Since we seem to still be getting a lot of new people without the Location field filled in...

[Dirtmechanic]

What do the temperatures mean? Is that how much hotter it gets or??

Cold cutoff zones. For example we have this fig tree that lets us know when we have gotten into the single digits by burning back to the ground. Conversely, there is some effort toward cataloging the number of heat days, that is 90f+. Here in 'bama we have more than a few. Heat can take out plants as well. It commonly gets so hot here that tomato pollen melts, so we get a summer off season to some degree (haha).

 

[Zeedman]

There are several versions of the U.S. Hardiness maps; to me, those that provide long-term lows are the most useful. If I am planting long-lived nursery stock (such as fruit trees or grape vines) I'm less concerned about "average" low temperatures, than I am about the once-every-10-years low which could potentially destroy my investment. IMO the Arbor Day map is overly optimistic. A couple winters ago, we reached -28 F. in my area, which they have listed as Zone 5 (and has even been listed as 5b by others). Many of those locally who planted peaches & grapes based upon those rosy predictions lost their stock.

Is there anywhere on TEG that shows a map of the different grow zones?
I don't know my grow zone ¯\_(ツ)_/¯ and I didn't see the importance until reading this thread.

For those in the Western U.S., the Sunset Climate Zones might be more useful. Those are "growing zones". They take into account such influences as elevation and ocean proximity. I'd really like to see such a concept adapted nationwide, since the Sunset Zones provide information on year-round climate, not just winter hardiness. For planning a vegetable garden, the USDA climate zones are just about useless. The Sunset map for California & Nevada is linked below, but there are more detailed regional maps:
Sunset Zones for California

 

[Dirtmechanic]

There are several versions of the U.S. Hardiness maps; to me, those that provide long-term lows are the most useful. If I am planting long-lived nursery stock (such as fruit trees or grape vines) I'm less concerned about "average" low temperatures, than I am about the once-every-10-years low which could potentially destroy my investment. IMO the Arbor Day map is overly optimistic. A couple winters ago, we reached -28 F. in my area, which they have listed as Zone 5 (and has even been listed as 5b by others). Many of those locally who planted peaches & grapes based upon those rosy predictions lost their stock.

For those in the Western U.S., the Sunset Climate Zones might be more useful. Those are "growing zones". They take into account such influences as elevation and ocean proximity. I'd really like to see such a concept adapted nationwide, since the Sunset Zones provide information on year-round climate, not just winter hardiness. For planning a vegetable garden, the USDA climate zones are just about useless. The Sunset map for California & Nevada is linked below, but there are more detailed regional maps:
Sunset Zones for California

I pondered the name "Sunset" growing zones until it registered that I might consider that the sun setting in the western US had something to do with the zones being western. I get there, just more slowly than some folks.

 

[Zeedman]

I pondered the name "Sunset" growing zones until it registered that I might consider that the sun setting in the western US had something to do with the zones being western. I get there, just more slowly than some folks.

The Sunset climate zones were created by Sunset Magazine. They also published a very comprehensive garden guide, the Sunset New Western Garden Book, which I used often when I gardened in California. It contains all of their more detailed climate maps, and a very comprehensive Western plant encyclopedia. My copy is very dated (1983) but I still use it as a reference occasionally. I believe the latest version is the 9th Edition. I highly recommend this book to anyone who gardens West of the Rockies.