If you’re new to gardening, the concept of hardiness zones can be a bit confusing. Fear not! We’re going to break it all down in this blog post.
Before we jump into hardiness zones, let’s cover some basic terminology to go over the foundations.
Hardiness refers to a plant’s ability to survive adverse weather conditions.
Plant hardiness is generally divided into 2 categories: hardy and tender. Simply put, hardy plants are plants that can survive the winter and tender plants are those killed by freezing temperatures.
What's a hardiness zone?
A hardiness zone is a geographical area that has similar weather conditions that are relevant to plant growth and survival. In Canada, there are 10 zones, which are numbered from 0 to 9. The higher the zone number, the warmer the climate.
The zone system was originally developed by the USDA in the 1960s, and has since been adopted and refined by Natural Resources Canada.
Canada’s most recent Plant Hardiness Zone map was developed based on climate data from 1981-2010 across the country. The model that determines each zone takes a wide range of climate variables into account, like minimum winter temperatures, maximum temperatures, rainfall, snow cover, wind, and elevation.
Why are they important?
Hardiness zones are important if you want your plants to thrive in your garden and survive the winters. Selecting plants that are best suited to your garden’s climate is key to ensuring your plants flourish. This is especially important for perennials, shrubs, and trees.
Since every plant has its own ideal climate conditions for growth, every plant has hardiness zones that correspond with those ideal climate conditions. The hardiness zones for plants should be identified in product descriptions and/or plant tags.
Practically speaking, when someone says, “This plant is hardy to zones 5-9”, it means that it will grow and survive the weather conditions in those zones year over year.
Perennials are plants that come back every year, whereas annuals only survive 1 year or season. To keep your perennials coming back every year, it’s important to plant them in zones that work best for them.
A perennial that’s planted in a zone that's much too cold will act like an annual since it won’t survive the winter. A good example of perennials are treated like annuals are garden mums. Some varieties are hardy to zones 7-11, so if you live in a zone 6, there’s a chance they won’t come back the following year.
Conversely, a perennial that’s planted in a zone that’s too warm might not come back the next season if it requires a period of cold to kick off their lifecycle. Tulips and other fall-planted bulbs are perfect examples of plants that require a period of cold in order to grow.
How do I know my hardiness zone?
You’ll also notice that the zone numbers are suffixed by ‘a’ and ‘b’. These letters simply provide more granularity within a given zone. The ‘a’ signifies the colder half of the zone and ‘b’ signifies the warmer half.
Why are there two different maps on the government’s website?
Canada's Plant Hardiness Zones map is Canada’s official plant hardiness map, which takes into account 7 climate variables. There are two versions of this map: an older version using data from 1961-1990 and a newer one from using data from 1981-2010. Given that Canada’s climate has changed over the decades, it’s better to rely on the newer map with more recent data.
The Extreme Minimum Temperature Zones map generates from the original USDA approach, which only looks at the average annual minimum temperatures for the country.
This is useful for businesses that are trading plants between the two countries and need to make comparisons between the two maps.
In general, Canada's Plant Hardiness Zone map is consider more accurate since it takes more climate variables into consideration.
Things to keep in mind:
- If you want to overwinter perennials outdoors in containers, choose plants that are hardy to 2 zones lower than your hardiness zone. For example, if your are in zone 6, choose plants that are hardy to at zone 4 and below. This is because containers tend to provide less insulation to plants and more extreme temperatures since they are out of the ground.
- Hardiness zones are meant to be used as a guide. It doesn’t take into account factors that would create micro-climates that would be different than the surrounding areas.
- Since we're working with mother nature, there will always unpredictability and anomalies with the weather. An unseasonal cold snap could kill off plants that would normally survive the winters and an unusual summer drought could kill off plants that usually get enough water from the rain.
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