My understanding is that USDA zones are based on the cold hardiness of a plant. Which is good if you live in an area where the winters get cold at all. Now if you lived in an area that sees very hot summers and mild winters you would probably be more concerned with how much heat and sun a plant could tolerate than with how cold it can get before it kills. I would assume that Sunset zone and heat zone are simialr but I'm not sure as I don't have to worry about that too often. Where I live I have seen it get as cold as -25 F. and so I purchase all my plants according to that temp. The new map that was previously presented last week shows that the zones are getting warmer for the most part but I will stick with buying for my former zone and that which is the same as the USDA as I believe that most garden centers and nurserys go by that number. I also want a little insurance that my plants will survive no matter what the winter temps are. If I am mistaken someone please correct me. That is just my take on it.

MeMo

My understanding is that USDA zones are based on winter lows. Heat zones are based on summer heat and humidity. Sunset zones pertain to the west coast.

A USDA zone designation change will not guarantee that a plant rated as 'zone 8' is going to survive in the 'new' zone 8 -- or even in some old zones 8. I still consider myself to be in zone 8b and I grow many plants down here near the Florida line that did not thrive in Atlanta where I used to live. Many plants that grow in the Pacific Northwest zone 8 are not hardy here because of the heat and humidity of our summers.

I like Southern Living's designations of Upper South, Lower South, Coastal South and Tropical South.

Another gardening concern not addressed by zones is soil difference: Alkaline soil vs. acid, sandy loam vs. red clay makes a tremendous difference in plant performance.

I just do what works, most times. I prefer a garden that grows lush and full rather than struggling with unsuitable cultivars to prove that I can just make them survive. (Never mind about my tulips; it's a character flaw.)

Nell

Nell makes some excellent points, especially about the relationship between heat and temp and also soil. This is something that I learned from a trip to Plant Delights. They have most everything planted in these raised bed berms, and the soil is superfantastic. It is amazing what that kind of soil can do for a plant, and it's ability to deal with stress and disease, let alone adverse summer conditions. Their plants are wildly lush.
My garden has always been lush here, and I grow many things rated to hardiness zone 9 or zone 8b or so. This is not always due to anything dealing with a hardiness zone. Sometimes it's just that nobody has really experimented with growing the plant in a variety of conditions or areas. That is why a place like gardenweb is so helpful! I can ask other plant crazy maniacs if they have ever experimented with a plant, and how it did for them.

But aside from that, think about the California and western coastal enviornments. Visit Annie's Annuals page and take a look at what they grow winter to summer on their slide shows. The biggest difference is that they do not usually experience the seasons as we do (which is tongue in cheek, because we don't experience much in the way of seasonal differences as they do in so many regions). For some reason many of us think of California being warm all the time. It really isn't in much of the coastal state. It's cooler, and clammy and the summers can be quite cool. It is quite unusual to have any length of time with unbearable humidity in combination with a high heat. It's also uncommon for them to have the freezing temps they just had for a continual hour length.

Does this help?
I moved to GA about 9 years ago. At about the 4th and 5th year I began to really experiment with growing plants that were considered not normally hardy here. Bulbs, herbs and many other things. I had great success by manipulating the soil and/or moisture in the soil (usually with bulbs and succulents, planted in pots which I move to a dry area during the winter. Often zone 9-10 succulents will survive outside here and be evergreen if they do not get any water).

It's a balance!

GGG

Great points everyone. Thank you! Discovering this garden forum is one of the best things ever happened to me! I am greatful.
May all your flowers be happy and lush no matter which zone they are in!

My old (1991) Sunset Western Garden Book says, "A [Sunset] plant climate zone is an area in which a common set of temperature ranges, humidity patterns, and other geographic and seasonal charactersistics allow cerain plants to succeed and others to fail."

For instance, my zone differs from zone 19 because it gets more ocean influence, and therefore is better for moisture loving plants. If you look at the annual and record lows mentioned in the book for these two zones, they 're just about the same.

I know that Sunset publishes a similar book for the eastern United States. Does anyone here use it, and do they find it helpful?

Interesting info. In regards to the heat zones, they are based on the average number of summer days that get above 88 degrees. It doesn't take into consideration HOW MUCH above 88 degrees it gets or for how long. Doesn't factor in humidity or, even more importantly, the night time lows during the summer. So, there are also a lot of flaws with the heat zones too.
Theresa

http://www.thegardenhelper.com/hardiness.htm
See if this site helps. It should be the USDA garden zones. HTH Nancy