hi,

The best thing to do is to cut off the bloomed out stalks AT THE GROUND. The plants will continue to push up new growth and flower more and look much better too.

Boca Joe

Without a photo, I can only make several suggestions.
Typically your hot temperatures do not cause cannas to look ratty, assuming they are well watered and fertilized. In your area cannas are often infested with one of the leaf roller species. That would require the application of a pesticide. Most canna cultivars grow and spread quite rapidly at your location and really need to be lifted or thinned out each early spring; every other year for sure. The overcrowded condition can create the possibility of rust on the leaves as well as other diseases.
Assuming none of the above apply; you will likely need to cut back all the bloomed out mature canes and leave only the newer canes to grow and bloom. As cannas grow and spread they will continue to produce new cane growth from around the base of the plant. The older canes typically produce only two to three bloom inflorescences and it is done. That particular cane can then be cut about 2 inches above the ground and discarded. So you certainly can remove all the old growth canes but I would leave any new growth that has not finished blooming.

Kent