Stevia is a small, leafy herb whose leaves taste intensely sweet. A single fresh leaf can sweeten a cup of tea. It grows well in pots and warm beds, and it is easy once it gets going. Seed is fussy, so most beginners start with a young plant.
Sowing and planting
Stevia seed is small, and germination is often poor. Many seeds are hollow and never sprout. Buy fresh seed and sow plenty. Look for dark seeds. Pale ones are usually empty.
Sow indoors 8-10 weeks before your last frost. Scatter the seed on the surface of damp seed compost and press it in lightly. Do not cover it. Keep the tray at 70-80°F (21-27°C) under a clear lid, in bright light but out of direct midday sun. Seedlings appear in one to three weeks. Grow them on in a bright window or under lights. Pot each into its own 3 inch (8 cm) pot once it has two sets of true leaves.
Cuttings are quicker and give plants identical to the parent. Take 3-4 inch (8-10 cm) shoot tips in late spring or summer. Remove the lower leaves and root them in damp seed compost or a glass of water. Roots usually form in two to three weeks.
Harden off young plants (get them used to outdoor conditions) over a week or two. Plant out one to two weeks after your last frost, once the soil is at least 60°F (16°C). Choose a sunny, sheltered spot with free-draining soil. A raised bed helps in heavy clay. In pots, use a container at least 10-12 inches (25-30 cm) across, filled with a good multipurpose potting mix.
Growing on
Water regularly so the soil stays lightly moist. Stevia has shallow roots and dislikes drying out. It also hates sitting in wet soil. Water deeply, then let the top inch (2.5 cm) dry before watering again. Pots may need water every day in hot weather.
Feed lightly. Too much nitrogen gives lots of soft leaves with less sweetness. Work some compost into the soil at planting. A balanced, low-nitrogen liquid feed every three to four weeks is enough for pots.
Mulch around plants with a thin layer of compost or straw. It keeps the shallow roots cool and moist. Keep the mulch clear of the stems.
Pinch out the growing tips when plants are about 8 inches (20 cm) tall. Pinch again every few weeks. This makes a bushy plant with more leaves. Tall plants can flop in wind or heavy rain. Push in a short cane and tie the stems loosely if needed.
Stevia grows well beside other warm-season herbs such as basil and sweet marjoram. Give it room and plenty of sun.
To keep a plant over winter outside zones 9-11, lift it or cut it back to about 4 inches (10 cm) before your first frost. Pot it up and keep it in a cool, bright room. Water sparingly until spring. Alternatively, take cuttings in late summer and grow them on indoors.
Stevia is killed by frost and rots easily in cold, wet soil. Keep it indoors or in a pot you can move until well after your last frost, and bring it in before the first frost in autumn.
Harvesting
You can pick a few leaves at any time once the plant is growing strongly. Leaves are sweetest in late summer and autumn, as the days shorten and flower buds start to form. Once the plant flowers, the leaves can turn slightly bitter.
For the main harvest, cut the whole plant back by about half just as flower buds appear. Cut stems with clean scissors, just above a pair of leaves. In mild areas a second cut is often possible later in the season. At the end of the season, before the first frost, cut annual plants at the base.
Use fresh leaves straight away in tea, fruit salads or cold drinks. To dry stevia, strip the leaves and spread them on a screen or tray in a warm, airy, shaded place. They dry in one to three days. You can also hang small bunches upside down. Store dried leaves whole in an airtight jar. Crush or grind them to a green powder when you need it. Dried leaves are sweeter by weight than fresh ones, so start with a small amount.
To make a simple syrup, steep a handful of fresh or dried leaves in hot water, then strain. Keep it in the fridge and use it within a week or so.
Saving seed is rarely worth it. Much of the seed is not viable, and seedlings vary in sweetness. Keep your best plant going from cuttings instead.
Problems and pests
Plants wilt and lower stems turn black
This is root or stem rot, caused by cold, wet or badly drained soil. Water less, improve drainage, and move potted plants somewhere warmer. Take cuttings from healthy tips if the base is rotting.
Brown or grey spots on the leaves
Leaf spot diseases, such as septoria leaf spot, spread in warm, wet weather. Remove affected leaves and water at the base, not over the plant. Space plants for good airflow and clear up fallen leaves.
Sticky leaves and clusters of small insects
Aphids and whiteflies suck sap from shoot tips and leaf undersides. Spray them off with water or wipe them away. If they persist, use insecticidal soap. Follow the label, test it on a few leaves first, and spray in the evening, away from bees.
Ragged holes in young leaves
Slugs and snails feed at night, especially on young plants. Check under pots and boards after dark and remove them by hand. Beer traps or copper tape around pots also help.
Seedlings never appear
Old, pale or hollow seed will not sprout, and cold compost slows the rest. Buy fresh seed, sow it on the surface, and keep it warm at 70-80°F (21-27°C).
Varieties worth growing
Stevia is usually sold simply as "stevia" or "sweet leaf" rather than by a variety name. Choose plants by how they were raised.
- Seed-raised stevia: cheap and easy to find; plants vary in size and sweetness.
- Cutting-grown plants: bought as young plants from herb nurseries; uniform and often selected for sweetness.
- Your own cuttings: taken from your sweetest plant; the best way to keep a good strain year after year.