Glycerin is a colorless, thick liquid commonly used in soap and pharmaceuticals. It is also used in floral preservation, as it is able to absorb water from the air. Glycerin pulled into a cut plant stem will remove moisture from the cutting while making the stems and leaves pliable for preserved floral arrangements. Preserve hydrangea stems with glycerin once the blossoms have matured on the plant, when the moisture levels are lower.
Collect hydrangea blossom branches that are 18 inches or less in length and have the desired shape and size. Choose branches with foliage that is not damaged from insects or tears.
Remove the lower leaves from the stem. Split or crush the bottom cut edge of the stem with a hammer to stimulate absorption.
Mix two parts warm water with one part glycerin. Stir the solution well and pour into glass containers at a depth of 4 to 5 inches. Mark the solution level on the outside of the container.
Place the cut stems into the mixture to start the preservation process.
Mix a solution of four parts water with one part glycerin. Fill the container to the marked line on the container as the solution is absorbed into the stem.
Continue to fill the container to the marked line until the absorption rate slows and the branches are preserved by glycerin. This will take one to three weeks, depending on the branch size.