Experts and BHG readers answer.
Trouble with Yarrow
Thanks for writing. If you wish to dry the flowers, you'll want to cut them while they're showing good color and hang them in a warm, dark place to dry them. If you leave them on the plant, they'll just turn brown as you've discovered. Also: By cutting the blooms to dry them, you stimulate the plant to produce more. If you keep cutting them, you can get three or even four flushes of bloom from some types of yarrow.
---Justin, Senior Garden Editor, BHG.com