We love the chintzy, venerable-dowager look of dried mophead hydrangeas, but the actual drying process can be a bit hit and miss.
The key to keeping the blooms plump is to pick them when they have already started to dry on the shrub - in the months of September and October. Any sooner and they will wither and fade after you cut them.
When you can see a deep red tinge on the florets (hydrangea-speak for the individual "flowers" - which are not actual flowers, but bracts), they are ripe for harvest.
Ideally, cut them mid-morning after the dew has dried (or realistically, after you've gathered them, just shake off the 10 gallons of rain that fell in the past 24 hours). Cut the stems to 20 or 30 centimetres, and strip off the leaves.
From here, you can take one of two routes. The first is to hang them upside down in a dry, airy place where they can gradually lose moisture. Don't expose them to direct sunlight, or they will lose colour. Or, put them into a vase of water, so that the water comes halfway up the stems. Leave in a coolish place (the hallway in many houses is perfect).
Don't top up the water: just let the stems drink it up. This allows the blooms to dry slowly, and, with just the right amount of moisture, to remain pleasantly rotund. A third, lazy person's route is to leave them to dry on the shrub, and to pick in late November or December. They won't be as sumptuous, but they are just fine for Christmas wreaths and garlands.