The Cox garden, like all gardens in the Comox Valley, was dressed in a shivering coat of snow last week.

Duchess of Dirt: The effect of freezing temperatures on plants

Winter weather came quickly in the Comox Valley

Leslie Cox

Special to The Record

The forecast for an earlier than usual snowfall – which indeed happened – and prediction of lower overnight temperatures had us scrambling to get the last few tender plants indoors for protection. Catastrophe in the plant world was looming.

Snowfall aside for the moment, freezing temperatures can be a real concern for the health and longevity of your plants. Especially the ones classified as Zone 9 and higher; the more “tropical” plants in the botanical kingdom.

And by the time a plant is showing signs of distress, the actual damage has already been done…under the plant’s skin, so to speak.

Basic biology: plants, like humans, are made up of many layers of cells. And like us, plants contain a lot of water. As temperatures drop into the freezing range of water, the water begins to turn to ice, expanding inside the plant’s cells and causing the cell walls to rupture.

But there is a double whammy. Plants hold water between their cells too, and when the water in this area starts to freeze, it sucks the water out of the cells through the walls, causing the cells to shrink and then rupture.

Once the water held within a plant has frozen, ice crystals which have formed inside and between the cells start to do their damage. And it can be intense – like hundreds of tiny needles poking holes into tender cell walls.

A very cruel form of punishment for any plant not deemed hardy enough for our region.

If you have selected only those plants which are hardy to Zone 8, for gardens in parts of the Comox Valley such as Comox, and Zone 7a, for Black Creek and other areas closer to the ski hill…then you have luck on your side. Plants “labeled” as suitable for this range of growing zones, and lower, are called “winter hardy.” (Most peony species, for instance, are winter hardy to Zone 2 or 3.)

Plants labeled as winter hardy actually have some protection against freezing because the water inside their cells is not pure water. It also contains dissolved sugars, salts, and a few other substances (such as calcium and ethylene). The resulting combination of all these ingredients forms a type of antifreeze within the plant’s cells.

But even with this antifreeze in their cells, plants can still develop deadly ice crystals should temperatures drop drastically all of a sudden. As ours did recently.

We were enjoying lovely, warm temperatures in the wind-up to the month of October, then bam! Within two days, the overnight temperatures dropped in our greenhouse from 5 C on Nov. 2 to -5 C on the Nov. 4. And -5 C is what they call a “killing frost.”

So drastic temperature swings aside, how do plants manage to prepare themselves for cold winter survival? Besides producing antifreeze in their cells, that is.

Some plants will slowly desiccate water out of their cells. Less water in their tissues means less likelihood of freezing. Other plants just move the water out of the living part of their cells as soon as temperatures start to drop in order to prevent ice damage.

Some plants drop their leaves. This means they can concentrate all their energy reserves in the roots. Conifers (pine, juniper, spruce, etc) do not need to do this as their needles have a waxy coating which prevents cold damage.

Evergreens use other defence mechanisms to protect from freezing. Rhododendrons, for example, will curl their leaves to prevent membrane damage.

All of these defences work wonderfully, in normal conditions. However, I do fear our sudden temperature drop this last week may have casualties. But we will have to wait for spring which now appears will be a long time coming.

Leslie Cox co-owns Growing Concern Cottage Garden in Black Creek. Her website is at www.duchessofdirt.ca and her column appears every second Thursday in the Record.

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