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Give your hydrangea stems a deep diagonal cut across the bottoms. A diagonal cut will help minimize damage to the stem ends Cutting the stems under warm running water will help prevent potentially damaging air bubbles from forming on the stem. Place a bit of alum powder on a plate, about 1⁄2 in (1.3 cm) deep. Once your hydrangea stems are cut, dip each individual stem in the alum powder. Then place the stem in a vase with water and arrange the flowers. This will help the blooms thrive for a longer period of time. You should be able to find alum in the spice section of any grocery store. Once you’ve cut the stems, you’ll need to get your hydrangea in water as quickly as you can. Fill a clean vase up with 3 in (7.6 cm) to 5 in (13 cm) of lukewarm or room temperature water. Do not crush the stem ends, as this can prevent your hydrangea from hydrating. Hydrangeas actually absorb more water from their petals than through their roots and stems. If you have cut hydrangeas that you want to keep looking nice, you’ll need to gently mist the petals with water each day to keep them hydrated. Use a spray bottle with a gentle misting stream so as not to damage the delicate petals. Fresh water will help your hydrangea blooms stay properly hydrated, and prevent them from wilting as quickly. Remove your cut hydrangeas from the vase and discard the old water. Refill the vase with fresh room-temperature water. Rinse out the vase to remove any debris before adding the fresh water. For hydrangeas that have begun to wilt, fill the vase with 2 parts ice and 1 part water. This will help cool the plants down if they are overheated. You should start to see improvements after one day. Feel free to continue the ice/water treatment for several days in an attempt to revive your hydrangeas. If your hydrangea blooms begin wilting, you can give them a bit of life support by submerging them in a bowl of warm water for up to 30 minutes. Be careful when lifting the hydrangea back up, as the water can make them quite heavy.

Summary:
Cut the stems at an angle. Dip the hydrangea stems in alum spice powder after cutting the stems. Place the cut stems in lukewarm water immediately after cutting. Mist the petals with water once a day. Replace the vase water every day. Add ice to the vase if your hydrangeas wilt when exposed to high temperatures. Dunk the flower heads in warm water if they start to wilt.