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
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.
1) Photo Source: public domain