Answer: Everyone I've talked to seems to have a different method of preserving freshly cut hydrangeas so you may want to try them all and see which works best for you.
Growers tell me that they condition hydrangea flowers prior to arranging them in a vase. To condition hydrangea's, dip cut stems into alum powder (this powder is found at your local drug store and sometimes used to preserve fruits) and place in very warm water. Remove and place in water at room temperature for several hours before working into your arrangement.
I've also heard that plunging the entire flower and stem into a large container of floral preservative for several hours will extend the life of the cut flowers. (available at florist's shops or Michael's craft shops).
Or, if you have a gas stove you can singe the cut end of the stem. If you don't have a gas stove you can plunge the cut end for about 30 seconds into boiling water (wrap the leaves and flower in newspaper or a towel to protect them) - the objective is to seal off the conducting vessels at the cut end. Then put the hydrangeas into a deep container and fill with cool water all the way up to just below the flowers. Mist the blooms, cover them with paper towels, mist the paper towels to moisten slightly and let them condition for 4 or 5 hours. The hydrangeas are then ready to use and should last a long time. Do not recut the stems if you use them in arrangements - if you think you are going to need stems of different lengths, make a good guesstimate of what you need and do the conditioning process with the stems cut to the various lengths.
I have heard that the wilting problem mainly happens with flowers that are not quite mature, and that once they reach maturity it won't happen. But, I have had what seem to me to be flower heads that are identically mature, and one wilts while the other doesn't. So, I've treated them all as noted above and almost never have one go limp.
Hope one of these methods works for you!
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