Stevia is a tender perennial grown for its leaves, which contain compounds far sweeter than sugar with no calories. In Zone 6b it cannot survive winter outdoors, so it is grown as a warm-season annual or potted and brought inside. Gardeners grow it to harvest fresh or dried leaves for natural sweetening.
Stevia needs warm soil and is slow from seed, so most growers start indoors or buy transplants. It is sensitive to cold and will not tolerate frost. Pinch growing tips to encourage bushy growth, and harvest leaves before flowering, when sweetness peaks. In fall, dig and pot plants to overwinter indoors in a bright window.
Aphids
Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.
Septoria Leaf Spot
Small tan to brown spots with dark borders on leaves, often with tiny black specks at the center. Spots merge and lower leaves yellow and die.
Remove infected leaves and avoid overhead watering. Space plants for airflow and rotate planting location each year. Apply a copper-based fungicide if spread continues.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.