Starting Stevia from Seed
By Jeffrey Goettemoeller
Stevia seeds not planted within a few weeks should be stored in a rubber-sealed glass jar placed in a cool, dark room or your refrigerator. A desiccant such as silica gel placed in the jar will preserve seeds even longer. 
Sow stevia seeds indoors in early spring, or any time if you plan to keep the plants indoors under lights. Transplants should be transferred to the garden at least two weeks after your last frost date. Plants take 8-10 weeks to reach transplant size.
If you are familiar with a seed starting method for bedding plants, go ahead and use it. In general, stevia should be treated similar to tomatoes when starting from seed. Here I describe my favorite method.
I like to use individual cells or pots such as 6-pack plastic inserts or other small containers with holes poked in the bottom for drainage. Place the cell packs or pots in a plastic nursery flat or some other container without holes. You will also need a clear plastic dome or cover. The larger plastic “clam-shell” containers for lettuce or spinach from the grocery store work beautifully for a small number of plants. They come with nice clear covers. We use the 16 ounce size.
Standard potting soil or seed starting mix works fine. If the soil is very dry, moisten it slightly. Fill cells or pots level without compacting. Place 2-3 seeds in each cell or pot. Seeds may be handled by pouring out on a sheet of paper, then pick up one at a time with the tip of a wet toothpick and transfer to the cell or pot. Barely cover (about 1/8 inch) with fine horticultural vermiculite (preferably) or a little of the potting soil.
Moisten gently with a spray bottle to settle the seeds. Put a little water in the bottom of the flat to be absorbed from the bottom. That should be enough water until plants are up and growing. Place the clear cover on next, slightly ajar to allow a little airflow. Put a thermometer under the cover. A plastic dial-type thermometer works best. Place the container under a fluorescent light left on all the time and hung so the height can be adjusted. Keep the air temperature inside the plastic dome between 72˚ 
F. and 80˚ F. by adjusting the height of the light. A heat mat underneath could do the job, but the light itself improves stevia germination slightly.
In 6-14 days, tiny seedlings should begin to emerge. Remove the cover when more than half the pots or cells have seedlings showing. Don’t leave it on too long or the humid air will encourage damping off (a disease). At this point, leave the light on all the time at about 4-5 inches above the plants. Pour about ¼ inch of water in the bottom of the flat for the pots or cells to soak up from below. Wait 3-5 days before watering again unless the soil dries out sooner. The goal is to keep the soil slightly moist, but not soggy. Avoid getting leaves wet.
If the potting soil lacks any compost or fertilizer, put some seaweed fertilizer or other weak, low nitrogen fertilizer in the water starting at about the third watering. Plants usually do fine without added fertilizer early on. When the seedlings are old enough to tell which one in each pot is strongest, cut the others off with a scissors. At about three weeks after sowing, begin to turn the light off at night. Maintain about 15 hours of light per 24-hour period. A timer is nice for this job. 
Transplanting
At about 8-10 weeks from sowing, the plants may be transplanted or left to grow a few more weeks. Plants may also be kept as potted plants, transplanted to larger pots. Harden off plants for at least 5 days before transplanting to the garden. Do this by placing plants outside in a protected area on nice days and bring them in at night when temperatures are forecast to drop below about 40° F.
Find more about growing stevia in Jeffrey's book, Growing and Using Stevia: The Sweet Leaf from Garden to Table with 35 Recipes.