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How to Protect Plants from Frost

It is that time of year again – cold and frost are coming to the north (I live in zone 5). Think about your poor plants – they will get cold – you must do something about this immediately – right?? Common advice all over the net is to wrap your trees and shrubs in burlap to protect them from frost and to keep them warm. Plants in containers on your porch and balcony need to be wrapped in bubble wrap to keep them warm. Let’s have a closer look at this common myth and try to understand the issues.

How to Protect Plants from Frost

Protecting plants from frost

How to Protect Plants From Frost

Before we look at what to do with plants to protect them from frost, let’s understand some basic physics. Don’t stop reading – this is easy stuff to understand.

What is Heat?

Heat is a form of energy. If something has more heat – it has more energy. As it looses energy, it gets colder. Warm soil in the summer has more energy than frozen soil in winter. However, even frozen soil has some stored energy – there is warmth in frozen soil.

This heat energy is not made by magic. If an object is to get warmer, it needs to absorb energy from somewhere – the energy source. When you come in from outside on a cold day you might stand beside a fireplace to get warm. The fireplace is a source of energy and your body absorbs the energy making you feel warmer.

How do Plants Warm Up?

Your plants get heat energy from three places. They have internal chemical reactions that produce heat. In winter plants are not chemically active and so they produce very little internal heat energy. The second source of heat is from the sun. Sun shining on plants transfers energy to the plant, and the plant gets warmer. In winter, nights are long and when the sun does not shine plants do not get heat energy from the sun. The third source of heat energy comes from the earth. The center of the earth is made up of molten lava, and the heat from this moves to the surface of the soil. The soil may be frozen but there is still some heat moving from lower levels to the top level of the soil. It then moves into the air.

Wrapping a Plant Does Not Keep it Warm.

Enough physics! What about the plants? Think about a potted plant sitting on a balcony. The advice is to wrap the plant to keep it warm. Where is the heat source? The pot is not in the ground so there is no heat coming from the soil. The sun may shine on the wrapped plant. If the wrapping is something like burlap the sun will have little affect in warming the plant. Any absorbed heat is quickly lost to the air. If bubble wrap is used, it might act like a small greenhouse. Sun traveling through the plastic would warm up the inside of the bubble wrap – but only during the day. Once the sun goes down, the heat will be lost, and the plant is exposed to the cold night temperatures. The truth is that this heating during the day can be very detrimental to the plant, but that is a topic for another day. Bubble wrap, if used, should be covered with a light proof material.

Some very small amount of heat may come from the chemical reactions in the plant, but in winter this is negligible.

Since the plant on the balcony has no heat source, wrapping it will not keep it warm.

What about wrapping trees and shrubs in burlap – does this keep the shrub warm?

The simple answer is NO. Except right at ground level, there is no heat source and so there is no heat energy to trap with the burlap.

In my post Are Wrapped Trees Warmer in Winter, I actually measured the temperature under wrapped plants.

What About the Effect of Wind?

We all know that if we stand outside on a cold windy day, we feel colder than if we stand in a spot that is sheltered from wind. Our bodies are wrapped in cloths – a nice heavy coat. This coat is an insulator just like the bubble wrap and it traps heat inside the coat. It is not a perfect system and some heat is always escaping. As the heat energy moves to the outside of the coat, the wind quickly blows it away. As this happens, more energy moves to the outside of the coat and it too gets blown away. If we stand in a sheltered spot, less energy gets blown away and more of it stays trapped inside the coat – so we are warmer. You might ask; Where is the heat source? It is coming from the chemical reactions in our body. Unlike plants, we don’t shut down in winter. Our bodies ‘burn’ the food we eat and as a result of these chemical reactions the body creates heat energy – we are the heat source.

In winter plants are not a heat source, and so wind has less of an effect on their temperature.

Plants and humans do have another problem with wind. The wind also removes moisture along with the heat. Not a problem for us humans – we just go inside and have a hot toddy to replenish the water lost to the wind. Plants can’t do this in winter when the soil is frozen. When the ground is frozen, plant roots absorb very little water, so a drying wind can be very harmful to the plants.

So what about wrapping plants in winter? The wrapping does not keep the plants warm. It does however reduce moisture loss due to wind. Moisture loss in winter can kill a plant. Wrapping a plant will reduce moisture loss and may help to keep a plant alive until spring.

Should you cover plants in winter? See my post Should Plants be Covered in Winter for a more detailed discussion.

References:

1) Photo Source: public domain

I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writting and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

9 Responses to 'How to Protect Plants from Frost'

  1. Codechimp says:

    I’m just curious how you looked at the “physics” because it seems like you’re looking at this all wrong. You’re making the argument that there’s no heat, but what you really should be looking at is the temperature. Just because something doesn’t feel warm doesn’t mean its not above the plants temperature hardiness threshold. If adding insulation causes the temperature to be just a few degrees above the outside temperature, that can make all the difference. In addition, rapid temperature fluctuations can be more detrimental to the plants roots/survival. Thus insulation is one way in which you can help mitigate large temperature changes and keep the plant from dying.

