zones?

on tomatos thread, there is mention of people being in various zones.

Can someone explain what they are, how they are defined, and maybe link to a complete list, so that I (non-US user) can translate! Thanks.

USDA zone info (with link to map):
http://www.usna.usda.gov/Hardzone/index.html
Map: http://www.usna.usda.gov/Hardzone/ushzmap.html?

Canadian zone info:
http://www.planthardiness.gc.ca/ph_main.pl
Map: http://nlwis-snite1.agr.gc.ca/plant00/

Hope that helps :)

Cold hardiness zones: Determined by the maximum cold that a plant might endure during a winter. For example, I'm in zone 4a, so my plants need to survive cold to -25/-30F (or -34.4/-31.7C). Of course, things like snow cover, length of cold snaps, and freeze/thaw cycles need to be taken into account, but the zones give an overall idea of how well a plant might do.

I also found this link for the UK. Read the text on the side - it's interesting

http://www.trebrown.com/hrdzone.html.

There are also heat zones, which are more applicable to the southern US than the northern part. For hotter areas of the country, it's more important to know what will survive the heat than the cold

http://www.ahs.org/publications/heat_zone_map.htm.

Edited to try to fix the funky links, which for some reason won't work correctly. Finally "de-linked" them.

Only the US zones are based on maximum cold - the Canadian zones take other factors into account also.

well I was gonna help ya out but seems like you got it well in hand! happy gardening!

But, of course, when growing tomatoes, the length of the growing season is much more important than the hardiness zone. My growing season is as long as your typical USDA zone 4, even though our winter is only as cold as a zone 5 or 6 (nowadays).

Well assuming the numbers are about the same pretty much all of the UK is zone 8a or b - but as noted we have very long winters compared to an equivalent zone in the US as we are much further north.

Thanks for the explanations.

Your winters may be long, but they are mild. When would you say your last frost (0C/32F) occurs in the spring? And when would you expect your first frost in autumn?

Last frost might be as late as mid-April, (although this year that was the warmest its been all year).

First frost - mid Oct maybe. Some years it doesn't seem to have a frost until after Christmas.... just dark (by 4pm-9am), wet and unpleasant.

Hard to say, the weather has been very odd recently.

We won't have your heatwave problems either, me may just about reach 30C one or two days a year.

I guess this blandness compared to US weather is why we don't normally think about weatehr zones.