What should you put in water to make roses live longer?

If you plunge them up to their necks in very hot water before arranging them it will make them last longer, and even revive them if they have wilted a little.

Bacteria in the water is what causes flowers to wilt and rot. This is something you DON'T want in the water. Other things to keep out of the flower's water are leaves from the stem. They will break down in the water and cause bacteria growth. You will want to change the water in the vase regularly, adding bleach or vodka (and flower food, aspirin or sugar each time).

Keep your flowers beautiful by keeping them cool at night (maybe even in the refrigerator) and away from any sources of heat. Trim the stems daily. A thin film grows over the bottom of a cut flower's stem, impeding their ability to take in water. Prevent this by regularly trimming the stems bottom. Flowers with woody stems, such as roses, should have the bottoms of their stems crushed slightly as well.