Place the container underneath the stevia plant.
Shake the stevia plant gently. Any ripe black or tan seeds will fall into the container.
Remove only the black seeds from the container for planting. According to a 1999 study conducted by the University of Purdue, tan stevia seed is likely to be nonviable.
Revisit the plants in four weeks and repeat Steps 1 through 3.
Visit the plants a third and final time in four weeks. Cut any branches that still hold seed. Shake them vigorously over the container to loosen any remaining viable seed.