Cut flowers keep fresh longer if you add one tablespoon sugar in lukewarm water. Do not change water every day; just top it up as needed.
The water in the vase with cut flowers will remain fresh and clean, if you put a piece of charcoal for grilling in it. Charcoal prevents the growth of bacteria and removes unpleasant odors.
Instead of charcoal for grilling, you can use a weak solution of warm water and potassium permanganate.
After use, rinse the vase with the solution of warm water and vinegar. Vinegar is the most effective way to kill bacteria, fungal spores and microorganisms.