Hardiness Zones – Gardening
What do those zone numbers mean… and how do they apply to my garden?
For gardeners, “being in the zone” matters—a Plant hardiness Zone, that is. An interactive online map of Canadian and USA zones allows gardeners to zoom in and discover their zone to Planting their veggies, plants, and flowers. But what do those seventeen numbers starting at zero, through 1a, 1b and culminating at 8a mean?
The first map and the “new” map
Canada’s original Plant Hardiness Zone map was created in 1967. However, as we gardeners know, times—and climatic conditions—are changing. Scientists from Natural Resources Canada’s Canadian Forest Service, Agriculture, and Agri-Food Canada, and Natural Resources Canada’s National Atlas of Canada collaborated to create the new Plant Hardiness Zones Map. Published in 2000, it used data captured between 1960 and 1990.
The new data incorporated information on elevation, so exposure effects are factored into the hardiness equation. The hardiness zone map is useful for home gardeners through to nursery owners and large-scale orchardists to refer to these zones prior to investing in plants.
What is the relevance of the numbers?
The nine major plant hardiness zone numbers describe Canadian growing conditions, where zero (0) describes the harshest environment and 8 represents the mildest. A simplistic way to remember the numbers is to think of zero being harshest, and 8 being very mild. When we look at the map of Canada divided into its zones, this approach works. All of Canada’s north including the Arctic region falls under zones 0a or 0b, the color is blue—icy blue.
Usefully, warmer zones go from green to yellow to red, where fuschia (zone 8a) represents the mildest growing conditions, found on British Columbia’s West Coast. The general rule of thumb is that when you’re buying plants, you buy plants that are tagged for your zone (or lower) because you know the plant will survive in your zone’s specific weather conditions.
Buying plants for your zone and what do ‘a’ and ‘b’ mean?
Buying plants for your zone
My husband Eric and I planted 24 fruit trees in 2011: 22 apples and two pears. When consulting the nursery operator from whom we purchased bare-root whips (meter-high saplings), we identified our location and hardiness zones (4a/4b) with an occasional zone 5 species surviving. He recommended varieties hardy from zone 3a to 4b. Species include Snow and Wealthy (3), Northern Spy and Winesap (4). Looking now at the Silver Creek Nursery website, I realize we could have experimented with Rescue, which is hardy to zone 2.
Why bother dipping into zones below the sub-zone in which you live? Because if the plant does well in zone 2 or 3, it will likely grow even better in zone 4.
What’s key is that as the numbers increase, so does the mildness of the zone. Clearly, not all lower zone plants would do well in milder zones: it’s prudent to be cautious and ask the experts at the nursery proprietors.
What do a and b mean?
The numbers from 0 to 8 are broken down into sub-zones, such that each number (except 8), has an associated a and b. These descriptors break the numbers into a finer set of sub-zones. Agriculture and Agri-Food Canada’s website informs us that these numbers were derived from plant survival data, as well as a number of climatic variables ranging from seasonal temperature fluctuations and frost-free periods to summer and winter precipitation, and wind speed.
The subzones come in handy to explain micro-climate conditions. According to the website, “some significant local factors, such as micro-topography, amount of shelter and subtle local variations in snow cover, are too small to be captured on the map. Year-to-year variations in weather and gardening techniques can also have a significant impact on plant survival in any particular location.”
A micro-climate example and “beyond the zones”
A micro-climate example
Over time, gardeners may recognize the subtleties of micro-variations in their gardens. Let’s look at an example. My farm, Spiritwood, is near Ottawa, but on the northern, more elevated (read: exposed, harsher) Quebec side of the Ottawa River. Whereas Ottawa is in hardiness zone 5a, according to the zone map, my farm, located 50 minutes northwest of the Parliament Buildings, is between zone 4a and 4b.
Interestingly, my growing season is different than that of some friends who live a 20- to 40-minute drive west of me, in harsher conditions. While my tomatoes ripen on the vine; theirs frequently must be gathered to ripen indoors.
Moreover, because of microclimates my husband Eric and I have created since 1989, we can extend our plant hardiness zone to (somewhat unreliably) include species requiring 5a.
Beyond the zones
Natural Resources Canada’s Canadian Forest Service is now calling on the public to tell them which plants from a comprehensive list are surviving in their zone. These will be added to range maps for individual trees, shrubs, and perennial flowers. Take a look at the website to discover how you could contribute data for this project.
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