As soon as the cut is made, the flower, like an astronaut without a temporary life support system, is in trouble. Biocides are chemicals that kill the bacteria, yeasts and fungi that feed on the sap that seeps from the cut flower stem. These bacteria plug the tiny straw-tubes that conduct water to the flower. As a result, buds fail to open, necks bend, and leaves wilt. The acid helps water move up the stem more easily and the sugar acts as a flower food.
Copper is a fungicide and the aspirin makes the water more acidic.
If you don't add anything to the water, change the water every couple days. Add two ounces Listerine mouthwash per gallon of water. Its acidity enables water to move more easily up the cut stem. PlanTea is also an excellent cut flower preservative. Mix 1 part liquid concentrate to 6 parts water.