Gardening: Sun: the little known contributor to winterkill

  • Mar. 11, 2020 12:50 p.m.

Winterkill, plants dying during the winter, is a common worry with Alberta gardeners. In the fall gardens are mulched and tender plants covered to try to protect them from changing temperatures. It is common to see evergreens wrapped in burlap or covered with a commercial bag to protect plants from the deep cold or fluctuating temperatures. Boxes or small pens made of chicken wire, filled with mulch help protect tender shrubs.

What gardeners tend to forget, it isn’t only the deep cold of winter or fluctuating temperatures that can kill plants.

The sun also is a culprit. Sunny days in late winter or early spring, causes evergreen plants to transpire faster than on cloudy days. As spring draws nearer, the suns heat increases, along with longer daylight hours add the reflection of the sun on the snow and evergreens can quickly deplete their moisture reserves. As parts of the branches, needles and scales dry out, they turn a dull green which often goes unnoticed. It isn’t until the season progresses the needles and scales turn brown that the damage is noticed. At this point the chances of recovery are slim and the branch often has to be removed.

Broadleaf evergreens have large leaves, more surface area to catch the sunlight, transpire therefore and are less likely to survive the Alberta winter. When broadleaf evergreens are stressed for liquid, their leaves curl backwards minimizing the amount that is exposed to sunlight. Northern Lights Azaleas and PMJ Rhododendrons are listed as hardy to -35 or -40 ˚ C which equated with zone 3 or 4. Given a preferred location and well drained but moist soil they can survive or even thrive.

Watering evergreens well in the fall to ensure the plants go into winter hydrated is a good way to prevent them becoming brown. Watering evergreens on warm days and during chinooks will provide the plants with some moisture as the water seeps through the top layer of soil.

Blocking the glare of the sun and or the reflection of the sun on the snow slows the plant’s transpiration rate. Wrapping the plant in question with an open woven material that allows the air to circulate works well.

Shoveling excess snow over shorter evergreen tends to provide protection from the sun and moisture keeping the plants hydrated. Removing excess snow from around the evergreen eliminates cools removes the reflection from the snow which also slows plant transpiration.

In years where there is a good snow cover, tender plants will survive the winter to die in the spring. Either the plant has remained green under the snow or a number of consecutively warm days has encouraged the plants to start to put out tender new growth. A few nights of very cold temperatures will kill these plants. Covering them with a mulch or leaves in the spring will help them survive.

Debating which plants to recover or cover in the spring? Start with the ones that weren’t expect to overwinter. Some of the common herbs which are perennials in warmer climates will overwinter in sheltered areas.

Do not worry about spring bulbs. They will start growing during the warm periods and stay static in the cold to grow again.

Spring and warm weather is on the way. Take the time to look at the garden and help plants where necessary.

Linda Tomlinson is a horticulturalist that lives near Rocky Mountain House. She can be reached at

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