How to Grow Stevia in Australia

Stevia (Stevia rebaudiana), also called sweet leaf, is a frost-tender perennial of the daisy family, Asteraceae, grown for its intensely sweet leaves. It comes from Paraguay and Brazil and thrives in the humid north, while southern gardeners grow it in a pot and shelter it in winter. The usual mistake is burying its tiny seed, which germinates poorly even in ideal conditions.

Three mistakes that matter

1. Burying the seed and giving up early. The tiny seed germinates better in light, so a covering of mix can hold it back. Press it onto the surface, keep it warm, and expect slow, patchy germination.
2. Leaving it in frost or cold, wet soil. Frost injures or kills the plant, and roots rot in cold, wet winter soil. Where frost falls, grow it in a pot that can move under shelter, and water sparingly in winter.
3. Letting it run to flower unpicked. Unpinched plants grow tall and lanky, and once flowering starts they make few new leaves. Pinch the tips regularly and take the main harvest just before the flowers open, as Pinching and Harvesting describes.

When to plant

Stevia seed germinates best with the soil at 20 to 25°C. Sow in spring once the soil warms, or in the tropics as the wet season eases. One sowing is usually enough, because a sheltered plant can live three to five years.

Where spring is slow to warm, start the tray indoors on a seedling heat mat and plant out once the last frost has passed. Seed more than two years old germinates poorly, so sow a fresh packet.

Site, soil and setup

A plant holds its spot for several years, so give it the edge of a herb bed rather than a place in the vegetable rotation. On heavy clay, read waterlogged soil before planting.

Container growing

Choose a pot at least 30 cm across; a standard 300 mm pot holds about 13.5 L. Fill it with premium potting mix. The store sells one stevia, with no separate compact form.

• Water: whenever the surface feels dry, and never let the pot dry out in summer.
• Feed: fish and seaweed blend at half strength, about 12.5 mL per 9 L, fortnightly through summer.
• Position: full sun, with afternoon shade in very hot districts; move it under shelter or indoors before the first frost.

Sowing Seeds

Start stevia in a punnet, where warmth, light and moisture are easy to control. A packet of the store's Stevia seeds holds 10 seeds: sow them all at once, since only some are likely to come up. Three or four plants are plenty for a household.

Starting in a tray

• Fill a punnet with moist seed-raising mix and firm the surface.
• Scatter the dust-fine seed on top, as sowing tiny seeds shows, and press it in lightly without covering it.
• Mist the surface and cover the punnet with clear plastic or glass to hold the moisture.
• Keep it in bright light out of direct sun, at the soil temperature When to plant gives.
• Seedlings usually emerge in 1 to 3 weeks, often unevenly.
• Pot on each seedling (move it into its own small pot) once it can be handled, disturbing the roots as little as possible.

Sowing direct

In the tropics and subtropics, seed can go straight into a fine, warm bed. Press three seeds onto the surface at spots 30 cm apart, keep them moist and weeded, and thin to the strongest seedling.

Planting out

Plant seedlings out once they are 5 to 10 cm tall with several sets of true leaves, and only after frost has passed. Harden them off first (set them outside a little longer each day, for a week or two, so they adjust to sun and wind). Water them in with seaweed solution at half strength, 15 mL per 9 L, and mulch to keep the roots moist.

Growing from cuttings

Cuttings strike readily, and cutting-grown plants are far more uniform in sweetness than seedlings. Over summer, take tip cuttings 10 to 20 cm long, strip the lower leaves, and bury each to half its length in moist potting mix. Keep them moist until they root; taking cuttings covers the method.

Care by growth stage

Warmth and steady moisture decide how well stevia grows, and it needs only light feeding.

If growth stalls in poor soil, scratch in all-purpose controlled release granular (NPK around 14-5-10) in spring, at half the label rate.

Pinching and Harvesting

Pinching keeps the plant bushy and full, and it delays flowering. The stems are brittle, so cut with scissors or secateurs rather than bending them.

