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Get in the zone

Choosing the right plants for your garden and landscape can be overwhelming. Plant hardiness maps with zones based on the average minimum winter temperature can help you select plants that are more likely to thrive in the area where you live.

by Melinda Myers

You can't control the weather, but you can increase your gardening success by selecting plants that survive and thrive in the cold of winter and the heat of summer. Unlike annual plants that grow, reach maturity and die in one growing season, perennial plants—those that grow for multiple years in the garden—need to survive the winter cold to return each year.

Plants in winter conditions

Canada’s Plant Hardiness Zone Map and map key.
SOURCE: “PLANT HARDINESS ZONE MAPS.” NATURAL RESOURCES CANADA, 2014. REPRODUCED WITH THE PERMISSION OF THE DEPARTMENT OF NATURAL RESOURCES, 2022.

Stay in the zone

A plant’s tolerance to winter temperatures is reflected in its zone hardiness. Most plant tags, seed packets and other gardening resources contain growing information on the sunlight, soil type, wind and, most importantly, climate that an individual plant needs to thrive. Compare this number to your area’s hardiness zone, which you can find on the U.S. Department of Agriculture’s (USDA) Plant Hardiness Zone Map (planthardiness.ars.usda.gov) covering the United States, Canada (located on the 1990 map) and Mexico.

Introduced in 2001, Canada’s Plant Hardiness Zone Map (see the map on page 25; planthardiness.gc.ca) considers seven climatic variables, such as rainfall, elevation, heat, wind and humidity, for its plant hardiness zones.

The USDA Plant Hardiness Zone Map is broken into 13 zones. Each zone represents areas within a 10 F (about 5 C) range of the average minimum winter temperatures. These areas are then subdivided into narrower 5-degree segments. The lower the zone number, the colder the average winter temperature.

As you look at the USDA Plant Hardiness Zone Map, you may notice islands of warmer or colder zones within the larger zone hardiness swath. Large bodies of water, urban heat islands, mountains and valleys influence the surrounding climate and growing conditions. These microclimates also exist within your landscape.

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Properly caring for plants will help them thrive throughout the year.
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Zone training

Niki Jabbour, a gardening book author and Costco member based in Halifax, Nova Scotia, considers both the Canadian and USDA plant hardiness zone maps useful tools for gardeners, even though they are based on different factors. There is also an extreme minimum temperature zone map that focuses only on extreme minimum temperatures based on the USDA zones.

Jabbour finds most Canadian gardeners use the U.S. and Canadian maps interchangeably. On the Canadian Plant Hardiness Zone Map, her garden is in zone 6a, but on the USDA Plant Hardiness Zone Map, she’s in zone 5b. The two maps are based on different factors, but the zone recommendation is similar.

Brian Minter, a grower and horticulture expert based in Chilliwack, British Columbia, also recommends using cold hardiness as a guideline but reminds gardeners to consider the impact of larger bodies of water, elevation, soil type and more when selecting plants. In his province, he works with gardeners growing plants in zones 7 or 8 around the city of Vancouver and islands off the coast of British Columbia to zone 2 in the far north. Even those gardening in zone 5 can be surprised by an arctic wind blowing through and causing a sudden and extreme drop in temperatures that can negatively affect plants rated hardy for that area.

Braving the cold

To help plants survive the cold of winter, Bob Polomski, a Costco member and horticulture specialist at Clemson University in South Carolina, recommends identifying pockets of microclimates in your landscape where the temperatures may vary by several degrees. This difference can mean life or death in cold weather. Microclimates are influenced by sun exposure, existing plants and nearby structures. Colder winter temperatures generally occur on the north and northwest parts of a property and in low areas where cold air settles. Tree canopies, overhangs, arbours and fences also create microclimates in your landscape.

