My first thought is too much sun. Just a guess.

And perhaps a tad short on water.

Also wonder if you fertilize. If not, you should.

Hi! I've added fertilizer and Magnesium but I'm not seeing a difference. And I have watered. The plants get a little over half of day of sun some of it being afternoon sun so maybe that's it. We have had a hot September. These plants have been in the same spot for years and I've not had this problem. Thanks so much for your suggestiongs !

There are numerous causes of plant leaves yellowing, known as plant chlorosis, ranging from nutrient deficiencies to the wrong growing medium pH. Perhaps this article might be of some help.

http://extension.illinois.edu/focus/index.cfm?problem=chlorosis

kimmq is kimmsr