Stevia is the plant behind the sweetener: its leaves are intensely sweet — many times sweeter than sugar — and a few plants supply a household's worth of leaves for tea and cooking. It is a tender tropical perennial, so in most of the United States it is grown as an annual from seed, started indoors while it is still cold outside.
Stevia seed is tiny, needs light to germinate, and demands real warmth — it will simply sit and do nothing in a cool tray. That combination makes indoor sowing the reliable route. This guide covers the surface sowing, the heat mat that makes the difference, and how to keep the seedlings alive until summer. Stevia is a start-indoors crop — its sowing method is "start indoors" — so the steps below are the standard route, not a workaround. A tray indoors gives you control over warmth, moisture and light that outdoor sowing cannot match. This crop is in the Asteraceae family, needs full sun, and takes 90–120 days to reach maturity, so the timing below is built around those facts, not guesswork.
Stevia at a glance
| Growing factor | What stevia need |
|---|---|
| Days to germination | 7–14 |
| Sowing method | start indoors |
| Days to maturity | 90–120 |
| Soil pH | 6.0–7.0 |
| Spacing | 18–24 inches |
| Sun | full sun |
| Cold tolerance | tender |
When to start stevia seeds indoors
In Zones 3–5, start indoors 8–10 weeks before your last frost — stevia needs a long warm season to produce leaves. In Zones 6–8, 6–8 weeks is enough. In Zones 9–10, start in late winter and plant out after frost. In frost-free Zones 11–13, sow any time; stevia is perennial there. Check your local extension service for frost dates, or use our frost-date calculator and seed-starting calculator.
As a rule, work backwards from your last frost: sow indoors 8–10 weeks before it if you are starting indoors at all, and check the seed packet against the germination time of 7–14 days. Our seed-starting calculator does the backwards maths for you, and the germination-time calculator helps you plan around that 7–14-day window. If you are unsure of your zone, the hardiness-zone finder will confirm it.
What you need
a seed tray or small pots, fine seed-starting mix, a clear lid or cling film, a spray bottle, a heat mat and a grow light (both genuinely useful for stevia), and labels. Stevia seedlings stay small for a long time, so small cells are fine at first.
You will also want a spray bottle or a watering can with a fine rose, and a notebook to record the sowing date — germination takes 7–14 days, and it is easy to forget when you sowed.
Step by step: starting stevia seeds indoors
- Fill your containers. Fill trays or pots with fresh, damp seed-starting mix — not garden soil, which is too heavy and can carry disease. Press it down lightly and level the surface.
- Sow the seed. Scatter stevia seed thinly on the surface of damp, fine seed-starting mix and press it down gently — do not cover it, as it needs light to germinate. Cover with a clear lid and keep the tray warm, ideally 70–75°F; a heat mat helps a lot. Germination takes 7–14 days. Thin to one seedling per cell once they have two true leaves, snipping the extras rather than pulling them.
- Cover and label. Cover the tray with a clear lid or cling film to hold humidity, and label it with the crop and the date. Put it somewhere warm and bright, out of direct sun until seeds sprout.
- Wait for germination. Germination takes 7–14 days. Check the tray daily, mist the surface if it dries, and be patient — uneven germination is normal, especially for slow crops. Our germination-time calculator helps you track the window.
- Remove the cover. As soon as seedlings emerge, take the lid off. Good airflow from this point on is what prevents damping off.
- Give them light. Move the tray under a grow light or to your brightest window immediately — seedlings stretch within a day or two if the light is weak.
- Thin and pot on. Thin to one seedling per cell once they have two true leaves, snipping the extras rather than pulling them. Pot on into larger pots when roots show at the drainage holes, keeping the seedlings at the same depth.
- Feed lightly. Once seedlings have two sets of true leaves, feed with a half-strength liquid feed every week or two. Do not overfeed — soft, overfed seedlings are more likely to damp off and less likely to harden well.
Light for seedlings
Stevia seedlings need 14–16 hours of bright light a day. Without it they stretch, go pale and stall. Keep a grow light 2–4 inches above the leaves. A windowsill alone is rarely enough in early spring — this is one herb where a light pays for itself.
Watering seedlings
Stevia has moderate water needs. Keep the mix evenly moist but never soggy — mist or water from below, and let the surface dry slightly between waterings once seedlings are up. Overwatering causes damping off, which is the main killer of stevia seedlings. Our watering calculator can help you work out how much water your seedlings need as they grow.
Hardening off
Stevia is tender, so harden off carefully over 10 days: short spells in warm, sheltered shade first, building to full sun. Do not leave plants out overnight until nights are reliably warm, and bring them in if a cold night is forecast.
Transplanting outdoors
Plant out only after all frost danger has passed and nights are warm, in full sun and light, well-drained soil. Space plants 18–24 inches apart. Our plant-spacing calculator works out the layout. In cold zones, stevia also grows well in a pot on a sunny patio. For the full picture — soil, feeding, pests and harvest — see our complete growing stevia guide.
Common problems and fixes
| Problem | Fix |
|---|---|
| Seeds do not germinate | The tray was too cool, or the seed was buried. Sow on the surface, use a heat mat, and keep the mix damp for the full 7–14 days. |
| Leggy, pale seedlings | Not enough light. Move the grow light closer and run it 14–16 hours a day. |
| Damping off | Stems collapse at soil level. The mix was too wet. Water from below, remove the lid once seeds sprout, and use fresh mix. |
| Seedlings stall and stay tiny | Stevia is slow for the first month. Keep it warm and bright, feed very lightly once it has true leaves, and pot on when roots show at the drainage holes. |
Seeds do not germinate
The tray was too cool, or the seed was buried. Sow on the surface, use a heat mat, and keep the mix damp for the full 7–14 days.
Leggy, pale seedlings
Not enough light. Move the grow light closer and run it 14–16 hours a day.
Damping off
Stems collapse at soil level. The mix was too wet. Water from below, remove the lid once seeds sprout, and use fresh mix.
Good companions for stevia include tomatoes, peppers, marigolds, cabbage. Watch out for aphids, slugs, thrips — check young plants regularly, and deal with pests early while plants are small. Our pest-identifier helper can help you work out what you are looking at.
Frequently asked questions
How long do stevia seeds take to germinate?
7–14 days in warm, bright conditions. A heat mat speeds germination noticeably.
Do stevia seeds need light to germinate?
Yes — sow them on the surface and do not cover them with mix.
Can I grow stevia from seed in a cold climate?
Yes, as an annual: start indoors 8–10 weeks before your last frost and plant out after frost. Or grow it in a pot and bring it indoors for winter.
How far apart should stevia plants be?
18–24 inches apart in full sun and well-drained soil.
When can I harvest stevia leaves?
About 90–120 days from sowing. Pick leaves just before the plant flowers, when they are sweetest.