Most plants are lost in the first two weeks — still in the cube, long before they ever reach the tower. The cause is almost never the nutrient mix. A seed carries its own supply for the first 5–7 days; what it needs is a stable temperature, moisture without saturation, and air. Get those three right and your germination rate realistically climbs from around 50 % to 85–95 %.
Substrate temperature, not room temperature
The thermometer on the wall measures air; the seed responds to the cube it sits in. Wet rockwool on a cold floor is often 3–5 °C colder than the room because evaporation cools it. So measure inside the cube with a probe thermometer. A 10–20 W heat mat under the tray solves the problem for every warmth-loving crop.
The opposite direction is just as risky. Above 25–28 °C lettuce enters thermodormancy: the seed locks germination down and refuses to emerge even when everything else is perfect. In summer that means sowing in the evening, in the coolest room, or chilling the seed for 24–48 hours at 4–6 °C beforehand.
| Crop | Germination temp. | Days to emerge | Light? | Into the tower after |
|---|---|---|---|---|
| Lettuce | 15–20 °C | 4–7 | light germinator | 12–18 days |
| Rocket / arugula | 15–22 °C | 3–7 | either | 10–14 days |
| Spinach | 10–18 °C | 7–14 | dark germinator | 16–22 days |
| Lamb's lettuce | 10–15 °C | 10–14 | dark germinator | 20–28 days |
| Kale | 15–22 °C | 4–7 | either | 12–18 days |
| Chard | 15–22 °C | 7–14 | dark germinator | 16–20 days |
| Basil | 20–25 °C | 7–14 | light germinator | 20–28 days |
| Parsley | 18–22 °C | 14–28 | dark germinator | 28–35 days |
| Coriander | 18–22 °C | 7–14 | dark germinator | 18–24 days |
| Chives | 18–22 °C | 10–14 | dark germinator | 25–30 days |
| Tomato | 22–26 °C | 6–10 | dark germinator | 25–35 days |
| Pepper / chilli | 25–28 °C | 10–21 | dark germinator | 30–40 days |
The last column assumes indoor conditions with decent light. In winter with weak lighting, add 20–30 % to those times — a seedling does not finish faster just because the calendar says so.
Light and dark germinators
Some seed needs a light signal to start at all; other seed will not tolerate it. In practice the only difference is sowing depth, and it decides everything: a light germinator buried 1 cm deep will never come up.
- Light germinators (lettuce, basil, celery, thyme, chamomile): lay the seed on the surface or cover with no more than 2–3 mm of vermiculite
- Dark germinators (tomato, pepper, spinach, lamb's lettuce, parsley, coriander, onion): 5–8 mm deep, in a rockwool cube all the way to the bottom of the hole
- Rule of thumb without a table: sowing depth = 2× seed diameter
- When unsure, sow shallow. Seed buried too deep is a far more common failure than too much light.
Sowing into rockwool or foam, step by step
Rockwool leaves the factory at pH 7,5–8,5, which is too much for young roots, so it is always conditioned before sowing. PU foam plugs need no soaking — they are pH-neutral and clean water is enough.
- 1. Adjust water to pH 5,5 and soak the cubes for 30–60 minutes until fully saturated
- 2. Lift them out and let the excess drain on its own — NEVER squeeze rockwool; crushed fibres no longer hold air
- 3. Two seeds per hole (insurance: bought seed germinates at 80–95 %, older seed far less)
- 4. Close the hole with a fingertip or 2–3 mm of vermiculite, depending on the germinator type
- 5. Dome or clear film until emergence: aim for 90–95 % humidity, but vent 5 minutes a day or you get mould
- 6. The moment the first sprouts break through, the dome comes off and the light goes on — even 24 hours of delay gives long, thin seedlings
- 7. Plain water for the first 3–5 days; introduce nutrients only with the first true leaf, at half strength
- 8. Keep the substrate moist, not wet: a cube standing in 2 cm of water runs out of oxygen and the root turns black
| Phase | Substrate temp. | Humidity | Light | EC / pH of solution |
|---|---|---|---|---|
| Sowing → emergence | per crop | 90–95 % | only needed for light germinators | plain water, pH 5,5–6,0 |
| Emergence → 1st true leaf | 18–24 °C | 60–75 % | 14–16 h, 100–200 µmol/m²/s | EC 0,4–0,8 |
| Before transplanting | 18–22 °C | 50–65 % | 14–16 h, 200–300 µmol/m²/s | EC 0,8–1,2 |
When a seedling may enter the tower
Read the plant, not the calendar. Three signs have to be present at the same time:
- Roots protrude 2–5 cm from the cube, ideally with more than one white tip — white means healthy, brown means it was kept too wet
- 2–4 true leaves (cotyledons do not count)
- The seedling stands upright, the stem is no thinner than a matchstick and does not flop — stretched seedlings rarely catch up with the rest inside the tower
Seat the 25 mm cube in the net pot so its lower edge touches the film of solution running down the wall, or sits at most 1–2 cm above it. During the first 48 hours the roots have not reached the flow yet, so the cube has to stay moist on its own. More on carrier materials in the article on substrates, more on light levels in the article on lighting.
A practical note for a 32-slot tower: do not sow all 32 at once. Start 6–8 cubes every two weeks. Otherwise everything matures in the same week and you have nothing for a month afterwards.
Three mistakes that keep repeating
- Full-strength nutrients from day one: an EC above 1,2 during germination pulls water out of the seed instead of feeding it. Those seedlings never get past the cotyledon stage.
- Leaving the dome on too long: after emergence, 95 % humidity leads straight to damping-off (usually Pythium). You recognise it by the glassy, pinched stem right above the substrate.
- Old seed with no test: lettuce keeps for 3 years, tomato 4–6, cucumber 5–8 — but parsley and chives drop off after 1–2. A germination test with 10 seeds on damp paper costs you a week and saves you a month.