Mine are blooming prolifically, but one clump that usually are dark red came up dark orange this year! I use no chemicals on mine, not even fetilizer.

Let me suggest digging up a few to see if anything is wrong underground.

Both under watering and over watering will cause yellow growth. So, it is hard to say. Have you fertilized them?

Thanks for the replies. I have fertilized them, and if I have erred on the watering, it would probably be on the under-watering side.

I'll try to dig one up and see what they look like at the root zone, but I'm not quite sure what I'm looking for. If they are going to become high-maintenance suddenly, I'm afraid they are destined to become compost. It's too hot out there for this ex-yankee.

Last year I got the scissors and trimmed all the leaves of my daylilies back to the stiff fan part of their growth. If there were any flower stems yet to develop, I avoided them.
I know nothing about day lilies, but when they start looking ragged, this is the way my neighbor does hers.

We've had plenty of rain, or at least we are not in a drought. My day lilies were extremely tall this year, some almost 4 feet bloom stems. And even the canna lilies in the same bed looked pretty good, after I dealt with those nasty leaf rollers. I doused the entire bed in Sevin dust.

Also in that lily bed I have my purple salvia and other perennial salvias, which give a purple/blue accent to the yellow and orange of the day lilies.

I keep mulch on this bed, which was created by a lot of mulch and then the compost worms dug the bed for me. Hard as a rock to begin with, but I covered it with leaves and other mulch and left it alone for over a year. When I went back to see if plants would go in, those busy little worms had worked the soil for me. That's when I began collecting day lilies to go in this hot exposed location.

I've had the same problems with some of my daylilies - the common old "ditch" lily - leaves have turned yellow and although they did bloom some - not like they shouild have bloomed. I fertilized lightly in early spring and don't think I over or under water - probably under if anything. The ones that have received more water because of location did much better - so who knows??