During the cold months frost is one of the main reasons regarding plant deaths. Freezes caused by low temperatures can harm and kill plants in a couple hours, so it’s important to be prepared when it gets colder.
There are many ways to protect your plants from frost damage, one way to prevent damage is to collect or preserve heat around your plants. Covering your plants before nightfall can be helpful. Watering plants before frost, and avoiding frost damage in the first place is another method to help your plants survive harsh conditions. Good planning of your crops can also prevent damage from occurring, there are a lot of cold-hardy plants to choose if you live in a very cold climate.
There are a lot of great ways to protect your plants from frost, which I will talk about later, but let’s first start with what frost is, and how it affects plants.
What is Frost and a Freeze?
While they sound very similar, they describe two very different things. Frost
Frost can present itself in many different ways, Surface Hoar, Rime, Black frost and Frost Flowers are some examples to how frost can occur.
Surface Hoar: Is the most common type of frost, it forms when evaporated water freezes and forms small ice crystals to cover the ground. They usually melt again as soon as the sun comes out and require temperatures just below freezing
Rime: Forms when supercooled water – water at temperature below freezing, but not in solid state – in form of fog or clouds touches a surface and crystallizes.
Black frost: or killing frost is not frost, is caused by very low temperatures and is more a kind of freeze then frost. It only occurs during low humidity, there will not be enough water in the air for frost to manifest, but plant tissue will freeze and die off, leaving behind black spots.
Freezes, on the other hand, are caused by very low temperatures (>32°F or 0°C), which causes water in the plant cells to freeze. A freeze does not have to form ice crystals and unlike a frost doesn’t require air humidity. Temperatures this low for several hours will kill most plant-live, so let’s first have a look how different plants react to these cold conditions.
How does Frost affect plants?
Frost or low temperatures will affect plants in one of two ways, first, the water in the plant cells will freeze and expand, causing them to rupture. Second, once the soil freezes, water won’t be able to reach the roots of the plant. Interfering with the water supply can also cause a plant to die.
The response to cold conditions is hugely dependent on the type of plant, root crops will react differently compared to annual or frost-tender plants.
How do plants react to and protect themselves from frost?
Let’s first look at the different types of reaction we can expect after temperature dips below freezing.
Frost-Tender plants: These plants are not able to survive cold conditions and will most likely just die of, when not protected. Tropical and other plants, suited for warmer climates will not adept to these harsh conditions and need extra care during cold months.
Annual plants: The Plant will die off, as soon as temperatures get too low, but will grow once the weather gets warmer. These plants survive by producing seeds, so don’t worry about them dying off completely.
Root-hardy-perennials: Root plants will in most cases survive freezing conditions. The overground foliage dies off during colder times, but the root system survives in a dormant state. During very cold conditions the root system might be affected as well, causing the whole plant to die.
Shrubs, Trees, Perennials: These plants will try to withstand cold conditions by entering a dormant state. It’s quite uncommon for these plants to die except for younger plants, which are far more sensitive to climate.
Damage to plants strongly depends on the intensity of the cold weather. Even minor changes of 1-2° will have a different outcome for the plant-survival rate. Most plants will recover from short dips below freezing, however, serious cellular damage will occur once temperature is below 28°F (-2.2°C). Temperatures below 25°F (-3.9°C) will cause severe freezes and desiccation (dryness caused by lack of water due to frozen soil)
A map was designed to get a general picture of the climate for every respectable area. Hardiness Zones show how low temperatures get and can tell you which plants are suited for your region.
What are Hardiness Zones?
This map can help you to decide which plants to grow and give you a general understanding of the climate in your area. It’s important to mention, that this map might not always be perfectly accurate, but give a good overall picture.
Hardiness zones is a very interesting topic in itself and I would highly advise you to read about them if you haven’t done so yet. Here is a Link to an article I liked giving a good overlook about this topic.
How can you protect your plants from frost?