• Pinch out the growing tip once a young plant has several sets of leaves.
• Pinch the tips of upright stems every four to six weeks, and use the tips in the kitchen.
• Nip off flower buds as they form to keep leaves coming a little longer.
• For the main harvest, cut the whole plant back by about half before it flowers.
• After the last autumn harvest, cut the stems back to about 15 cm.
• Replace a plant with a rooted cutting after three to five years, as vigour fades.

Companion planting

Stevia suits a sunny herb bed or vegie patch, among plants that want the same steady water.

Grows well with

• Basil, which wants the same sun, warmth and moisture.
• Chives and Coriander, which aphids tend to avoid.
• Alyssum and Dill, which draw ladybirds, lacewings and hoverflies that eat aphids.

Keep apart

• Mint, whose runners spread and can crowd it out.
• Rosemary and Lavender, which want drier, leaner soil than stevia's steady watering allows.

Harvesting Leaves

Pick leaves from about 9 to 13 weeks after planting out. The leaves are sweetest just before the tiny white flowers open in late summer or autumn, so take the main harvest then.

• Pick in the morning, when the leaves are sweetest.
• Cut whole stem tips with scissors rather than stripping single leaves.
• Keep watering steady through picking, as the care table gives.
• Wait a few days after a liquid feed before picking, and rinse the leaves before use.
• Cool autumn weather can sharpen the flavour, so leave the final cut late, but finish before the first frost.

To dry the harvest, hang small bunches upside down in a shaded, airy place; in good weather they dry in about 10 days. Store the leaves once they are brittle, in airtight glass jars in a dark cupboard, for about 12 months. If condensation forms inside a jar, dry them longer.

Two or three leaves sweeten a cup of tea, and dried leaves grind to a powder for stewed fruit. They do not brown or crystallise like sugar, so they will not work in meringues.

Common problems

No seedlings after three weeks. Seed that was covered, kept cool or left to dry out often fails; see Three mistakes that matter. Read more

Tall, weak seedlings. Seedlings stretch towards the light in a dim spot, so move the punnet into bright light as soon as they emerge. Read more

Plant wilts while the soil is wet. Wet soil in cool weather rots the roots, as Three mistakes that matter warns, so replant on a mound or in a pot.

Brown spots on the leaves. Fungal leaf spots are common in warm, humid weather, so give plants their full spacing and water the soil, not the leaves.

Plant dies back in winter. Losing its leaves or dying back to the crown in late autumn is normal, and a sheltered plant usually reshoots in spring. Read more

Pests and diseases

Stevia has few serious pests, and most damage falls on seedlings and soft new growth.

Slugs and snails, including garden snail (Cornu aspersum). They feed at night and after rain, chewing seedlings to the ground. Search with a torch about two hours after sunset, and ring pots with copper strips. Read more

Aphids (Aphididae). Small green or yellow sap-suckers crowd onto soft tips and leave them sticky and distorted. Squash small groups or hose them off, and spray the undersides of leaves with insecticidal soap, as its label directs, if they persist. Read more

Damping off (Pythium species and Rhizoctonia solani). Seedlings rot at soil level and topple over, most often in wet, crowded punnets kept warm and covered. Sow into fresh mix and water from below. Read more

Prevention first

• Sow into fresh seed-raising mix in clean punnets.
• Lift the cover off the punnet once seedlings appear, so air can move.
• Give plants their full spacing so leaves dry quickly after rain.
• Check soft tips each week for aphids through spring and summer.

Tips

1. Seed-grown plants vary in sweetness, so taste a leaf from each and keep the sweetest going from cuttings.
2. Root a few cuttings in late summer and keep them on a bright windowsill as insurance against a hard winter.
3. Rely on cuttings rather than saved seed: stevia is largely self-sterile, so a lone plant rarely sets good seed. In warm regions it can still self-sow.
4. For a syrup, simmer two teaspoons of dried leaves in 1 L of water for ten minutes. Let it stand, strain it and keep it in the fridge.