Starting seeds in seedling trays is the most reliable way to get strong, even seedlings, instead of sowing straight into the ground and waiting to see what comes up. A tray lets you control the growing mix, water and temperature, and move the seedling to the ground with its whole root ball and no transplant shock.
This article walks you through it step by step: choosing the tray, the right mix, sowing and watering, and when to transplant.
How to choose the right seedling tray
Cell size depends on the seed type and how long the seedling will stay in the tray:
| Cells | Cell size | Suits |
|---|---|---|
| 288 | Very small | Lettuce, onions and herbs, seedlings moved early |
| 105 | Small | Lettuce, cabbage and broccoli |
| 72 | Medium | Tomatoes, peppers and aubergines |
| 54 | Large | Cucumbers, courgettes and watermelons |
| 20 | Very large | Large seeds and seedlings kept longer |
Rule: the bigger the seed or the longer the seedling stays, the larger the cell.
What is the best mix for seedling trays?
- Do not use garden soil; it is heavy and carries fungi and weed seeds.
- Best is fine peat moss mixed with perlite at roughly 3 parts peat to 1 part perlite.
- Peat holds moisture, and perlite adds air and prevents root rot.
- If the peat is raw, seedlings need light feeding after the first true leaves. If it is enriched, it is ready to use.
For details, see Agricultural Perlite.
Steps for sowing seeds in a tray
- Moisten the mix before filling: it should feel like a wrung-out sponge, not dripping.
- Fill the cells and level off the excess, pressing only lightly.
- Make a small hole in each cell two to three times the seed's size deep.
- Place one or two seeds per cell; scatter very small seeds on the surface.
- Cover the seeds with a thin layer of peat or vermiculite.
- Mist with water so the seeds are not washed out of place.
- Label the variety and sowing date on the tray.
Germination conditions: temperature, humidity and light
- Temperature: most vegetable seeds germinate best between 20 and 30 °C. Tomatoes and peppers like warmth.
- Humidity: cover the tray with a clear lid or plastic until germination, then remove it straight away.
- Light: once seedlings emerge they need strong light, or they stretch and weaken. Place them in morning sun or under grow lights.
- In our summer: keep trays out of direct midday sun and use shade netting.
The right way to water
- Bottom watering is best: stand the tray in a basin of water for 10 to 15 minutes so it soaks up through the drainage holes.
- If watering from above, use a fine mist.
- Do not let the mix dry out completely, and do not flood it.
- Overwatering is the main cause of damping off and fungus gnats. See Damping Off and Fungus Gnats.
Feeding seedlings in the tray
- Do not fertilise before the first two true leaves appear (after the seed leaves).
- Then use a balanced soluble fertiliser at a diluted rate as per the label, about once a week.
- For stronger roots, see Best Fertiliser for Strong Roots.
When to transplant seedlings
- When they have 3 to 5 true leaves.
- When roots hold the mix and the seedling slides out of the cell as a complete plug.
- Harden off for a week before transplanting: put them outside for longer each day to adjust to sun and wind.
- Transplant in the late afternoon, watering the ground before and after.
- Protect young seedlings from cutworms in the first days. See Cutworms.
Common seedling mistakes
- Using garden soil instead of clean peat.
- Sowing too deep so the seed cannot emerge.
- Leaving the plastic cover on after germination, so seedlings rot.
- Weak light, producing tall, thin seedlings.
- Overwatering, causing damping off.
- Transplanting too late, after roots have circled the cell.
Products from Orkida
- Stender raw peat moss (Germany) 300 L
- Enriched peat moss TS3 (Germany) 210 L
- Agrolite agricultural perlite 100 L
- Seedling tray 72 cells
- Seedling tray 54 cells
- Seedling tray 105 cells
- Seedling tray 288 cells
- Seedling tray 20 cells
Seedling tray FAQs
How many days do seeds take to germinate?
It depends on the crop: cucumbers and courgettes 3 to 7 days, tomatoes 5 to 10 days, and peppers up to two weeks, especially in cool weather.
Can I reuse seedling trays?
Yes, but wash and disinfect them before reuse so you do not carry over seedling diseases.
Why are my seedlings tall and thin?
Usually because of weak light or high temperature with little light. Move them closer to light and reduce warmth after germination.
How many seeds per cell?
One or two. If both come up, snip the weaker one at soil level instead of pulling it out.