Las Vegas is generally 9a... you will have no issues with D.A. roses as many are zone 5/6 USDA and Las Vegas just doesn't get that cold.. the heat and English Roses..well that is a different story... most will definitely need afternoon shade.

I think they use USDA.

It's the USDA hardiness zones. These only pertain to the coldest temperatures a plant can withstand.

Sunset's hardiness zones are not just about cold tolerance. They really describe the climate. Sunset Magazine is a lifestyle magazine for the western U.S. which includes mountains, deserts and seacoasts. The climate on one side of a mountain range is wetter than on the other side, and it can be warmer in areas between the mountains.

All gardening advice and zones originated on the East Coast, nobody ever gives advice directed to West Coasters. That is how Sunset zones came about, Sunset began as a magazine directed toward Californians, because the East Coast gardening knowledge was usually wrong for here. For example, all advice will say "hardy down to 15 degrees", for example, but but nowhere do you find "hardy UP to 105 degrees". Only recently did agriculture develop "heat zone degrees", which are much more important for most west coast states than hard frosts. It always cracks me up when lazy local news uses Washington Post articles about gardening and home repair , discussing roof ice dams and cool September mornings. I feel sorry for newbies who plant their annuals as directed in "full sun" in May, because full sun in Maryland is only a whisper of Sacramento sun for 106 degrees in June.
So you learn living in the west to use "9" as a rough estimate, but use the Sunset number to really determine your actual likelihood of climate, and the most accurate is to be asking local gardeners.
Carla in Sac

I would imagine USDA zone but it is an interesting question and I hope someone can help you out.

Or you could call David Austin directly and Texas and ask them.

In the US Catalog, they are probably using the USDA zones. There are differences in zones even with the same # though. For example, there are going to be DA's that are better for dry zone 9's and DAs better hot & humid zone 9's, so its good to ask for recommendations from forum members for your specific part of the country. I hope that helps :)

I am also inclined to believe DA is using USDA. They categorize their roses under Hot and Dry Climate (2019 Catalogue) and that's where I echo Carla in Sac's comments. Hardy 'up to' what temps in hot and dry climates? Here, in LV, we hit 120 quite easily in July.

My DA exercise in LV will be trial and error based on sun zones around my own home(courtyard vs backyard).

Thank you everyone for your time and responses.

"All gardening advice and zones originated on the East Coast"

Well, that's just not true at all :-) If anything, until recently, most published gardening advice originated from the UK, not the east coast! And one needs to make the distinction between the USDA hardiness zones, which only measure winter cold tolerance, and climate zones (like Sunset), which take into consideration many other factors. But again, few growers or catalogs will ever publish or list suitable Sunset climate zones. There are too many (45 total - 33 just for the west coast) and they are not that widely used or accepted.

Best to ask locally what works or doesn't work in your location. I can't imagine too many Austins that wouldn't work - I see them all over SoCal!

Austin Roses for the West Coast

Gardengal

Since you are correcting me, are you saying gardening advice written in England is appropriate for Las Vegas?
The link you provided says NOTHING about gardening in heat, at best it suggests roses might get "large" in "warm areas".

I gave a specific example of common advice written not for the hot and dry West, here are several more:
Rodale Gardening books-- one of the earliest bibles for gardening, out of Pennsylvania
The Vegetable Bible, now leading on Amazon, out of Vermont.
Old Farmer's Almanac, published in New England
Better Homes and Gardens--Des Moines Iowa

I have no problems admitting when I am wrong and learning new things, but next time you are going to publicly dismiss me once again, please give specific (and correct) reasons why I am wrong.
Carla in Sac

I didn't get that from the comment at all. If anything just the opposite, but I didn't read the link. I would think that a lot of general rose advice from more modern sources would reference Ca growing conditions since so many HTs are grown there. That's one reason these forums are so helpful.... we can read what does well for others in our zone! However, I admit, seeing a rose grow beautifully in Ca or OR does make me more apt to get it which is very silly. I also got a skewed version of things since my Mom got her subscription to Sunset long after we'd moved out East and It never occurred to me that's why it was created.

I wasn't dismissing anyone! I was merely pointing out that it has been the case for many years that most gardening advice came from the UK....the books you cite are relative newcomers, all things considered. I have a pretty extensive personal horticultural library and probably at least half the books are UK publications. Because that's where all the serious horticulturists lived. And I am NOT saying that is more appropriate for Las Vegas or the west coast.....heck, 95% of it is not necessarily appropriate for most of the US!!

And that link is from the Pacific Horticulture Society, which is a California based organization...........therefore it addresses west coast gardening issues, including ALL of CA.

I also tried to make it clear that USDA hardiness zones have nothing to do with how well any plant will survive in different locations other than to address winter cold tolerance - that is all the USDA hardiness zones were intended to do. Upper numbers are meaningless....if a plant survives a zone 5 winter, of course it will survive a zone 8 or 9 winter!! You have to look at other data for that information, which is why I also said to check locally, as virtually no growers assign Sunset zones - which admittedly would be very helpful - to their plants.

Please accept my aplogies if that was too difficult to understand.