Although not true for all flowers, putting sugar in the water for cut roses has proven to increase the life of their blooms (1). Photosynthesis, the process by which plants receive nutrients from the sun, normally produces sugars on which they thrive (3). Roses are biologically equipped to respond to the presence of sugar, but it is important to know the right amount to add.

...
Add sugar to the water of your home-grown roses and help prolong bloom life.

Cut Roses

Since cut roses are literally cut off from their source of food, they last longer in the vase if they have the right nutrients (1). A cut flower cannot feed itself through photosynthesis, yet it can absorb sugar and acid through its stem (3). Sugar has the effect of providing the rose with a little bit of energy.

Potted Roses

Though sugar water seems to benefit cut roses, sugar should not be added to the water with which you would ordinarily water your potted plants (1). Adding sugar to soil creates an environment that breeds bacteria and fungi (1). It also has the effect of drawing water out of your plants and back into the soil, which can leave your plants wilted and listless (1).

Things To Remember

Be sure to not make the sugar solution too strong. MasterGardeners.Org recommends 1 tablespoon of sugar to 1 quart of warm water.
Adding sugar will not cancel out any other harmful things you might be accidentally feeding your cut flowers—dirty vases or salty water will harm your blooms. Different types of roses are hardier than others. Adding sugar might make hardier roses last longer than more delicate roses.