Late summer/early fall is the perfect time to dry some hydrangeas.  It is usually the ‘mophead’ types that dry so well .  These are the Hydrangea macrophylla cultivars.  The big white Hydrangea paniculata cone-shaped flower clusters commonly know as ‘P.G.’ types also dry well if not harvested too early. (They should be tinged rose and feeling papery before they will dry if cut.) This applies to the ‘mophead’ types as well.  Newly opened flowers will just shrivel and wilt , even in water. They do not dry nicely and  just look like dead flowers.
To dry hydrangeas well, I have found it is best to arrange the cut stems, remove every leaf, place them in a vase of water and simply let them drink it up . Do not add any more water, … just let it run out. There is no need to add anything  to the water. It takes a few days, but the flowers will dry in situ and last for years !!  Some people hang their flowers upside down and dry them that way. You might get lucky if you start with very leathery blooms that have aged on the bush, but in general I find the blossoms shrivel right away and don’t dry perfectly.
So, gather (check out abandoned houses!!) or purchase some gorgeous hydrangea blossoms right now and enjoy them, first in water, then all winter as dried beauties !