Stevia

Sow into plug cells about seven to eight weeks before your transplant date, several seeds per cell, then thin to the strongest seedling once they are up. Use only the black or dark seeds and discard the clear to tan ones. Better still, root stem cuttings taken from a mother plant you have tasted and know is sweet. Stevia seed germinates poorly and slowly, so direct-seeding into a bed usually fails, and the pale seeds are typically sterile. Seedlings also vary plant to plant in sweetness and bitter aftertaste, while a cutting carries the mother plant's sugar profile exactly.

Plant into well-drained soil rich in organic matter once all danger of frost has passed. In field rows, growers in eastern North Carolina use 30-inch rows with plants 6 to 7 inches apart. Avoid any site that floods or puddles, and avoid saline soils. Stevia is a tender perennial with no frost tolerance, and its shallow roots rot in ground that stays wet.

Take the main harvest when plants are mature and blooms have just begun to form. Small plots are usually cut by hand; large plantings are cut at ground level with a harvester. Dry the cut plants with low heat and good air circulation for 24 to 48 hours, then thresh to separate the sweet leaves from the bitter stems and store the dry leaf in airtight containers. Sugar levels decline after flowering, while cool temperatures and short days intensify sweetness beforehand, so the window just as buds appear is the peak. The stems are bitter, which is why threshing the leaf off matters.

Keep a consistent source of moisture through the season, ideally by trickle irrigation, in frequent shallow waterings rather than occasional deep soakings. Do not let the soil stay saturated. Stevia's feeder roots sit near the soil surface, so a shallow, frequent pattern is what actually reaches them, while standing wet soil brings on root rot.

Keep any feeding through the growing season modest, and do not push the plant with high-nitrogen fertilizer. Follow a soil test rather than a fixed schedule. Research in Ontario found stevia has low nutrient requirements, and excess nitrogen drives profuse leafy growth with poor flavor. You get more leaf and less sweetness, which is the wrong trade for this crop.

Through the growing season, pinch out flower buds as soon as you spot them. The leaves are sweetest before flowering occurs, so pinching buds off as soon as they appear protects the quality of the leaf you are growing the plant for.

Check growing tips and leaf undersides regularly, especially on greenhouse or indoor plants. Aphids, thrips and whiteflies can become a serious problem under cover. In the field, watch for Septoria leaf spot, southern blight, stem and root rot, and tomato spotted wilt virus. Field stevia is not known to have serious insect pest problems, but protected transplant production is where these pests build up fast and can significantly impact the crop before it ever goes out.

Outside zones 10 and 11, lift plants into a container and move them indoors ahead of the first frost, or simply grow them as an annual and replant next spring. Stevia is only winter hardy in USDA zones 10 to 11. Everywhere colder it is grown as an annual and brought indoors before the first frost.