I bought two perennial Coreopsis from a home hardware chain three weeks ago for my deck.
They were dry & flagging so I soaked them overnight & have been watering daily.
They now have lots of buds but they do not open fully & shrivel up before opening.
Can you advise please?
Expect transplant shock, especially in mid-summer. The plants were probably left over from spring so were rootbound and not in good shape, as you said. Roots are now struggling to get established. Blooms take a lot out of a plant energy wise and are the first thing to suffer in transplants. I would actually trim back the entire plant to get rid of buds. Let it focus on roots, then green growth, then flowers. Coreopsis doesn't like a lot of water or fertilizer. Is this in a pot? Only water when the top 1" of soil is dry. Your goal is damp, not wet, soil. Provide shade for the rest of this week; hot sun will dry the leaves and blooms faster than the roots can replace water. Unlike annuals, perennials often don't show much top growth their first year.