yarrow with brown blossoms
Just planted a yarrow whose blossoms turned brown despite proper care. This was a red yarrow whose greenery still looks good and new green growth occurring in the center, but the blossoms all turned brown without any blooms or buds.
This can happen with newly planted perennials. The blossoms may be nearing the end of their lifespan and transplant shock can cause the flowers to brown and fade. Dead head the spent blossoms, and water your new plants daily for the first 2 weeks, then you can cut back.
You may receive new flowers again, but most likely you will need to wait until next summer for the plant to bloom again.
Here is a link about growing Yarrow.