East coast. 4 seasons.
Why Does the Foliage of Daylilies Turn Yellow?
Certified GKH Gardening Expert
If they have finished flowering for the season, then it is likely going dormant. The plants could even enter dormancy early as a result of the hot temperatures affecting much of the country. For additional causes of leaf yellowing, this article should help: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm