Stevia plants originated from Paraguay and have provided a natural, non-caloric sweetener for centuries, though only recently gaining popularity in the United States. Classified as a herb, the stevia plant produces leaves up to 30 times sweeter than refined sugar. Because of its tropical origins and sensitivity to frost or cold weather, stevia is considered an annual in all USDA Plant Hardiness Zones except zone 11. Stevia is difficult to grow from seeds, so if you have plants you will want to save them from year to year. Save and process the sweet leaves to use in cooking and drinks.
Saving Stevia Plants
Dig up stevia plants from your garden two or three weeks before any frost or cold weather below 32 degrees F is expected.
Plant the dug-up stevia plants in clay pots large enough to accommodate the plant's root ball. Use a well-draining, commercial, organic potting soil to plant your stevia plants in for the winter. If you grew your stevia plants in containers outdoors over the summer and moving them inside for the winter, check that those plants are not root-bound and need re-potting.
Find a warm indoor location for the semi-tropical stevia plants to over-winter. Stevia plants also need 14 to 16 hours of light a day. Set your stevia plants under grow lights or fluorescent lights for the required amount of time. The lights should be 5 to 9 inches above the plants.
Cut back stevia plants to 2 to 3 inches when they reach 7 to 10 inches tall. This will promote a bushier plant with more branches and vigor. Your stevia plants may look devitalized by spring, but will quickly rejuvenate once outdoors again.
Fertilize with a low-nitrogen, organic fertilizer, according to the manufacturer's directions, in the spring before moving your stevia plants outdoors. This should be the only feeding your stevia plants need during the growing season. Trim off any developing flower heads on your stevia plant. Once a stevia plant flowers, it will die.
Saving Stevia Leaves
Harvest stevia plant leaves just before the plant flowers, when the sweetness of the leaves is at its highest. Flowering usually occurs in late summer or early fall. Leave about 6 inches of stems on your plant when you harvest the leaves so the plant can recover and continue to grow over the winter.
Place the harvested leaves in a dehydrator with good air circulation, set at 100 to 110 degrees F. If it is a warm, low-humidity day, lay the leaves on a drying screen to dry. The leaves dry in 8 to 12 hours. If left longer than that, the leaves will lose some of their sweetness.
Save dried, whole leaves from your stevia plant in clean glass jars with tight fitting lids. Dried stevia leaves will keep their sweetness for years when stored in this manner.
Grind dried stevia plant leaves by loading a coffee grinder with metal blades half-full of dried leaves. Run the grinder until the leaves are reduced to a powder. This is called "green stevia powder" and is used in stevia recipes. Or save the powder in clean glass jars with tight lids.