Stevia is a tender perennial from Paraguay grown for its leaves, which carry compounds many times sweeter than sugar. A leaf chewed fresh is sweet with a faint liquorice edge; dried and crumbled, or steeped, the leaves sweeten tea and drinks. It grows 24 to 30 inches and is a perennial only in zones 8 to 11; elsewhere it is a warm-season annual or a pot brought inside.
It needs warm soil and is slow and erratic from seed, so most growers buy transplants or start indoors ten weeks before last frost. Set plants out 18 inches apart once nights stay above 50°F, in full sun and average soil that drains, and keep them evenly watered; it wilts in drought and rots in wet ground. Pinch the growing tips through early summer for a bushy plant.
Harvest the leaves before flowering, in late summer when the sweetness peaks, and dry them quickly in a warm, airy place. In fall, before frost, dig and pot a plant to overwinter in a bright window, or take cuttings in late summer, which root easily. The small white flowers are visited by small bees. Deer leave it alone.
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.
Aphids
Aphididae →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.