Freezing and Thawing: top plant killer

A couple of weeks ago, gardeners in northern North America were worried about the buds breaking prematurely. Gardeners on social media were wringing their hands in worry. But really it’s not a huge problem. Sure, there may be a few less flowers in spring, but most shrubs and trees will survive. The real problem is the numerous freeze/thaw cycles, which damage plants if garden beds haven’t been mulched.




We are having a strange winter in southern Ontario this year. Weather experts explain that a La Nina year combined with a positive Arctic Oscillation phase, have led to less snow and more freezing rain, warmer temperatures as well as the fluctuations above and below the freezing point in southern Ontario.

Usually at this time of year the temperatures stay below freezing, and most years a blanket of insulating snow covers the ground. Now, if you have covered your garden beds with two inches of insulating mulch, and used evergreen boughs over the more delicate shrubs and perennials, your plants will probably make it through this winter.

And if you haven’t mulched your garden? Well, you could lose a few plants this winter. But it is not all doom and gloom as some hand-wringing melodramatic “garden experts” would lead you to believe.

A decade ago, I lived in Calgary, Alberta. Not only are the winters colder (Zone 3 versus the milder 5B zone in the GTA), but the temperatures also fluctuate dramatically. Calgary is slap-dap in the middle of Chinook belt. Every winter, the warming Chinook winds blow down the Rockies raising temperatures as much as 20-25 Celsius degrees (36-45 F degrees) in an hour. Despite the harsh conditions, Calgary gardeners create fantastic gardens filled with resilient plants.

Without realizing it, some of the plants we use in out GTA gardens are “Chinook-proof.” So these plants will come through our winter unscathed.


Cedars (Thuja spp.) and firs (Abies spp.) are borderline in Chinook conditions, but most spruces (Picea spp.) and larches keep on ticking.

Some species of pines (Pinus spp.) do well (The bristle-cone pine, eastern white pine, Jack pine) but the limber pine, red pine, Scots pine, Swiss Stone pine and white bark pine won’t do as well.

Deciduous Shrubs & Trees

Paper birch (Betula papyrifera) come through a Chinook winter with flying colours, but the river birch (Betula nigra) and weeping birch (Betula pendula) won’t do as well.

A big plus for chokecherries (Prunus virginiana), but black cherry (Prunus serotinia) and muckle plum (Prunus x nigrella ‘Muckle’) may not do as well.

Northern American maples will do OK, but Japanese maples (which you can’t grow in Calgary) may kick the can this year (sorry…Japanese maples are borderline in southern Ontario to begin with).

Elders (Sambucus spp.) and false spirea (Sorbaria spp.) will come through with flying colours, but some of our dogwood shrubs (Cornus spp) may be slightly damaged.

Most lilacs thrive in Chinook country, with the exception of a few cultivars.

Potentillas and spireas are the ultimate in tough plants. The same can’t be said about mockoranges (Philadelphus spp.).

Rosa rugosas are planted in most Calgary parks for a reason. Tough plant!



Yarrows (Achilleaspp.),ajugas, hollyhocks, lady’s mantle, columbines, artemisias, astilbes, masterworts, bergenias, bellflowers, bleeding hearts, purple coneflowers, globe thistles, queen of the prairies, blanket flowers, sweet woodruff, cranesbills, daylilies, coral bells (Heuchera spp.), hostas, gayfeathers (Liatris spicata), beebalms, peonies and sedums.


Find more Chinook-hardy plants on ChinookScape 


Fun  Facts

The greatest Chinook temperature jump ever recorded occurred on January 22, 1943, when a Chinook shot the temperature in Spearfish, South Dakota, from a chilling minus 4oF (-20oC) at 7:30 AM to 47oF (8.3oC) just two minutes later! And, in Pincher Creek, Alberta, a Chinook jacked the temperature 21 Celsius degrees (37.8 F degrees) in four minutes on January 6, 1966.     The Weather Doctor 


And how does the constant freeze/thaw harm plants?
Warming up the soil, activates the root cells, once the soil freezes again, the active cells cool off and become inactive again. Constant flux of plant cells going from active to inactive stresses plant root cells. Most species of plants can’t take this yo-yo effect…and the plant is either damaged or dies.

Mind you if you mulched the garden bed in the fall with wood chips, you don’t have to worry too much. The mulch acts as an insulator and minimizes temperature fluctuations.

Written by Cristina da Silva
Thursday, February 9, 2012 in Plants & Soils

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