurs. This is when the temperature falls below 28 degrees Fahrenheit, when only truly hardy vegetables will survive – and with some help from you.
If you’d like to know more about the differences between frost and freezes and all about dew points and relative humidity, this article explains in a lot more detail.
How Frost Affects Your Vegetable Plants
When temperatures fall and frost or freezing occurs, water in the plant cells turn to ice crystals and expand, causing the cell walls to burst. This results to damage and possible death to that part of the plant. Leaves and new growth tend to be the first affected, so the plant itself may survive with some damage.
The more cold-hardy vegetables, however, can survive light frosts and freezes since they start to produce sugars when the temperature drops. Sugar water freezes at a lower temperature than water, so the water in the plant cells does not freeze and no cell damage is caused.
The slower the weather cools down the better since it takes a while for the plants to produce sugar. This sugar defense in cold weather is why frost-tolerant vegetables improve their flavor after a freeze or two.
How to Protect Plants from Frost
For a short light frost or freeze, hardy plants do not need to be covered. However, you can take these measures as an extra precaution:
Water Your Plants
Keep an eye on the weather forecast and water your plants a few days before the temperature drops to keep the soil is moist. This seems counter intuitive, but damp soil holds the heat better and as the water in the soil cools at night it releases the heat and protects the plant.
If you want to go a little further, place a jug of water beside each plant to give off more heat as the temperature drops.
There are many benefits to using mulch, but one of the main reasons to mulch in winter is to protect from freezes. For that reason, the best winter mulches are loose, slightly coarse materials that will provide a protective cover but will also allow water and air to flow through.
Wood chips, straw, fallen leaves, various barks and pine needles are examples of suitable winter mulch materials. Spread a layer about 2 to 3 inches thick leaving a space around the base of the plant. Remove the mulch as soon as the threat of a frost has gone to allow the soil to warm up as spring approaches.
Semi-hardy plants will need some kind of cover, as will hardy vegetables when temperatures drop below 28 degrees Fahrenheit.
Cloth or Synthetic Cover
The simplest and easiest way to protect your plants from frost is using something you have in your home to cover the plants overnight, such as old sheets, fabric tabletops, towels and plastic. Make sure they are clean and store them in a place where vermin can’t get at them.
Put stakes in the ground and drape the covering over them so they won’t touch the plants. Then, remove them by mid-morning so they will benefit from the daylight.
Floating Row Covers
You may find it easier to buy covers designed specifically for this purpose. They are called various names such as floating row covers, garden fabric and plant covers, and they’re made of non-woven materials such as polypropylene or polyester.
Row covers are lightweight, breathable and durable and they allow air, light and water to reach the plants. Therefore, they don’t need to be removed during the day which is a necessary hassle with heavier covers. You will need though to make a point to check underneath row covers for pests and weeds from time to time.
If you want to get a little more sophisticated, you can use PVC pipes to create arches and lay the row cover over them to create a mini hoop house. You’ll need to attach the cover to the PVC with clamps to stop them blowing off and to the ground with some heavy items like large rocks or bricks. By raising the covers in this way, airflow is increased, and plants have room to grow without risking damage.
As for individual tall plants, they can be wrapped in row cover material like a bandage, or covered with plant covers designed specifically for this purpose.
So, floating row covers save you time and help your plants at the same time. They are available in different weights and sizes, so be sure to check the package carefully before you buy. Your plants will do best with row covers that let in at least 80% of daylight. You can always double up for extra protection on extra cold nights.
Cloches and Bell Jars
Cloche (pronounced like posh) is the French word for “bell.” The original cloches were made of glass and were bell-shaped to be placed over plants overnight. Nowadays cloches or domes are made of plastic and come in various shapes and sizes to cover an individual plant or a small group of plants. A pack of heavyweight plastic cloches are inexpensive to buy.
Because some plastic materials are light, some cloches may need to be pegged down in windy weather. You may need to remove them during the day if the weather is warm since they will trap the heat. You probably won’t need to water your plants as often, but you should still check your plants daily.
Alternatively, there are many sources of cloches that may already be in your home. Depending on the size of your plants you could use cake protectors, large drinking glasses, flower vases, buckets, plastic bottles or jugs with the bottom cut off.
This video demonstrates 5 low cost frost protection ideas:
Cold frames are bottomless boxes with a transparent “roof” on hinges to let the light in. A typical size might be 3 feet x 6 feet with a depth that slopes from 12 inches at the front to around 18 inches at the back. But the size will depend on how much space you have and what vegetables you want to grow in them.
Cold frames can be constructed from wood or plastic or any material such as bricks that can sit on the soil and form a protective frame. If you have a raised bed, you can even place a sheet of glass on top to create a temporary cold frame.
If you’re like most people and not inclined to make your own cold frame, you can purchase a cold frame kit. They come in many shapes and sizes, some are tall like mini greenhouses, and they are all fairly easy to assemble.
How to Protect Plants in Containers from Frost
Move vegetables and herbs in containers indoors for the winter months. However, if that’s not possible – if there is not enough light, for example – there are a few steps you can take to protect them from the frost.
- Move the containers so they are next to a wall or fence.
- Lean a sheet of material such as wood or glass against the wall, as you would a ladder, to protect them.
- Place the containers on bricks or large stones to raise them up from the ground to allow for drainage.
- Wrap containers that are liable to crack in insulating materials such as bubble wrap or burlap.
- Wrap tall plants in row cover material like a bandage, or cover with individual plant covers.
Tender herbs, like basil, chives, cilantro, dill, mint, parsley and tarragon will not do well in cold weather. If you don’t have space for them indoors, its best to harvest them and preserve them by drying or other means.
Don’t be caught out by sudden cold nights that could ruin your crops. Use the information in this article to decide how to protect your plants from frost then gather together the materials you’ll need to protect them so you are prepared. You will be rewarded with a healthy, bumper harvest in the winter or spring.
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