How to - Gardening Basics

Hardiness zones demystified

By
Katharine Fletcher

What do those zone numbers mean... and how do they apply to my garden?


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 (metre-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.”

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