The days are getting shorter. They can still be quite warm but evenings cool down and night time temperatures are approaching the freezing point. It’s time for perennials, shrubs and trees to get ready for the challenges of surviving the deep freeze of our prairie winter. When spring comes these plants will need a reserve of energy to re-emerge or to leaf out and begin the next year’s growth.
We think of plant hardiness as a plant’s ability to survive a certain minimum temperature. Temperature is important but many other factors play into the hardiness of trees, shrubs and perennials. A better understanding of these factors will help us to improve the winter survival of plants in our gardens. A plant’s natural adaptation to the climate and to the planting site, the health and nutritional status of the plant, the availability of nutrients and moisture through the summer and fall, the freeze-thaw cycles and moderation of soil temperature by snow cover or mulch can all affect the plant’s survival.
Plants that are adapted to our climate have survival strategies to cope with long cold winters. In Edmonton where the temperature usually remains cold throughout the winter, plants can go into winter hardiness state in the fall, lasting a relatively short time - perhaps two or three months. After that they remain dormant because of the cold temperatures: if temperatures rise above freezing they may start growing again. In Calgary, many plants that would survive in cold Edmonton succumb to winter injury because they begin to grow during a winter chinook.
However, some plants have genetically adapted to conditions of temperature fluctuation, such as Calgary experiences, by remaining physiologically winter hardy for a longer period. (They may not be able to tolerate temperatures as cold as Edmonton’s, however.) Let the chinooks come: they can tolerate rising temperatures.
Species and cultivars must be tested and evaluated to discover which adapt best to a climatic area. Gardeners should plant proven hardy cultivars for the “bones” of their garden. Experiment by planting more tender ones in protected areas.
Climatic variations act on a smaller scale in our gardens. Shelter from wind will protect trees from its chilling effects and will help to preserve snow cover. Trunks of trees that are protected from winter sun will be less likely to suffer from sunscald injury. Good drainage will promote root health and maturing of growth in the fall. Mulching of the soil will reduce freeze-thaw cycles and keep plants dormant through brief warm periods in early spring.
Healthy plants store energy that the leaves manufacture over the summer. To promote winter survival, gardeners should ensure that plants have had adequate water, nutrients, and air for respiration and strong root growth. Attacks by insects or diseases that defoliate a plant should be controlled. Good maintenance practices are especially important for plants that are newly planted and becoming established.
To prepare for winter, the plant needs to shut down food production in the fall and move its food stores to the roots until growth resumes in the spring. These processes proceed more effectively if nitrogen levels in the soil are allowed to decline. In nature this happens in late summer as soils become drier and vegetation takes up nitrogen from the soil. Don’t apply nitrogen fertilizer after mid-summer and reduce the frequency of watering in late summer.
Although roots are hidden below ground, they play a crucial role in a plant’s winter survival. Healthy roots act as storage organs to hold the sugars needed to start growth and produce the first leaves in the spring. Plants take advantage of spring moisture to make root growth in early summer; most also grow new roots in the fall to store the sugars and take in added nutrients and water that will support spring growth. We often get rainfall in September to provide for this root growth but if soil conditions are dry then we should supply additional water. Just remember that roots need air as well as water to grow. Thoroughly soak the root ball once or twice but allow the water to drain away between waterings. Too much or too frequent watering can be as detrimental as drought.
Plant roots are much less tolerant of cold temperatures than are the tops. Temperatures even a few inches below the surface of the soil seldom fall below -10 degrees, and plants should be protected against extreme cold that deviates from the norm and against a warm surge. Snow cover is very good insulation for the soil but we can’t always count on it at critical times. Adding a mulch layer will have a similar effect in moderating soil temperature and protecting the roots from freeze-thaw cycles. If you have plants in pots that you want to over-winter outside, you should dig a trench and cover them with soil or with a mulch material like straw or shavings.
Tips for over-wintering perennials, trees and shrubs:
- Choose plants that are adapted to the conditions that exist where you will be planting them.
- Develop a wind-protected microclimate.
- Use mulch to moderate soil temperature and reduce freeze-thaw cycles.
- Promote plant health with good maintenance practices.
- Allow nitrogen fertility to diminish and reduce watering frequency in late summer.
- Make sure the roots have adequate moisture going into winter by watering in fall.
- Protect the roots of container plants by digging a trench to hold the containers or protecting them with mulch materials.
With these actions you can have healthy and vigorous plants that will survive the winter and bring increased beauty to the landscape.
Hugh Skinner operates the Frank Skinner arboretum near Roblin, Man., and has for 30 years been involved in the testing and nursery production of a wide variety of hardy plants.
*Originally published on Alberta Gardener 2007 Fall issue
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