Heucheras need excellent soil drainage and the right exposure to thrive, and are also very prone to getting root weevils attacking the roots and boring up through the center of the stems and killing the plants. You will probably have better luck with the species native to southern California and Arizona if you are planting them in areas with really hot summers. I'd suggest you pull up some of your dying plants and see if you see signs of whitish grubs eating out the centers of the stems. They seem to particularly favor plants in heavy clay soils, or those that are under stress from heat or drought.

I know I tend to avoid this plant as a group because it is also problematic with insect attacks here in the SF Bay Area.

I've had some here for the past 2 years, and they seem to be doing well along the coast. Mine are native/native hybrids, though, and my soil is pretty sandy, so drainage is not an issue. They are in a shady, unirrigated strip between houses, and my neighbor is impressed with them for hanging in there, as delicate as they look.

Try H.maxima native to the Channel Islands. Has a huge flower spray.And did very well for at least two years I can tell you that. But,same here for the hybrids-I gave up.

They need to be split after year two and rejuvenated. You dig the whole thing when there are short new "trunks" coming off the main old "trunk". You break those new "trunks" off, cut off the leaves (but not the stems of the leaves! The leaf stems contain a lot of food for the plant!), dust the end of the "trunk" with rooting powder, and stick them back in the ground. About 80% of them will grow into nice new plants.
You throw out the old main stem.

The guy from Terra Nova, can't remember his name offhand, demo'ed how to do this on a "Gardening By The Yard". I tried it and it works great. That was one of the most useful segments I ever saw on that show.

I have 'Amethyst Mist' which seems to be a very strong cultivar. I have 5 along the side of a shed, the 3 center ones get almost no sun, and the 2 on either end get an hour or 2 of sun, and all get almost no summer water. They're about 5 years old and still look great.
I haven't had good luck with any of the other cultivars.

wanda

I'm not in S. Cali, nor am I near the coast, however I've had my hybrid Heuchera ('Dolce Peach Melba') for at least 4 years - dappled sun all day, watering every 2 weeks during the hottest/driest part of the year, feed once a year in late spring. It bloomed 3 times for me this year, and it only seems to get better the older it gets.

Brenda

Thank you everyone for the outstanding comments and advice. So there are some successes out there. It looks like the best bet is to create excellent drainage for these plants and choose varieties that are best suited for my area. I'll keep trying.