Winter Protection in the Garden
Joe Gadbois, October 22, 2024
It’s no secret that Alberta is a challenging climate to garden in. There are a number of reasons for this, but the biggest one is arguably our long and harsh winters. One of the most common questions we get here at Greenland at this time of year is, “What plants should I protect for the winter and how should I do it?”.
A Primer on Winter Hardiness
Plants vary in their ability to withstand cold depending on what part of the world they originate from. A common misconception is that plants grown in Alberta are more winter hardy than plants grown in, say, BC or the US. Winter hardiness actually has to do with the genetics of the plant, not where it was grown. That means a species of plant that grows wild in similar climates to ours should survive our winters, regardless of where it was produced. Hybrids and cultivars of plants with a hardy genetic background should inherit the hardiness of their parents as well. Frequently, more northerly species of plants will be included in breeding programs to improve the winter hardiness of their offspring.
It’s important to understand that whether a plant survives the winter is not a black or white thing and there are many factors at play. There are many popular garden plants that simply will never survive our winters – generally we grow these as annuals. For hardy or borderline hardy plants though, cold alone is not the only factor. More and more we are seeing extreme temperature swings in winter, and this has been proven to be more detrimental than extended periods of very cold temperatures. If winter starts abruptly, or ends abruptly, or interrupts spring, that can also cause problems. Plants going into the winter dry are more likely to suffer winter damage as well. Dry winter winds can cause some evergreens such as cedars to desiccate. Sun reflecting off snow and siding can have a similar effect. If plants are growing in a poorly drained area which floods in the spring as the snow melts, they may rot before they start into growth. All these things can cause damage on even the most cold-tolerant plants.
We use hardiness zones as a guideline to help consumers make the right choices for their climate. Unfortunately, while Natural Resources Canada and the USDA create hardiness zone maps based on climate data, there is no central authority on zone ratings for plants, which means if you research a plant, you will likely find multiple different opinions on its hardiness. Growers and breeders will trial plants and print hardiness zones on their tags accordingly. Unfortunately, Alberta is a very small market compared to the US, or even southern Ontario, so these companies will rarely trial colder than zone 4 or 5, meaning even plants that have been grown successfully in zone 2 or 3 for decades are often labeled zone 4 or 5. All of this combines to create confusion and skepticism amongst gardeners.
The hardiness zone maps themselves are updated every couple of decades and are predominantly based on minimum expected winter temperatures based on historical data. The easiest way to find the current hardiness zone for your municipality is to use Natural Resources Canada’s Plant Hardiness by Municipality page. If you’re in Greater Edmonton, you are zone 4a. Outside the city you’ll be in the zone 3 range. The idea is that plants rated for your zone or lower should be winter hardy, while those rated for higher zones will be tender. At Greenland, 95% of the perennials, trees, and shrubs that we carry are winter hardy for the Edmonton area, but we do carry a few tender plants for those who want to experiment and those are always clearly marked. Plants that are one, sometimes even two zones higher than yours can often be overwintered successfully outdoors with some winter protection.
What Plants Need Winter Protection?
Anything rated zone 5, and perhaps even zone 4 plants depending on where you live, should get winter protection. Some shrubs that fall into this category include rhododendron and azalea, boxwood, tender roses (such as hybrid tea, floribunda, and David Austin types), Endless Summer hydrangea, and daphne. If you are in a rural area, sumac and smokebush would also qualify.
On the perennial side, hybrid clematis, tree peonies, certain types of cobra lilies, and most corydalis would be examples of tender plants in Alberta.
Usually, tender trees won’t survive because they are so difficult to protect. Borderline hardy trees and evergreens like sugar, red, and Norway maple, and cedar, can be very successful in established urban neighborhoods and sheltered areas but may experience winter damage in exposed rural settings.
You may also have a plant that you really don’t want to lose, and it might be worth giving some extra protection even if it’s not tender!
Planting Location
Plant more tender things in areas that are sheltered by trees, buildings, fences, etc. Nooks where snow tends to accumulate are ideal. Snow is the number one insulator, but we can’t rely on its presence all winter. Throwing snow on top of tender plants whenever possible makes a big difference.
Raised beds/planters can work for hardy plants but are not suitable for tender plants. Even the hardiest plants will not overwinter reliably in pots outdoors in Alberta.
Hybrid clematis should be planted against a heated foundation, 6 inches deeper than pot level. Tender roses and tree peonies should be planted with the graft union 6-8 inches below the soil surface. Note that hardy/species clematis, hardy roses, and herbaceous and Itoh peonies should be planted at pot level and do not require winter protection.
Watering
Make sure none of your plants go into the winter dry. All your perennials, trees, and shrubs (even hardy ones) should be watered deeply just before the ground freezes. This is especially true of any plants that don’t lose their leaves in fall (conifers i.e. spruce and cedar plus broadleaf evergreens like rhododendron and boxwood). A deep root feeder is an invaluable tool for getting water where it needs to be.
Mulching
Mulching for winter is the practice of deeply covering a plant with natural material to provide extra insulation. Materials you could use include bark mulch, straw, compost, peat, or leaves. Bark mulch and leaves are my favourite choices for winter mulching because they are less likely to become too wet and cause rot. Make sure leaves you use are free of insect pests or disease – leaves from trees or shrubs that experienced these problems this season should be thrown in the garbage and not used for mulch or compost. If you chop up your leaves with the lawnmower, they can be worked into the soil in spring to add nutrients. 2-6 inches of mulch can be piled over perennials. To mulch shrubs, ideally, you’ll want to cover the entire shrub; you can use a box with no top or bottom as a frame to hold the mulch in place around the branches. Wait until daytime temperatures are consistently below zero before mulching! Mulching too early can cause rot and other problems. In spring, you will want to remove the mulch in layers as it thaws. This will prevent rot and plants breaking dormancy early.
Burlap Screens
A great way to protect larger shrubs, trees, and evergreens is to use burlap screening to shield the plant from cold, dry winds and sun reflection. You don’t want to wrap your plants as many people do as this restricts airflow and can cause problems. Drive stakes into the ground around the plant while the weather is still warm and put up your screens once temperatures are consistently below zero during the day.
Anti-Desiccant Sprays
These products (such as Wilt-Pruf) are designed to be applied to conifers to help prevent browning over winter, but they must be used correctly. In Alberta, we find the spray must be applied in midwinter, when the plants are completely dormant. If applied too early, the product can cause damage. You will need to apply on a day during a midwinter thaw when it’s above zero.
Overwinter Indoors
Tropical and subtropical plants can often be stored indoors over winter relatively easily. It’s common for people to keep bedding geraniums indoors, for example. Non-hardy temperate plants (like Japanese maples), however, are trickier, because they still need a cold winter dormancy, yet won’t survive the extreme cold of our winters. For these plants, you need a heated garage kept cold, a cellar or crawlspace, a heated sunroom kept cold, or some other space you can keep at around 5C over winter. You don’t want it above 10C or below -10C. Sometimes, an insulated unheated garage will work, especially if it’s attached. Plants overwintered indoors will need to be checked for water once or twice monthly. They should be kept just shy of totally dry. Detailed instructions on how to overwinter indoors would warrant a separate post!