Growing seedlings is one of the most rewarding investments in the organic garden. 🌱 Good seedlings mean a more reliable harvest, earlier cropping and healthier plants. Based on the ÖMKi (Research Institute of Organic Agriculture) guide, we show how to raise strong, hardened seedlings using biointensive methods.
🌡️ When to sow? — Temperature needs by species
| Plant | Optimal temperature | Seedling period | Size at planting out |
|---|---|---|---|
| 🌶️ Pepper | 25°C | 8–10 weeks | 7–10 true leaves |
| 🍅 Tomato | 22°C | 4–6 weeks | 4–5 true leaves |
| 🥒 Cucumber | 25°C | 2–4 weeks | 2–3 leaves — do not prick out! |
| 🍆 Aubergine | 25°C | 6–8 weeks | 6–7 true leaves |
| 🥦 Cabbage, broccoli | 13°C | 4–6 weeks | 3–4 true leaves |
| 🥬 Lettuce | 16°C | 4–8 weeks | 3–4 true leaves |
| 🌿 Celery | 19°C | 9–10 weeks | 4–5 true leaves |
💡 Golden rule: “Warm feet, cool head” — root zone temperature should be ~3°C higher than air temperature. This promotes faster root development (ÖMKi 2022).
🪴 Choosing containers
| Container type | Best for | Advantage |
|---|---|---|
| 🔲 Cell tray (40, 66, 104 cells) | Lettuce, cabbage, leeks | Space-saving, can be mechanised |
| 🪴 Individual pot (4–10 cm) | Tomato, pepper, cucumber | Seedlings individually removable |
| 🟫 Soil block | Tomato, pepper, brassicas | No container cost, biodegradable |
⚠️ PROHIBITED in organic production: Raising seedlings in rockwool cubes — EU Organic Regulation 848/2018 explicitly forbids soil-free cultivation.
🌍 Growing media composition
Simple home mixes based on scientific recommendations:
- 🌿 Small garden: 50–60% green compost + 40–50% peat + 1.5–2% granulated organic fertiliser (min. 3:3:3 NPK)
- 🏡 Peat-based: Peat + 2% fertiliser. Correct acidic peat (pH 3–4) with lime (30 g per 10 l).
💡 Peat use is environmentally controversial — European peatland area shrank from 495,000 km² to 187,000 km². Coir, wood fibre or home compost are good partial substitutes.
🌱 Sowing and pricking out
Sowing depth: 1–2 times the seed diameter. Light-germinators (basil) do not cover. Prick out at 1–2 true leaf stage. Suitable for pricking: tomato, pepper, celery, lettuce, brassicas. Not suitable: cucumber, squash, melon (root-sensitive).
🛡️ Common problems and solutions
| Problem | Cause | Solution |
|---|---|---|
| 🌿 Leggy, drawn seedlings | Light shortage or too high temperature | More light, lower night temperature |
| 💀 Damping off | Overwatering + poor light + Pythium | Less water, better airflow, mature compost |
| 🟡 Yellow leaves | Nitrogen deficiency or pH issue | Liquid feeding (100 g organic fertiliser / 10 l water) |
| 🦟 Fungus gnat larvae | Overwatering + undecomposed plant matter | Steinernema feltiae nematode drench |
🏋️ Hardening off — essential before planting out!
Planting-ready seedlings must be hardened off over 8–10 days: gradually acclimatised to outdoor conditions — sunlight, wind and temperature fluctuation. Use shade netting initially, bring back under cover at night. Hardened seedlings withstand transplant stress far better.
📱 How BioGarden365 helps
- 🗓️ Sowing calendar: The app calculates sowing dates from your planned planting-out date.
- 🌡️ Frost alert: Exactly when seedlings are most vulnerable.
- 📷 Disease identification: Strange spots? Photograph and the AI tells you what it is.
✨ Summary
The secret of successful seedling raising: right temperature, good growing media, measured watering and hardening off. Get those right and you’ll raise strong, healthy seedlings — which is half the harvest won. 🌱