Like humans, plants burn sugar as a source of energy, and, indeed, the entire purpose of a plant's leaf is to manufacture sugar for the plant's growth and collect sunlight. It may stand to reason, then, that adding sugar to water you use to water your plants would help foster its growth. But keep in mind that gardening is not immune from urban legends that range somewhere between science and myth. Determining if adding sugar to a plant's water affects its growth in anyway takes some careful consideration.
The Myth
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When a plant drinks water, porous plant cell membranes let in the water because of the water's high sugar and salt content. The cells store the sugar so that the plant's roots can use it, combined with salt leached from the soil, to grow further down into the soil. Because of this, some gardeners believe that mixing a small amount of standard table sugar into the water you use to water a plant or to the water in a plant vase can make for a more sugar-rich watering solution and thus provide more food to the plant. The desired result is quicker and more efficient plant growth.
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The Reality
Adding sugar to plant water is a favorite science fair project for middle high school and high school science classes, and the results generally confirm the myth. Plants grown with a small amount of sugar added to its water generally appeared healthier and grew more quickly than those that were grown with regular water. The effect is much more dramatic in plants grown outside of soil in controlled environments (because younger plants may not have yet developed as effective sugar producing and storing capabilities as older ones), and in plants harvested from the soil and kept in a container, like a vase, because such containers lack the naturally occurring sugar in outside soil.
Microbial Growth
The important thing to remember when adding sugar to plant water is to use it in moderation. If you notice that your plants grow more effectively when sugar is added, you may be tempted to add more and more sugar continually in order to double the effects. But an abundance of sugar in the water that a plant drinks can be a perfect breeding ground for micro-organisms that can threaten the health and the very life of your plant. To combat this, add a small amount of a biocide, like bleach or Physan, a kitchen disinfectant.
Other Considerations
The amount of sugar you should add to the plant water as well as the ratio of biocides to sugar varies widely, depending on the type of plant being watered, the age of the plant, and many other important considerations. Therefore, it is impossible to provide general guidelines that apply to all plants. For example, some small, flowering, decorative plants respond well to a solution of 10 gr of sugar and 1 mm of bleach to 1 quart of water, but plants in the very same family can show no growth improvement or can potentially be damaged by this very same solution. The best option is to first determine what plants you are planning to cultivate and then research what sugar and biocide solution works best for that particular varietal.