Plant frost resistant plants: Planting crops suited for your region is the easiest way to prevent frost damage. There are a lot of cold-resistant versions of plants, which are normally very sensitive to colder conditions. It’s also a good idea to keep plants, which you know are more sensitive in containers.
There is a wide range of edibles which are resistant to frost: carrots, winter greens (spinach, kale), broccoli, leeks, beets, Brussels, cabbage, cauliflower, celery, collards, green onions, parsnip, Mustard, Rutabagas, reddish, turnips, Swiss Chard.
Plant in more frost resistant places: Another good way to prevent severe damage is to grow your crops in places, where they are more resistant towards cold conditions. So tips which can help to keep your plants alive is to grow them:
- in unshaded areas facing south,
- having a wall on the north or west side to absorb and radiate heat,
- on elevated ground to avoid frost pockets (depressions in the ground where cold air collects)
- in less sandy soil, Sand is a very bad insulator.
These are all very useful steps to take but have to be done preemptively, you won’t be able to simply build a wall a day before frost falls. Add these steps to your plans for the next summer and you’ll be rewarded in the winter
Know your weather: One very important step to prevent serious crop damage is to be prepared, knowing your weather can help you to predict colder nights, so you can prepare your plants for the incoming frost. There are a lot of important factors to knowing your weather, like the first and last day of frost, cloud formations, climate zone and predicting the time-frame in which temperatures will dip below 32°F (0°C).
To find out your first and last day of Frost take a look at this site, just type in your location and it will give you the exact dates.
Learning about cloud formations is a bit harder and require some time to build confidence in, here is a very basic introduction to the most important frost related patterns:
Cover Plants with cloth: Covering your plants with cloth will collect radiating ground heat. The best time to cover your crops is shortly before sunset, the most heat has been absorbed by the earth at this point and you’ll loose a lot of that heat if you wait until it’s dark.
It’s important to extend the cover all the way to the soil and fixate it with rocks or other heavy object. Do not tie the cover together, because the heat from the soil will not be able to get inside your cover.
I prefer using old Bed sheets and blankets but you can also buy specific gardening cloth for a reasonable price (Check current price on Amazon).
After the frost has thawed in the morning remove the cover to release the humid air and prevent your plant for breaking the dormancy.
Covering your plants with cloth will protect them from light frost, using an additional plastic cover will add more protection.
Additionally with Plastic: Adding a plastic cover over your cloth cover can help to store even more heat for very cold nights. Never cover your plants directly with plastic, as the plastic will damage the plant.
Remember to also remove the cover as soon as frost has thawed in the morning. Otherwise, your crops will be damaged. This will help you bring most of your plants safe through the winter.
Give your plants a hot-water bottle: Warming up some water or stones and adding them under your cloth cover can warm up the surrounding air around your crop. The constant heat will help to keep your plant alive and prevent serious damage during really chilly nights.
I would only use this method as a last resort to bridge a very cold night. It’s very labor intensive and only provides short term heat, it’s less of a general way to prevent freezing damage but rather a last resort solution, to keep a very precious plant alive.
Heavy Mulching to protect roots: Mulching can help to protect the roots of your plants. Adding more soil will increase the time needed for your soil to freeze and limit water flow to your plant, causing it to die of dehydration.
This is also more of a last resort method, so if you think your plant won’t survive and want to focus your effort on the roots of your crop, mulching might be a good option for you.
Build Insulation Barriers or build a Cold-frame: This is just a more advanced version of the cloth cover. Construction an Insulation Barrier, which in essence is a small Greenhouse or a Cold-frame will keep your plants warm.
You can either buy them at a local nursery or in a DIY style by cutting away the bottom of a plastic bottle or using milk jugs, etc.
Wrap medium sized trees: If a tree is to big to cover completely with a cloth, wrapping a cloth around the trunk will also help to prevent frost damage. Younger Fruit trees have very thin bark, which will often split in harsh conditions. This is far less labor intensive, as you can just leave it on until winter is over.
