Preserved flowers provide graceful reminders of summer blooms. Most preservation methods dry flowers, but preservation with glycerin replaces the water in the plant with a compound that maintains the supple texture of foliage and blooms. Glycerin is a more expensive choice for preserving plants and is reserved for materials where other methods fall short. It excels in preserving foliage and large or delicate flowers like hydrangea or bells of Ireland. Since glycerin-preserved foliage and plants fade, cake coloring, which contains glycerin and food colors, is often used to maintain natural hues.
Cut flower stems up to 6-inches longer than you want the finished stem, using garden shears. Using your fingers, strip leaves from the lower part of the stem, leaving only a few at the top. Remove all leaves if you plan to wrap the stem with florist's tape. Store prepared stems in plastic bags in a refrigerator.
Smash up to 6 inches from the bottom of the stem with a mallet or hammer. You're laying open the phloem, a set of channels that, like veins, circulate nutrients through the plant. In order to keep the damaged phloem completely open, keep the prepared stems in a vase of hot water while you prepare the glycerin solution.
Boil a quart of water, then allow it to cool to between 150 degrees F and 180 degrees F. Fill a pitcher or carton two-thirds full with the water. Add up to 2 cups of liquid glycerin (available at craft stores or pharmacies) to fill the receptacle within a few inches of the top.
Plunge the stems into the glycerin solution. Mix the solution by swishing it around with the stems or a long spoon. Do not crowd stems; use more pitchers or cartons to give foliage and blooms plenty of room.
Allow stems to take up the solution for two to six weeks, depending on the size of the bloom, amount of foliage and length of stem. Add water to keep liquid at the same level; if soaking for more than two weeks, add a three-part-to-one-part mixture of water and glycerin to fill. When the flowers are completely preserved, they will have a rubbery feel but be completely pliable. Bells of Ireland, for example, may be ready in a week or two, but large or thick blooms like hydrangea or substantial ones like roses may take three weeks. Flowers with foliage attached to the stem will take longest; the glycerin must work its way up the stem and fill all of the plant cells.