The USDA hardiness zones only apply to expected winter cold temperatures. They have nothing to do with how well a plant can tolerate summer heat or humidity. Assigning a hardiness range is meaningless.....if they will withstand a zone 4 winter, then of course they will withstand a zone 9 one!!

To determine if a plant is able to withstand your summer conditions, you need to look at other sources.......like the heat zone map. Or even better for your situation, the Sunset Western Garden Book. Nepeta x faassenii 'Walker's Low' is suitable for ALL Sunset zones (as are most other species and hybrids of Nepeta).

'Walker's Low' is a little misleading in its name :-) It is not low growing at all (named after the garden in England where it was discovered) but rather a big, vigorous plant, typically growing to 3' tall and as wide. There are other cultivars that would be more suitable for a groundcover or low growing edging plant. I'd look at 'Litte Titch', 'Little Trudy', 'Purrsian Blue', 'Cat's Meow' or 'Junior Walker'.

And it enjoys full sun and does well in dry, xeric gardens once established.

What about calamintha nepeta? It's a ground cover. It's tough. Grown easily from seed or cuttings. 'Marvelette Blue' bloomed for me first year. It's supposed to, but I thought I'd endorse that it really really did so. It's in my full baking sun part of the yard. I can't imagine it'd do any worse in your area since it seems to love heat abuse. It's really low growing. Truly a ground cover. And beautiful flowers for a long long time. It's really hard to take its picture. In real life it's paler and the foliage is quite lovely. Very compact. For $8 you could have 25 plants. It's worth a try, as an experiment

Thank you both! I thought Walker's Low looked sizable- I love the color of Purrsian Blue and Cat's meow (although it drives me crazy the way they adjust the color on the photos)! And I love, love the way the Marvelette Blue looks. I think a trio of the nepeta, calamint, and santa barabara daisy would look really, really nice.