Use winter mulch to provide added insulation for plants that are not reliably cold hardy in your area and to discourage early sprouting during winter thaws. Cover susceptible plants with a layer of evergreen boughs, straw or marsh hay after the ground freezes. Use nearby plantings, screens and fencing to protect evergreens subject to injury caused by winter winds and sun. Burlap and other plant-protecting fabrics can also be used to shield sensitive plants from winter wind.

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Caring for plants may include sheltering them from cold weather; watering plants to keep roots moist.
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Beat the heat

These resources can also help when considering the impact extreme heat has on plants. When temperatures exceed 30 C, many plants suffer short- or long-term damage such as withering, leaf and flower drop, stunted growth and more. The longer a heat wave persists, the greater the risk of damage.

Minter notes that heat tolerance has become even more important in the last few years. Areas in British Columbia experienced temperatures of 43 to 48 C during the summer of 2022. Gardeners are faced with finding plants that tolerate both extreme heat and cold. Plant breeders are working feverishly to select and develop plants suited to this change of climate.

Another solution, says Polomski, is providing afternoon shade and mulching the soil to keep plant roots cool and moist. When designing a new landscape, selecting the most zone-appropriate plants and arranging them to help create a more desirable environment can help.

Drought and water restrictions also make selecting plants suited to your area’s rainfall more important than ever. Once established, drought-tolerant plants require less frequent watering. That means less work for you once these plants have formed a robust root system. Minter recommends creating a well-drained soil so plants develop deeper, more-drought tolerant roots. These plants are better able to survive winter cold, summer heat and drought conditions.

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Covering seedlings to protect them from the sun and birds; replacing faded annuals with those better suited to the weather
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Successful shoulder seasons

The average last spring and first fall frosts also influence plant selection and planting times indoors and out. The number of days between these two dates defines the frost-free growing days in your location. Compare this to the plant tag, seed packet or plant description. Most of these resources spell out the number of days from planting to harvest or bloom. Match the number of frost-free days in your area with those required by the plant.

Overcome this limitation by growing varieties that require a shorter season, start plants from seeds indoors or purchase starts to transplant for those plants that require a longer growing season.

Toronto-based Steven Biggs, host of The Food Garden Life Show, recommends choosing cold-tolerant plants so gardeners can enjoy gardening in the shoulder seasons. Planting dates, indoors and out, are also based on a plant’s ability to tolerate cold soil and air temperatures.

Jump-start the growing season or extend it past the first fall frost with season-extending strategies. Use structures, such as cold frames, cloches and floating row covers, to protect young seedlings and transplants—or your harvest—from chilly temperatures and frosts. Jabbour uses garden covers such as polytunnels, cold frames and mini tunnels to create microclimates for vegetables, allowing her to extend the harvest into winter, but also to overwinter tender perennial vegetables.

Many gardeners choose to incorporate native plants, which are better adapted to their local climate conditions. Biggs recommends talking with nearby gardeners to find out what works in your immediate neighbourhood, as hardiness differences can exist within a small area. Visit your local botanic garden, seek plant recommendations from local university sources and consult with plant experts to help with plant selection.

With a little planning, your landscape can be an extension of your lifestyle, values and sense of ambience that helps reduce the stress of daily life. Wherever you garden, select plants that match the growing conditions, space, your design and the time you have to care for your landscape. That way you will have time to relax and enjoy the efforts of your labour.


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Water wise

Water new plantings often enough in the first few weeks to keep the roots moist but not soggy. Gradually reduce frequency so you will be watering thoroughly but less often. Watering frequency varies with the soil, climate and plants you are growing. Proper watering helps plants develop robust, more drought-tolerant root systems. Spread a layer of leaves, evergreen needles or wood chips over the soil surface away from tree trunks, shrub stems and the crowns of perennials. These organic mulches help conserve moisture, suppress weeds and improve the soil as they decompose and moderate soil temperature.—MM


Melinda Myers (melindamyers.com) has a master’s degree in horticulture and is an author, columnist, TV and radio host.

Costco Connection: You'll find a variety of plants and garden supplies in Costco warehouses and at Costco.ca.

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