Answer for Plant Hardiness Zones (climate, snow, hot, warm)

[Winter-40c]

Iam in the colder side of Plant hardiness zone 5 and its from -23.3c to -28.9c but its gotten colder than that in the area its been down to -35c and the trees didn't die they survived, and it was not just one day , doesn't that mean our trees are hardy to Zone 3b.

[B4U]

USDA Map of North American hardiness zones, issued 1990 (for data 1974-1986). An interactive version of this map is available at http://www.usna.usda.gov/Hardzone/ushzmap.html




The hardiness zones are a general guide. Other factors are your microclimates, and wind chill & how many hours of sunshine, water/snow, and such.
I'm in 10a, but my NE side of the house is probably a 9 and is always shaded. The SW side is probably a 10b at the hotest part of the day.

[ColdCanadian]

All they are is a measurement of the AVERAGE coldest annual low.

I've seen forecast lows as low as -40 C, which is also -40 F.
I've seen forecast highs as low as -36 C, which is maybe -33 F.
(same day as the -40 F low, but it was overcast too )

I live in the warmer end of zone 5B, which means annual average lows between -23 C to -26 C. (-10 F to -15 F)

Some of our native plants might have suffered a little damage,
however none of our native plants have been killed by even our severest winters...
just a lot of non-native "garden" plants.

[deneb78]

Hardiness zones as a rule are rather useless, especially here on the west coast. I live in Vancouver, BC which is hardiness zone 7b/8a but there are many plants from the southeastern US which is also rated 7b/8a which would wither and die here...our summers just don't get hot enough. As one poster mentioned, there are many factors which affect plant hardiness such as amount of summer heat, length of growing season, length of cold spells in winter, precipitation, moisture content of air and soil as well as snow cover or lack thereof among other things....
As another example, the southwest coast of the UK and southern Ireland is rated hardiness zone 10a but you would never see coconut palms grown there such as in zone 10a in Florida...