Herbs are clever plants. Just in case their good looks aren't enough to pique our interest, they offer something extra, such as unique flavorings or scents. One herb, Stevia rebaudiana, goes a step further by appealing to our sweeter side. This plant packs so much sweetness into its leaves that they can be used in place of sugar, and plant extracts are commonly used as sweeteners in Japan, China, and South American countries. One dried leaf, ground, is 10 to 15 times sweeter than an equal amount of sugar, and powdered extracts made from the leaves are up to 300 times as sweet — without the calories.

Stevia is a semitropical perennial shrub of the daisy family, native to the mountains of Brazil and Paraguay. The people there have used it for many centuries as a sweetener. Stevia first came to the attention of Europeans in the 1800s, yet it remained relatively obscure until it was planted and used in England during the sugar rationing of World War II. Japan took up research into stevia's potential after the war and remains a major grower of and market for the sweetener. There, it is approved for use in many food products, including cereals, teas, and soft drinks. Stevia is also grown in South America, Canada, Europe, Australia, China, and the United States.

The U.S. Food and Drug Administration banned the sale of stevia products in 1991, but three years later approved their sale as "dietary supplements." In FDA jargon, dietary supplements can carry claims of providing health benefits, but they cannot be marketed as conventional foods or food additives. Thus, stevia cannot be sold for use as a tabletop sweetener, which is considered a conventional food, or as a sweetener in teas or other products. Nonetheless, people can buy stevia powder and use it in place of sugar at home.

The substances responsible for the plant's sweetness are chemicals called glycosides, primarily one dubbed stevioside, which are concentrated in the leaves. These and other related chemicals are at their peak just before flowering, which is triggered by the shorter days of late summer and early fall.