Astile like moist soil. If the soil is too dry, the leaves turn crispy brown. I had this happen every summer, they even went dormant. I moved them to a new location and they are happy!

I have found that they are very heavy feeders too. Try keeping it moist, and give em some miracle grow,,,
Good luck!!

We have had plenty of rain. My others are not turning rusty brown on the tips. And since I just planted them I can't imagine they need more fertilizer. Any other ideas?