My “Jethro Tull” coreopsis appears to be dying – I was using a compost tea which appeared to work well do you think I may have overdone the feeding. The tea is composed of aged horse manure, screened compost from my compost pile, trace amounts of powered limestone & “ Indian River “ fish emulsion
I suppose it is possible you overdid it although you don’t mention how often you’ve used it. I question the addition of powdered limestone unless the pH is out of whack. Coreopsis will benefit from regular deadheading (the removal of spent flowers). If you let it produce seeds, it will put its energy into that rather than new growth and more flowers.