Gadenin日本語Get the appExpect a poor strike rate from seed
Stevia seed is one of the least reliable herb seeds you can buy. A packet holds fine, hair-thin achenes with a tuft of bristles on one end, and a large share of them are empty husks that were never fertilized. Even fresh, fully filled seed rarely gives better than one seedling in four, and a packet more than a year old often gives nothing at all. If you sowed a whole packet and got two plants, nothing went wrong on your side.
The second problem is that seedlings are not uniform. Stevia is an outcrossing plant, and seed-raised plants differ in leaf size, growth habit and, crucially, in how sweet the leaf is. Out of ten seedlings you might get two that are genuinely sweet, six that are middling, and a couple that taste mostly of green leaf with a bitter finish. That is why almost everyone who grows this seriously keeps a good plant going and takes cuttings from it rather than sowing again.
Sow seed if you want to find a plant worth keeping, or if you cannot get cuttings. Once you have one plant whose leaf tastes sweet and clean, stop sowing and propagate that plant instead.
Sow on the surface and keep it warm
The seed needs light to germinate and is far too small to push through a covering. Sow it on the surface of moist, fine seed-starting mix, press it down with a fingertip so it makes contact, and do not cover it at all. Warmth is the other half of the job: 70 to 77F (21 to 25C) at the compost, held steadily, day and night. Below 65F (18C) germination drops away to almost nothing, and a cold night after a warm day undoes a warm day.
- Sow thickly Because so much of the seed is blind, scatter it far more thickly than you would for any other herb. Twenty or thirty seeds across a four inch (10 cm) pot is reasonable. You will thin later, and you probably will not need to.
- Press, do not cover Firm the seed onto the surface with a flat fingertip or a small board. Contact with damp compost is what matters. A covering of even a few millimeters of soil will cost you most of the seedlings.
- Water from below only Stand the pot in a shallow tray of water until the surface darkens, then lift it out. Watering from above rolls surface-sown seed into the corners and buries it under washed compost.
- Cover the pot, not the seed A clear lid or a bag over the pot holds humidity so the surface never dries, while still letting light through. Take it off as soon as the first seedlings stand up, or they will damp off.
- Keep it on gentle bottom heat A heat mat set to 75F (24C) is the single change that most improves results. A windowsill that swings from 77F in the afternoon to 59F (15C) overnight is not warm enough, whatever the daytime reading says.
The first four weeks after germination
Stevia seedlings emerge as tiny, near-round cotyledons and then sit still for what feels like a long time. Two to three weeks with barely any visible growth is normal. What is happening is root growth, and pushing them with feed or extra water at this stage does more harm than good. Give them the brightest light you have, keep them warm, and be patient.
The seedlings are shallow-rooted and have no reserves, so a single dry-out kills them outright. At the same time they rot quickly if the compost stays sodden. Aim for compost that is damp all the way through but never glistening, and check it twice a day rather than watering on a schedule.
| Stage | What you see | What to do |
|---|---|---|
| Germination | Nothing, for one to three weeks | Hold 70 to 77F (21 to 25C), keep the surface damp, keep it in light |
| Cotyledons | Two tiny round leaves | Take the humidity cover off, move to full light immediately |
| First true leaves | Small toothed leaves, growth still slow | Weak liquid feed at quarter strength, once |
| Two to three pairs of leaves | Roots showing at the drainage holes | Pot up individually into three and a half inch (9 cm) pots |
| Six inches (15 cm) tall | Bushy, stem firming at the base | Pinch the growing tip, then harden off before planting out |
Pot up individually, then pinch
Transplant into individual three and a half inch (9 cm) pots as soon as the seedlings have two or three pairs of true leaves. Handle them by a leaf, never the stem, which crushes easily. Use an ordinary free-draining potting mix; stevia does not want anything rich at this stage, and high nitrogen makes soft, floppy growth that snaps in the first wind after planting out.
When a seedling reaches about six inches (15 cm), pinch out the growing tip above the second or third pair of leaves. This is not optional. An unpinched stevia plant runs up as a single thin stem, flowers early and gives you almost no leaf. Pinching forces side shoots from the leaf axils, and it is those side shoots that carry the crop.
Do not plant out until nights are reliably above 50F (10C) and all frost has passed. Stevia is tender: it will not die at 41F (5C), but cold nights check it so badly that a plant set out early is often still smaller in midsummer than one planted three weeks later.
Taste your seedlings, and write down which was which
Once plants have five or six pairs of leaves, pick one leaf from each and chew it. The difference between a good seedling and a poor one is unmistakable: a good one is intensely sweet within a second or two and stays clean, a poor one is grassy with a bitter, lingering aftertaste. Number your pots and note the result before you plant out, because you cannot tell them apart by looking.
Keep a short record for each seedling: sowing date, the date it germinated, the date you first tasted it, the verdict, and where in the bed you planted it. Next year that note is what tells you which plant to take cuttings from, and it saves you sowing a whole packet again to find the same thing out twice. Also record how many seeds you sowed against how many came up, so you know whether your seed source is worth reordering from.