Florists for years have been extending the life of their fresh cut hydrangeas by dipping the freshly cut flower stem into common alum. Cut hydrangeas tend to secret a sticky sap that can clog up the stem and prevent water uptake. Alum seems to help dissolve that secretion.
After cutting a section of the hydrangea head off with a sharp knife, dip immediately into powdered alum. You can put a small section out on a shallow dish, as the alum will tend to harden into crystalized blocks much like salt. You can crush these blocks with a spoon and return to the alum container after dried.
(Note: never use alum for consumption after using it for your cut flower arrangements. Cross contamination with flower fertilizers can occur.)
Insert the flower directly into the flower foam head, starting on the sides first.
If any of the petals shows browning (sometimes occurring when the petals are pressed too closely together), pinch off the browned petals before inserting into your bouquet.
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