The day after Valentine's Day, flower bouquets from sweethearts around the world begin to fade. A rose's vibrant red dulls to dried-out brown, and flowers begin to droop. Some say adding a citrus-flavored soda, such as 7-Up or Sprite, or an alcohol like vodka to the vase of water will lengthen the time these flowers remain beautiful.

According to floriculturists, they are right; if the mixture of soda and water is in the correct proportion, a bouquet will remain bright, because the combination provides the flowers with the water and food they need. "The 7-Up formula works really well," says Susan Han, a professor in the plant, soil and insect science department at the University of Massachusetts Amherst. Vodka also works as a flower preservative by interfering with the plant's ripening process but it is less practical to use.

Cut flowers, like those in a bouquet, are separated from their roots and no longer make food for themselves. Instead, their environment provides everything necessary to keep them fresh. Slightly acidic water travels up the stems to the flowers more rapidly than neutral or basic water, keeping the flowers hydrated and fresh. But in addition to water, plants need sugar for food.

This necessary combination of acidic water and sugar describes 7-Up or Sprite, says William Miller, a professor of horticulture at Cornell University. One can of either is loaded with citric acid that lowers the pH (increases the acidity) of the soda and contains about 38 grams of sugar. But, while water and sugar keep the flowers fresh, this mixture also encourages bacterial growth, which can harm the flowers. "So add bleach," Han says. A little bleach kills the bacteria without damaging the flowers.

When brewing this concoction, which is essentially what flower food packets hold, Miller and John Dole, a professor of horticulture at North Carolina State University, recommend one can of water for every can of soda added to the vase. Han, however, prefers a three-to-one ratio of water to soda, along with a few drops of bleach to kill the bacteria.

Vodka has a different effect on a bouquet of flowers: When added to a vase, it preserves them, probably by inhibiting ethylene production, Dole says. Ethylene is a ripening gas emitted by plants that helps them mature. Inhibiting this gas could slow wilting. Vodka, however, is not a very viable preservative. Plantslike many peoplecan only tolerate small concentrations of alcohol, up to eight percent, Dole explains, and 80-proof vodka from the liquor store is 40 percent alcohol. To be effective rather than harmful, the liquor would need to be dilutedInstead, growers use a more effective preservative, silver thiosulfate, which also inhibits ethylene, he says.

Other flower care strategies can help a bouquet stay beautiful. When the flowers arrive home, half an inch to an inch of the stem should be cut off. This prevents air from being sucked into the stem and creating a blockage in the plant's uptake system, Miller says. Also, removing the ends physically removes any bacteria that might be growing there, he adds. Roughly every three days another inch should be cut from the stem bottoms and the bouquet should be placed in a clean vase. "If you wouldn't drink out of your vase, don't be putting flowers in it," Dole advises. After fresh food and water are added to the clean vase, the bouquet should be placed in a bright, cool spot.

No matter what efforts are made, of course, over time the flowers will slowly die. "It is perfectly okay to go through and pluck out the dead flowers," Miller says. That way the remaining bouquet looks bright and pretty.

Sodas like 7-Up and Sprite may keep flowers from a sweetheart fresh but what keeps sweethearts fresh is not yet known to sciencethough vodka and soda might be key ingredients there, too.