Stevia (Stevia rebaudiana) is a source of natural, calorie-free sweetness derived from steviol glycosides found in its leaves. These compounds are hundreds of times sweeter than table sugar, making Stevia popular among health-conscious gardeners. Growing Stevia from seed requires specific conditions and must begin indoors well before the final spring frost to ensure a strong start.
Preparing for Stevia Seed Starting
Starting Stevia seeds indoors is necessary because they require a long, warm growing period. Begin the process approximately eight to ten weeks before the last expected frost date in your region. Necessary supplies include sterile seed starting trays, a fine-textured seed mix, and a warming mat, which is highly recommended for temperature control. A clear plastic dome or plastic wrap is also useful for retaining moisture and creating a humid microclimate.
Stevia seeds have a characteristically low germination rate, often ranging from 30 to 50 percent even under ideal conditions. This poor performance is a natural trait of the species, which is why commercial growers often prefer propagation from cuttings. To account for this, sow multiple seeds for every plant you desire to ensure enough seedlings successfully emerge.
Use fresh, high-quality seeds, as viability tends to decrease the longer they are stored. Storing seeds in a cool, dry location and using them within six months of purchase helps maximize the germination potential. Providing the correct environment from the start is the most effective preparation for overcoming the initial hurdle of poor germination.
The Sowing Process
Sowing Stevia seeds requires attention to detail, starting with the planting medium. Use a sterile, fine-textured seed-starting mix that offers good drainage, as Stevia roots are sensitive to overly wet conditions. A successful mix often contains peat moss, vermiculite, and perlite, and it should be slightly firmed into the trays before planting.
Stevia seeds must be surface-sown because they require light for germination (photoblastic). Sprinkle the small seeds across the prepared surface, pressing them gently into the mix without covering them with any additional material. This ensures they receive the necessary light stimulus to break dormancy.
Maintain consistent moisture by using a spray bottle to mist the surface, preventing the seeds from being dislodged or washed away by a heavy stream of water. Immediately after sowing, cover the trays with a clear plastic dome or plastic wrap to trap humidity, mimicking the plant’s native subtropical environment. This high humidity is crucial for successful germination.
Temperature is a non-negotiable factor for successful sprouting. The planting medium needs to be kept consistently warm, ideally between 75°F and 85°F (24°C to 29°C). A heat mat placed beneath the tray is the most reliable way to maintain this elevated temperature, which significantly improves the germination rate and speed. Seeds typically sprout within 7 to 21 days when optimal warmth is provided.
Nurturing Seedlings and Transplanting
Once the tiny Stevia seedlings emerge, the requirement shifts from high heat to strong light to prevent legginess (weak and elongated growth). Place the trays directly under a grow light source or in a very bright, south-facing window, ensuring 12 to 16 hours of light daily. If using fluorescent lights, position them just a few inches above the seedlings and adjust the height as the plants grow taller.
After the seedlings develop their first set of true leaves, they are sturdy enough for the next stage of care. Thinning is necessary to allow the strongest plants to thrive; snip the weaker seedlings at the soil line, leaving only the healthiest plant in each cell. Early pruning of the growing tips, once the plant reaches about six inches in height, encourages side branches, resulting in a bushier plant with increased leaf production.
The process of “hardening off” is required before transplanting and involves gradually acclimating the young plants to outdoor conditions over one to two weeks. Begin by placing the seedlings outside for a couple of hours in a sheltered, shaded location, protected from strong winds and direct sun. Each day, increase the duration outdoors and slowly introduce them to more direct sunlight and wind to minimize transplant shock.
Final transplanting into the garden or a larger container should only happen after all danger of frost has passed, and nighttime temperatures remain consistently above 50°F (10°C). Plant the established seedlings into well-draining soil at the same depth they were growing in their trays, spacing them about 12 inches apart to allow for mature growth.