Don't feel guilty about not getting out there in the garden to dead-head your Coreopsis, they will become seed heads and goldfinches will appreciate it! I knew not to dead-head the Echinachea, but I didn't know about Coreopsis too. I've been dead-heading the plants to keep them producing flowers for all the small pollinators, but I decided to stop as flower production slowed down, as an experiment to see if any birds would be interested in the seed heads. I wasn't even sure if it was going to make seed since it's just a cultivar (Coreopsis grandiflora Rising Sun). I think it's pretty amazing how the birds know that those plants are a food source, even if they've never seen that plant before. I have a full feeder of thistle seed in the backyard, easy pickings, but this bird would rather hang on a milkweed stem and pull seed out of the coreopsis. Because it's natural!