    • Robert Pavlis says:

      I don’t understand the statement “You’re making the argument that there’s no heat, but what you really should be looking at is the temperature “. Temperature is just an arbitrary measurement of the amount of heat present. A wrapped tree, as in the picture, has no heat source except the sun and air around it. It can’t get warmer because there is no heat source.

      I agree with the statement ” If adding insulation causes the temperature to be just a few degrees above the outside temperature, that can make all the difference”. However, it is a big “IF” and the If condition is incorrect. Insulation does NOT cause the temperature to be just a few degrees above the outside temperature. Insulation slows down the loss of heat lost, but does not create heat–it is not a heat source.

      You can test this for yourself. Take a cup of hot water and put it into a cooler. Then put the cooler outside for a while. After some time the water will be the same temperature as the outside air. All the heat is lost to the air, and since there is no heat source it never warms up again. That is basic physics.

      Things like burlap are a poor insulator in any case.

  2. Woody says:

    So plants should be wrapped anyway to keep them from losing water. Right?

    • Robert Pavlis says:

      Once the ground is frozen, the roots of woody plants have difficulty getting water to the upper part of the plant. Not so much of a problem with deciduous plants, but evergreens do loos water through their needles. This is the cause of winter burn – a drying out of the needles.

      The problem is that wrapping a plant in something like burlap will have limited effect on the water loss – but it does help somewhat by reducing the air movement around the plant.

      What is more important is to water in late fall. Most people stop watering when summer is over – say the start of September. But woody plants will still grow for quite some time and it is important to keep them watered right up to freeze-up.

      Bottom line: if you bought the right plant, and put it into the right location, it does not need to be wrapped to preserve water.

      • Don says:

        I have a number of cedars on my property, and had no problems for years, with care limited to shearing each year, keeping the ground moist during the summer months, and “watering in” before the ground froze.

        A few years ago, we added several cedars along the border of our property. They got off to a great start and wintered well. The following year I followed the same procedures, one fertilizing in spring, keeping the ground moist from spring to fall, right up to freeze up. The following spring, winter damage was evident on all trees, very severe on a third of them – on these, the top portions were dead. In researching the problem, I decided to follow one expert’s suggestion and last year discontinued watering in the fall, to allow the cedars to harden off. The explanation being that in encouraging succulence into freeze up, that growth would likely die in winter. I thoroughly watered in just before freeze up. This spring, following a winter that was the most severe since planting, there appeared to be no winter damage.

        I realize that one year may not be a fair test, but this approach might be an alternative to wrapping in some cases. The expert that provided this advice suggests that in our area you should stop watering trees in September, with one last soaking just prior to freeze up.

        Off topic, this expert suggests a similar approach for watering lawns, and recommends against a late fall fertilizing – contrary to many who state that is the most important application of lawn fertilizer. I followed this advice, and my observation is that the lawns fared at least as well, or better, than in years when I made a late fall application of fertilizer.

        • Robert Pavlis says:

          I am going to check on the watering of cedars with some expert, and will post my results once I have them.

          As far as fertilizing grass goes you are sort of correct. Ontario Agriculture (OMAFRA) recommends fertilizing in in early fall (from mid-August to mid-September) and again in late October. I assume that is for zone 4-5, but Ontario covers a lot of zones and OMAFRA should be more clear on this. It does go on to warn that you should not fertilize in late fall while the grass is still growing because it leads to succulent growth that does not overwinter very well. The late October fertilizing is after the grass has stopped growing. To be honest i am not quite sure how one tells this, except that it no longer needs to be mowed.

          ref: http://www.omafra.gov.on.ca/english/crops/facts/08-027w.htm

          The following is from this reference:

          Nitrogen is taken up by the roots even though shoot growth has ceased. This is because roots remain active at cooler temperatures than shoots.
          Nitrogen enhances fall colour and hence increases chlorophyll content.
          Increased chlorophyll content means increased photosynthesis.
          Increased photosynthesis means increased sugars. Since turf is not growing at the time of the fertilizer application, the sugars that are produced are not used for growth but are stored to enhance winter survival and spring recovery.
          These sugars make the grass plant less susceptible to freezing. A good analogy is that a bottle of juice in the freezer will take longer to freeze than a bottle of water. The grass plant cells are full of sugars and hence take longer to freeze, and freeze at lower temperatures.
          Late-season nitrogen applications promote deep rooting during the fall, so plants go into the spring and summer with deeper, healthier roots.
          Spring green-up is early, because the nitrogen that is stored in the roots is ready when shoot growth resumes.

          • Robert Pavlis says:

            Should watering be withheld in fall to help trees harden off and get ready for winter? This was the question asked as one of the comments.

            The answer is no. This is a myth that has been discussed by Dr. Linda Chalker-Scott in http://puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Winter%20watering.pdf The short explanation is that plants in temperate climates use day length as a signal to go dormant. This makes sense. Imagine if this were not true, and we had a rainy fall. Trees would not go dormant and would then die in winter. They would not survive in nature for long.

  3. A says:

    The point is to protect them from cold not keep them warm.

    • Robert Pavlis says:

      Cold and warm are the same thing. Cold is the lack of warmth. When heat energy is lost, the temperature drops and we refer to it as being colder. So to protect plants from cold means to keep them warmer.

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