Considered a tender perennial herb, stevia (Stevia rebaudiana) is hardy in U.S. Department of Agriculture plant hardiness zones 9 through 11. The plant’s leaves are dried and ground into a sugar substitute with zero calories. What makes the leaves so sweet are compounds called steviol glycosides. Because the levels of these compounds vary in plants grown from seed, you may have one plant with quite a lot of sweetness in its leaves and another plant with leaves that have very little. Another problem with growing stevia from seed is that it doesn’t’ readily germinate. With careful planting, nurturing and lots of patience, however, you may be able to grow several stevia plants successfully from seed.
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Soil and Container
Stevia seeds require consistently moist, very warm soil to germinate and they will rot if the soil is wet and cold. Use a soilless seed-starting mix for the seeds to avoid fungal pathogens that might be present in garden and even potting soil. Any container can be used, as long as it has holes in the bottom to allow excess water to drain. Because the seeds require humidity to germinate, consider using a flat with a plastic dome, such as this one. The soil must remain loose, so don’t pack it down as you fill the container with soil.
Viable stevia seeds are either dark brown or black. All white and off-white seeds should be discarded. Lay the remaining stevia seeds on their sides on top of moist planting mix, and barely cover them with sand or vermiculite. Squirting the covering layer with water from a spray bottle settles the stevia seeds into the mix without washing them away. When the soil begins to dry during the germination process, place the planting container in a shallow pan, and fill the pan with water until it reaches halfway up the container’s sides. Remove the container from the water when the top of the soil is moist.
The soil must remain 70 to 75 degrees Fahrenheit for the stevia seeds to germinate. Either place the container on a heat mat (the domed seed starter linked to above comes with a heat mat) or 6 inches beneath grow lights. Inserting a thermometer into the container, monitoring the temperature and making adjustments as needed helps to ensure the soil remains warm. Adjust the temperature by using the heat mat’s thermostat or by raising or lowering the grow lights. If all goes well, the stevia seeds should sprout within one to two weeks after planting.
Stevia seedlings grow slowly and take about two months to grow to a size at which it’s safe to transplant them. In the meantime, give the stevia seedlings six to eight hours of sunlight per day, and keep their potting mix moist. Wait until they have their third or fourth set of leaves before transplanting each seedling into its own, small planting pot. They should stay in those pots until they’re ready to be planted outside – after all danger of frost has passed and the soil temperature is at least 65 degrees Fahrenheit.