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This Reader's Digest article says:

Increase cut flowers’ longevity Freshly cut flowers will stay fresh longer if you add 1/4 teaspoon bleach per quart (1 liter) of vase water. Another popular recipe calls for 3 drops bleach and 1 teaspoon sugar in 1 quart (1 liter) water. This will also keep the water from getting cloudy and inhibit the growth of bacteria.

Is there any proof? I would imagine bleach would be toxic to flowers.

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  • 2
    My wife also adds a penny; apparently the copper helps in some fashion. Note that tap water has some minor amount of chlorine in it already to kill microorganisms.
    – Bryce
    Jun 20 '12 at 10:31
  • The amount of chlorine in tap water depends heavily on your water source. I was also under the (possibly incorrect) assumption that if you left tap water to sit uncovered overnight, the chlorine would evaporate out of it. @Bryce, Copper is a antimicrobial but who knows how much is required for a significant effect.
    – Sam I Am
    Jun 20 '12 at 12:41
  • This might also be an appropriate question over at Gardening & Landscaping.
    – Flimzy
    Jun 20 '12 at 16:45
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Scientific American:

[...] while water and sugar keep the flowers fresh, this mixture also encourages bacterial growth, which can harm the flowers.

"So add bleach," says Susan Han, a professor in the plant, soil and insect science department at the University of Massachusetts Amherst.

A little bleach kills the bacteria without damaging the flowers.


The Royal Society of Chemistry:

Bleach kills bacteria.

The flower stems have water channels. Bacteria clog up these channels - the bleach kills the bacteria.


University of Nebraska:

The acidity of the lemon-lime soda or vinegar lowers the pH of the water so bacteria don't grow well; the chlorine bleach kills bacteria on the stems and the sugar provides food for the flower.


More:

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  • Note: Click the links to learn how much bleach should be used
    – Oliver_C
    Jun 20 '12 at 21:40

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