Among the Top 10 questions Toronto gardeners ask at Humber Nurseries is one about Canadian hardiness zones. In case you didn't know: Canada is different from the U.S. Here's why.
Both countries map their landscape into zones to denote plant hardiness -- where a plant will survive, especially over winter. Yet, Canada and the U.S. use different methods to define them.
Canada uses a complex formula of seven variables, and the product of that formula determines the zone number. Variables include lowest mean temp of the coldest month, highest mean temp of the hottest month, precipitation, number of frost-free days and other factors. Natural Resources Canada explains this all in greater detail at this link [Ed: link updated Feb. 28, 2014]
The U.S. Department of Agriculture or USDA system is simple: it's based solely on average annual minimum temperatures. (This mimimum temperature system is followed by most other countries.)
Although Canada considers a larger range of factors than the USDA, we still talk about zones in terms of temperatures: the lower the number, the colder the climate. Both systems also subdivide zones with a letter (a or b). So, Zone 6a is little "colder" than Zone 6b.
In Canadian terms, Toronto is broadly Zone 6; in USDA terms, it's Zone 5. Our closeness to the Great Lakes moderates temperature highs and lows and increases precipitation.
Within the GTA, the specifics of your landscape can push you higher or lower. Close to Lake Ontario, you're more likely to be Zone 6b and might have a microclimate (a pocket that is a zone unto itself) that's Zone 7. In a more exposed situation, or farther from the lake, you might be Canadian Zone 5a or 5b.
Get to know your garden. You might even have a sheltered spot or two where you can cheat and use plants rated for a slightly warmer zone than your area. Or you might have a windy corner that goes the opposite way.
If in doubt, the zone number should be your zone or lower. If you are in Zone 6 and the plant is rated Zone 5 hardy, it will likely work in your garden. If you see Zone 11, drop the plant (gently) and walk away. [I should qualify that: walk away if you hope it will over-winter outdoors in your garden. Many plants that aren't zone-hardy in Toronto are grown as annuals or brought indoors for the winter, including pelargoniums aka geraniums, coleus, fuschsias and impatiens, to name a few.]
Know who you're dealing with. Canadian suppliers usually use Canadian zones. International websites or gardening books are likely to use the USDA system.
Remember: Cultural requirements are another thing entirely. You may be able to cheat the zone, but not the food and drink. If your current object of desire asks for moist, fertile, loamy, acid soil in full sun, and you give it dry, sandy, lean soil in deep shade, the only zone you're living in is the Twilight Zone.