You'll never have to guess when it's time to dry hydrangeas, they'll let you know. They turn from the brightest pink to dusty pink. Maybe they'll go lilac, maybe lime tinged with pink. As they lose their pigment, some may even go spotty. But they will be a dusty version of what they were in spring. The blossoms start to feel a bit papery, even a little leathery. They're done growing and intend to show off their fall colors which makes it the perfect time to dry them.
The time to pick hydrangeas for drying happened early this year, even in California when everything in the garden seems to happen at warp speed anyway, this is still extremely early. Maybe our drought is the reason? Maybe I need to remind my garden it's still summer.
Drying them couldn't be easier. After picking trim off all the green leaves. Place them in a vase less than half full of tap water. Give them some space and leave them alone, they will look pretty as they dry. In a few weeks the water will have evaporated and your hydrangeas will be dry and ready for fall arrangements. They're pretty.