Hydrangeas absorb water not only through the stem but also through the petals — that is their quirk. If the heads start to droop, don't despair.
Emergency rescue. Fully submerge the hydrangea heads in a basin of cool water for 20–30 minutes. Then cut the stems under water and return them to the vase. Within an hour or two the flowers will revive.
Prevention. Hydrangeas dislike warm rooms. Keep them away from radiators and air conditioners. The ideal temperature is 16–20°C.