So I guess I have to either move them back even more or to get rid of them... Or perhaps go ahead and cut the leaves back as they die off. I'm surprised that your other perenials don't get high enough to hide the foliage -- the foliage seems to wilt down to the ground quite quickly once they begin to die off.

Lower leaves are the oldest leaves and will drop for any number of reasons but in heat and water stress, roses will sacrifice old growth for new. An annual dropping of leaves isn't uncommon as we enter a period of natural dormancy or significant weather changes that impact uptake or availability of nutrients to the plant. Increasing the water during periods of heat and drought will limit the leaf-drop but also erecting some type of shade structure will limit it substantially as the intensity of the sunlight and heat is reduced. Limiting or reducing fertilizer applications during extremely hot weather will curtail some new growth and not stress the bush as much. Keep in mind that when leaves turn yellow and fall in a forest, we call it Autumn and expect the trees to go dormant as winter moves in. Some roses react the same way as they sense changes in humidity and temperature. In this area, we do a light pruning to remove summer growth and encourage new growth and fall blooms then do a hard-pruning in January. At that time, we remove all leaves anyway. I believe blackspot is a tough disease for arid climates to have, especially when the average daily temperatures are over 90F and humidity less than 15%. Phoenix even saw a couple days where humidity was less than 0% this summer.

"In California, consider the local soil and water; wherever azaleas are difficult to grow and suffer from salt buildup in the soil, Japanese maples will show burn on leaf edges. Give same watering treatment as azaleas -- flood occasionally to leach out salts." - Sunset 'Western Garden Book'

~~~ Explain thermal protection? ~~~ A piece of rigid foam insulation might help if the gravel or concrete is getting too hot and cooking the roots. By watering the one pot twice daily you are cooling it much more than the others. The plants own respiration may be helping with that as well. You're stuck. Without the plant using the water, you're at high risk of over watering but without being able to water more frequently, you appear to be making tomato root stew. Not a happy situation to be in. The one with the tomato looks especially sorry and I'm wondering whats holding it erect. Except as an experimental to figure how to prevent this in the future, I'm doubtful of that plants value.