I know that with daylilies, too much summer heat, when in pots, plus fertilizer, plus water, equals rot. Been there, done that. Shade helps some, but this is TX we are talking about.
At a minimum, shade (at least AFTERNOON shade), and no fertilizer until cooler fall temps, is my recommendation. And to be honest, given the horror of TX summers, if it's going to get hot very soon, I'd consider growing indoors under lights until fall. I only say this because I've lost several new irises potting them in the summer, and we don't have the heat that you do.
Best of luck!
