Harvesting should be done as late as possible, since cool autumn temperatures and shorter days tend to intensify the sweetness of the plants as they evolve into a reproductive state. While exposure to frost is still to be avoided, covering the plants during an early frost can give you the benefit of another few weeks’ growth and more sweetness.
When the time does come to harvest your stevia, the easiest technique is to cut the branches off with pruning shears before stripping the leaves. As an extra bonus, you might also want to clip off the very tips of the stems and add them to your harvest, as they are apt to contain as much stevioside as do the leaves.
In harsher climates, however, it might be a good idea to take cuttings that will form the basis for the next year’s crop. Cuttings need to be rooted before planting, using either commercial rooting hormones or a natural base made from willow tree tips, pulverized onto a slurry in your blender. After dipping the cuttings in such a preparation, they should be planted in a rooting medium for two to three weeks, giving the new root system a chance to form.