What is the best homemade flower preservative?

This simple recipe works well for cut flowers in a vase. Use a clear, non-diet lemon-lime soda, such as a generic lemonade, because the sugar is essential. Mix one part soda with three parts cool water in a clean vase. Add two or three drops of household bleach per litre of water. Stir gently. The sugar gives the stems energy, the citric acid helps water move up the stem, and the bleach keeps the water clear of bacteria that clog the stems.

Change the water and re-cut the stems every two to three days. Remove any leaves that sit below the waterline, as they rot quickly. Keep the vase out of direct sunlight and away from ripening fruit, which releases ethylene and speeds up wilting.

If you do not have soda, a simpler alternative is one teaspoon of sugar and a few drops of bleach in a litre of warm water. Some people add a crushed aspirin or a teaspoon of vinegar, but the soda version tends to work more reliably. Always use a clean vase, as leftover bacteria will undo the preservative's benefits.