How do you go about protecting plants from frost damage?
Before you do anything else, look at the average minimum temperature for your area as listed on the plant hardiness zone map for your area.
Compare that to the minimum temperature requirements of your plant. You can often find that on the plant's label, online, or in plant information books.
It may be that your plant (like the kale in the photo) doesn't need protecting at all!
The bigger the difference between
your plant's minimum temperature requirements and your area's hardiness
zone, however, the more protecting you'll need to do in order to keep your plant
alive during the winter.
Some hardy plants
But the main strategies plants use to survive the winter have to do with protecting their roots and sending those roots below the frost line. As long as the roots remain unfrozen, even if the upper part dies back, the plant will return when the weather warms again.
So you want to plant early in the year to give your plant the best chance to get its roots deep into the earth before cold weather comes.
Also, put your plant in the warmest area of your yard.
This will usually be in full sun, next to a south-facing (or north-facing, for those of you in the Southern Hemisphere) wall, with wind protection to prevent wind chill.
Brick or stone walls are best, because they will retain the heat of the day and release it overnight.
You can potentially get a good five to ten degrees Fahrenheit (2.8 to 5 degrees C) rise in the temperature surrounding your plants by using these strategies -- up to a whole zone improvement!
This isn't as much about protecting plants from frost as in improving the microclimate your plants are in to fit their needs.
Use raised beds in areas with a lot of snow. The ground will drain better and warm faster in a raised bed than in a regular bed, especially if you use dark-colored material to build the raised bed from.
If your plants can tolerate being planted deeply, do so. Some plants are very sensitive to how deeply they're planted, but some don't seem to care. Some, like tomatoes, prefer to be planted deeply. The soil temperature is more stable the farther down you go, so if you can plant more deeply, it's a good idea.
Mulch, mulch, mulch!
Mulching puts a layer of insulation between your plants roots and the freezing weather. The deeper the layer, the warmer your plant will be, just as if you put a thick coat on versus a thin sweater. This can go a long way in protecting plants from frost and freezing.
Dark-colored wood/compost mulch and rock/gravel mulch are both retain heat -- which you use will depend on your plants and what they need. Rock and gravel are better if you need good drainage, while dark wood or compost mulch will add nutrients later on.
Both soak up the sun's heat and transfer it to the soil, warming it and protecting plants from frost.
Row covers are rolls of special fabric used to cover your plants. While mulch covers the roots, floating row covers protect the tops of your plants from the wind yet allow light to go through.
You can either cover a whole row of plants at once, or cut the row cover to fit just one plant. Use ground staples to secure your row cover material from being blown by the wind.
This is an easy and simple method of protecting plants from frost that can be used along with other methods as well.
Usually row covers are temperature rated. I've found that doubling or tripling the row covers works just as well in protecting plants from frost as buying a whole new roll of higher-rated stuff, but I would err on the side of caution for protecting expensive plants or perennials going through their first winter.
Between mulching and row covers, you can get another zone or two worth of frost protection. If your plants are covered with snow, this may be all you need, because snow is a great insulator.
Check plants under the row covers during the winter to see if you need anything more as the temperature drops. Signs to look for are leaf damage and wilting.
Sometimes when the row cover touches the plant, it allows the cold to penetrate because there's not that bubble of warmer air for insulation. For a plant that's on the edge of freezing, that can be enough to cause damage.
If you find that happening, constructing a hoophouse type frame to raise the row covers above your plants can help.
If you live where the temperature drops below 0 degrees F (-17 degrees C), you might want to go further in protecting plants from frost.
There are two methods you might use:
The cloche is a bell-shaped glass cover -- used in gardens where hail isn't likely -- to overwinter delicate plants outdoors or start seedlings early.
These can be very useful in areas with relatively mild weather yet short growing seasons.
Some people construct plastic cloches out of leftover bottles to help in protecting their tender seedlings early in spring. If you find a source of attractive bottles, this can be a good alternative to glass cloches in areas that do get hail.
A cold frame is generally a wood box covered with glass on top. These is basically a mini green house.
Cold frames are a good alternative for people with small yards, useful to start seeds in winter for later planting, to overwinter small potted plants, and to extend the season for plants that tolerate shorter day lengths but can't tolerate freezing.
In most areas, the heat of the sun warms a cold frame sufficiently, but in extremely cold areas, you may need to add a heat mat underneath your cold frame to maintain its temperature.
Fruit and nut trees are a particular challenge to keep warm. The critical time for setting fruit is when a tree blooms. If the temperature drops below freezing at that time, then the tree will drop its flowers and you won't get any fruit. This can be a real issue in high latitudes, where the last frost date may be in mid-summer.
One tip I learned while researching this question is to string incandescent "Christmas" lights in blooming fruit trees during freezing weather. These bulbs warm trees quite nicely!
There are many types of greenhouses (which is a generic term meaning a building used to grow plants in). These provide the most protection for your plants.
As with the cold frame, most stand-alone greenhouses are heated by the sun alone, but greenhouses in areas where there is very limited daylight (especially the extreme north) or frequent cloud cover, under floor heating or a green house heater may be necessary.
Hoophouses come in many sizes, from a few feet high to huge commercial green houses the size of an airline hangar (also called high tunnels). These are just a row of hoops set into a rigid base and covered in row cover, canvas, or most commonly, plastic sheeting. These are easy to set up, are safe for areas that have hail, and work well in protecting plants from frost.
The disadvantage is that the cover needs to be replaced every few years due to the sun's effect on it. Also, a hoop house needs to be securely fastened to the ground in areas with high winds.
Conservatories (sometimes called patio rooms, sunrooms, solariums, a hothouse, or Florida room) are rooms with glass walls (and often a glass roof as well), attached to the house on one side. This arrangement allows you to take care of your plants without having to go outside.
You can turn any room in your home with multiple sunny windows into a sunroom for your potted plants and seedlings. I use a corner of my garage in that way.
The biggest challenge with plastic and glass frost protection methods is overheating. The temperature should be monitored and a way to ventilate should be added (a temperature sensor that opens a window at the top of the greenhouse when the temperature rises too high can be used)
Cold Climate Gardening has a nice list of resources for those looking for help with strategies for protecting plants from frost.
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