Summarize:

Hydrangeas produce a sap that makes it hard for them to absorb water. This means that cut flowers, if left untreated, will quickly wilt. You can solve this issue by boiling away the sap. You only need enough water to cover the ends of the flowers. You can leave the boiling water in the pot while you dip the stems. Use a timer to make sure that you don't leave them in the water too long, which could damage the ends. Put just the bottom .5 in (1.27 cm) of the ends into the water. If you think you damaged the ends, you can cut above the damaged part and try again. The stems should immediately go into the cool water to stop the heat from continuing to affect the ends of the stems. Once they're in the vase, you can arrange the cut flowers until you like the look. As an alternative, you can dip the freshly cut ends of the stems of your flowers into the spice alum, which helps prevent the sap from clogging the stem. Cut off the tips of the stems, then dip the ends into the spice. You can then arrange the flowers as you normally would. Fresh water will keep your flowers looking nice longer. If you leave the old water, bacteria will be able to grow, causing your flowers to wilt sooner.
Boil a pot of water. Dip the ends of the stems in the hot water for 30 seconds. Plunge them into the cool water in the vase. Try alum to keep your flowers from wilting. Replace your water every other day.