D3

Bob Osborne

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It seems that every winter, no matter how cold or warm, something in the garden suffers.

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photo contributed

Weigela is among the plants which suffered winter damage last year thanks to the freeze-thaw cycle.

Last winter, we saw winter damage on burning bush, weigela and many of the old garden roses. Why is it that these would suffer yet less hardy species of plants came through with no damage?

The answer is both complicated and often not entirely understood. First we need to understand a bit about how plants shift from active to dormant.

As the amount of daylight shortens and temperatures fall, plants begin a process of transferring water from within their cell walls to spaces between the cells. This critical redistribution of water prevents the water in the cells from forming sharp ice crystals and thereby rupturing the walls. As the temperatures decline, this process continues until, by early winter, there is virtually no water left in the cell.

There are many important 'organelles' inside a cell. These sites are similar to the organs in our bodies and perform critical functions. If there is no water at all, these organelles would die. The tiny amount of water that remains in the cell forms extremely thin coverings around these organelles. Strangely enough, when water becomes this thin, it remains flexible to temperatures far below the freezing point.

Each species of plant has developed a certain level of proficiency for this 'supercooling' as it is called. Those that live in areas where it never freezes have little or no ability to withstand frost. This is why tropical plants we grow in our gardens such as tomatoes or squash wither if the temperature hits the freezing point. In the Arctic, there are plants that have developed their abilities to the point that they can withstand temperatures of -60C or lower. In our latitudes, most of the plants that grow here can withstand temperatures up to and even slightly below -40C.

While this process seems straightforward, there are many other factors that complicate it. As we all know, the descent into winter is not a perfect slope. Plants will begin the process in fall then have it interrupted by a mild spell, such as is happening to us now. As temperatures rise again, more water flows back into the cells. This shifting of water can occur several times as the plant becomes dormant.

The danger is that a mild spell can be followed by a sudden and dramatic change of temperature. If the plant does not have time to remove the water, serious cell damage can occur. This is particularly true of plants that require a longer season to begin dormancy. A good example is the peach tree. Peaches tend to grow late into the season and are often damaged well before true winter begins.

To make matters more complex, once plants achieve their highest level of dormancy, they can be brought out of dormancy by high temperatures. If this happens in mid-to-late winter, as often happens in our 'January thaws,' the plant usually cannot return to the same level of deep dormancy as before, thereby making it more susceptible to frigid late winter temperatures.

As if this is not enough, there are other dangers for plants. The location of a plant can determine how much winter injury it sustains. For instance a rhododendron planted on the east side of a house will receive morning sun but not late afternoon sun. Often in late winter when the sun intensity is increasing the leaves will heat up, moving water into the cells, only to have them freeze very quickly as the sun sets. If that same rhododendron is planted toward the south and west, it can sustain devastating damage.

Some trees are very susceptible to this phenomenon, which is called 'southwest injury.' Norway maples and lindens, for example, often crack vertically during the winter and this crack will be directly in line with the direction of sunset when it occurs. Placement of trees can prevent such injury or the growing of shrubs to the south of the trunks will shade them. Some people place a board on the south side of the trunk or even paint the trunk white.

Wind can also cause significant damage. If temperatures are very low and a plant is open to strong winds or is planted in a 'wind tunnel,' where wind is squeezed through a narrow opening, the dry air can literally suck water from the leaves of evergreens or even the stems.

Knowing as much as possible about these influences can help you choose and place plants to lessen the impact of winter, but sometimes we can never know why a certain plant suffers. The amazing thing is that these delicate crystals of water and little else make it at all.

* Bob Osborne is the owner of Corn Hill Nursery Ltd., a member of Landscape New Brunswick, an author, farmer and gardener. His column appears each Thursday.