How plants breathe and how you can prevent winter burn

It is estimated that the total water content of the atmosphere is recycled through the stomata of plants every half year!  That’s an incredible contribution to the health and well being of our planet and underlines the importance of trees and their stomata-rich leaves to our environment.

It is also estimated that high carbon dioxide content in the air decreases the number and density of stomata, presumably because the more carbon dioxide in the air, the easier the job of the stomata in their daily role.

So how do these microscopic plant “pores” actually do this massive job?

A stoma is a tiny opening in the skin of a plant that allows the plant to breathe or exchange gases and water needed for life. Plants draw their energy from the sun in order to accomplish photosynthesis, the process of manufacturing plant- building materials from carbon dioxide drawn from the air and minerals, and water drawn up from the earth. In the process of opening, the stoma releases water vapour and oxygen as it draws in carbon dioxide.

On opposing edges of the oval-shaped stoma are two guard cells that trigger its opening and closing. The guard cells themselves are driven by a process involving humidity and light intensity, which, when just right, cause a proton pump to drive protons from the guard cells, increasing the cell’s negative electrical impulse and causing an uptake of potassium ions. In the process, osmosis of water into the cell is triggered by turgor pressure (the pressure exerted on a semipermeable membrane of a cell from both inside and outside).

This process obviously stops for the winter in deciduous trees when their leaves fall, but it continues, albeit at a much slower rate, in evergreens. That is why it is so important to make sure these trees are well hydrated before freeze up. Creating enough moisture deep beneath the surface of the earth allows the roots to have a source to get them through the winter as the needles slowly expire moisture, a process which speeds up in the early spring as the days get longer and the sun gets warmer.

By March, young evergreens are at severe risk for winter burn, which is really desiccation caused by lack of available moisture (the roots don’t go that far down yet) and the drying effect of the wind and sun.

This is why, especially for recently planted trees with a southern exposure, it is important to provide a wind and sun break. This does not mean wrapping the tree like a baby in a blanket, a mistake made by some amateur tree service people. Indeed, this practice can actually harm the tree because the burlap wicks away moisture from the needles. If you must wrap, make sure to build a tripod that will keep the burlap from touching the leaves, but a screen is all you really need.

Such a device on the south-facing side of your fruit trees (or any with thin bark) can also keep the tree from splitting in the early spring. In some orchards in Europe, they whitewash the trunks to provide this protection and to protect from insect infestation, but we face much harsher conditions. Once again, as the March sun heats the bark during the day, especially when it is reflected from nearby snow, cells expand. When it freezes at night, the cells contract. This expanding and freezing can result in ruptures in the cells and ugly wounds in the bark of young trees. (This is also what happens to the roots of perennials in winters with low snow cover).

It is not too late to protect your trees this spring. You still have time to erect a wind and sun break using some stakes and burlap. To interrupt sun burn, it can help to rough up the snow on the sunward side of the tree in early March to prevent reflection.

Dorothy Dobbie is the owner of Manitoba Gardener Magazine, giving Manitobans gardening advice for 21 years. For a subscription to the magazine call 204-940-2700 or go to