I live in the mountains of northern California and yes, the temps for summer are in the high 90s to low 100s.
I only grow one Austin, 'Perdita'. Even with regular deep watering, the rose just kind of puttered along. Never failing, but also not thriving.
Three years ago, I found that if I added chelated iron to all of my roses, ALL of them performed much, much better.
Back to 'Perdita', the rose is now thriving. It has more foliage than it has ever produced in prior years and is covered with buds.
My roses are just starting to pop. We had a long, snowy spring and everything is behind schedule, but as I look at the roses, I am totally awed by their performance.
My soil had tested just slightly alkaline and should not have needed the additional iron, but I did a bit of research and found that the iron that was already in my soil was "locked up" and not available to the plants because of this slight ph variance.
The iron is needed to help plants produce chlorophyll which is needed for photosynthesis.
Something to think about.
Smiles,
Ly