Q: Last year one of my Coreopsis turned brown and appeared dead. The plant came back this spring and appreaed healthy. It is now turning brown again. The one next to this one has started to turn brown also. I have another bed of Coreopsis, these plants (so far) have not shown this problem.
Sounds like stem rot. Coreopsis do best in fast draining soil that is not overly rich. The like to run on the dry side. A bit of compost in spring is all the fertilizer they need. Bottom line, you may be killing it with kindness. Remove all the dead material. Best And Happy Yardening, Nancy.