This information was supplied by our customers. I had two of them visit me at the San Francisco Garden Show at our booth. They both happened to be from the Sacramento area. As you know, they get well over 100 degrees in summer. Keep in mind that there is little humidity, unlike the Southeastern United States. There is little concern for fungal disease - just survival in the blistering heat.
They both swore (on a stack of Bibles) that they could successfully grow any heather as long as they are on some sort of irrigation system. One had sprinklers, the other drip irrigation.
I also recommend trying only a few colored foliaged varieties to start as they don't have as much chlorophyll to protect them from the intense sun. Some of the golden or red foliaged cultivars can scorch their needles or foliage in full sun during long periods of 100 plus weather.
What I do not know is if they are free of scorching on an irrigation system. My hunch is they are certainly less prone to it with regular watering.
Heather is very drought tolerant - to a point. In the arid climates, it is clear that an irrigation system with regular watering is essential for long term survival and good looking plants.
One more note that we have observed here at the nursery is that during our 3 month summer dry spells, Erica mackaiana and Daboecia look better and flower much better with more watering than the rest of the heaths and heathers.
We had virtually no rain for three months and many days over 90 degrees in the summer of 1998. We even had three days at 100 degrees (very rare for the Puget Sound region). I watered one area of about 600 heathers only three times. I lost about 3 heathers from drying out. This planting was almost three years old at the time. The surrounding forests in Mason County had thousands of young Douglas fir trees up to about 10 years old die from the drought. This is something I had never seen before in my lifetime. So, heather is indeed drought tolerant.