Prepare the soil: Soil plays a huge role when it comes to frost damage. Healthy soil will be able to hold a lot of water and is far more insulated then poorer and more sandy soil. Organizing your growing area into a raised bed also helps to avoid frost pockets.
Water Plants: Giving water to your plants shortly before the temperatures dip helps your plants to stay hydrated for a longer time, if the soil starts to freeze.
A well watered piece of soil is also more cold-resistant and rarely freeze to a solid block, increasing the time it takes until your crops are in danger of desiccation.
However, don’t give to much water to your plants, otherwise frost heave might injure your plants. Give your plants and the soil enough time before the temperatures go down to minimize the risk.
Use a Sprinkler: Keeping a flow of constant water on your crops can help to prevent them from freezing. Water will provide some heat to your crops, but it needs to be a constant flow, of not to much so it doesn’t simply start to freeze. This can be quite tricky to achieve and is not a very good long-term strategy. Alternatively Anti-Transpirant functions similarly and can be bought at a nursery, or online. You simply spray the plant you want to protect and it will create a humid film ontop of said plant, which will help to protect it.
Bring Plants Indoors: The most simple and successful way to prevent frost damage is to simply take your plants indoors. This is only really works for container plants, and means a lot of work for any plant you want to move from your growing area to a pot.
It might be worth to put in the effort, if you have some plants you really want to survive the cold period. Otherwise using another method is more efficient.
Use a heat source: Another way to prevent the cold from getting to your plants is by using heat sources like light bulbs or electric heater. You can easily prevent most damage to your plants, but your electricity bill might not like it.
Harden off Seedlings: It’s a good idea to give your Seedlings some time to acclimate them, they are much likelier to suffer damage, if you just put them outdoors after they stayed in a Greenhouse.
Gradually start to expose your seedling to the outdoor conditions (place them in a warm spot without to much sun exposure) for about 2-3 weeks before your want to transplant them. Only start to introduce your Seedlings to outdoor conditions once the temperature reaches about 45°F (10°C)
Sacrifice parts of your plants: Sometimes it might be impossible to keep all your plants over winter, it’s important to keep track of the general situation, so you know which plants might not make it, so you don’t waste any time on them.
Sacrificing the above-ground foliage is also no big loss, if the plant itself can survive. Plants are actually quite resistant, so don’t give up on a plant, which might have suffered some damage due to frost. It might be able to regenerate and grow back next spring.
What to do with plants after frost damage
You might be able to save your plants in less severe cases. This really depends on the plant and the extend to which the plant is damaged.
For frost cracks on Trees: Remove torn and loose bark with a knife, afterwards smooth the edges out. The tree will now be able to regenerate itself and might survive.
Potted Plants: Can be moved away from direct sunlight in
Damaged Plants: Do not try to prune damaged parts of the plants. Only cut away the damaged parts once spring comes along or you keep the plants indoor. Completely, prune dead stems, but only the damaged part of live ones. You might need to prune soft-stemmed plants as soon as temperature rises above 32°F, otherwise the stems will start rotting, which can cause the whole plant to die.
Try to give your damaged plants a surplus of fertilizer and water, as the crops will take a lot to recover from the damage.
What to do with frost damaged plants? Plants damaged by frost are best to be left alone until spring. Pruning them during cold weather will further increase harm to the plant and eventually kill it off.
Potted plants, which only have been exposed to frost for a short duration should be taken inside. Give them a good amount of water and fertilizer so they can start regenerating. It’s not guaranteed that a plant damaged by frost will survive, even if exposure time was relatively short.
How does Wind affect freezing risk? Wind helps circulating the air, mixing up colder and warmer air. In cold nights wind is your best friend and helps to keep your plants a little bit warmer, but this effect is most of the time insignificant, because the wind doesn’t keep your plants warm enough to protect them from harsh conditions. It just help to lessen the damage a little bit, that’s all.