Just because a flower is cut does not mean it is dead. A cut flower has many of the same nutritional needs as a growing flower that has not been cut. A cut flower is like a straw. When it is placed in water, it draws that water up through the stem and into the flower at the top of the stalk. Adding sugar to the water will help preserve the petals of the flower and make them last longer.
All plants have some sugar in their cells already. When plants absorb sun through their leaves and stems, they go through a process called photosynthesis. During photosynthesis, plants manufacture a sugar known as sucrose. This sucrose is transported to other parts of the plant to help it maintain its energy. However, flowers that are cut typically have their leaves removed and are kept out of direct sunlight to preserve them. Because of this, a plant is not able to sufficiently produce sucrose.
The sugar water placed in a vase helps to replace the sucrose lost due to the cut flower's inability to photosynthesize. Because of this, many floral preservatives use some form of sugar. Using a sugar-based preservative can extend a flower's life span two or even three times as much. The cut flowers absorb the sugar through the xylem, or the open end of the stem. If you were to place a drop of food coloring or dye into the sugar water, you could trace the path of the liquid up the stem and into the veins of the flower's petals.
Not all flowers respond well to a sugar water preservative. Roses and Snapdragons respond best to this type of solution. For this reason florists often package floral preservatives with these types of flowers. Most flowers respond better to a solution of 2 percent sugar mixed in water. Plants such as Zinnias respond better to a solution containing 1 percent solution, and carnations do well without sugar in their water at all.
According to the University of Massachusetts at Amherst, flowers that are treated with sugar water respond better when other additives are added as well. Two of the most important additives are a biocide to kill microorganisms and an acid to regulate the pH of the water. Two of the most common biocides are bleach and Physan. Physan is a disinfectant used commercially in restaurants. Citric acid is a common product used to lower the pH of water for cut flowers. Water with a lower pH is more acidic and helps to move the sugar water up the stems and to the petals faster, which helps prevent wilting.
One method that florists use to revive wilting flowers before marketing is to use the pulse method. The pulse method loads wilting flowers up with sugar water by soaking the stems in a high concentration of sugar overnight. Not all flowers respond to the pulse method; however some flowers that are pulsed have a longer vase life and bloom more fully.