NEW : stevia has zero calories and no impact on blood glucose levels. It is native to the altitudes of Paraguay and Brazil in South America, and has been used as a sweetener for hundreds of years. Traditionally, it was used to sweeten teas, and for its medicinal properties. Refined stevia products are available for a more standardized sugar substitute, but with a little creativity, you can use dried leaves or extracts in many different recipes. No Frost Tolerant. The most important tip for harvesting your stevia plant is to pick the leaves before the small white flowers appear. Once the plant starts putting energy into flowers, the leaves become less sweet. (You’ll find this with most garden plants, such as other herbs and lettuce. Flowers = bitter lettuce and less fragrant herbs.)