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For ordinary seed sowing, remove the propagator lid when seedlings appear. Do not wait for a set number of true leaves. Before germination, use the vents to manage excess heat and moisture while keeping the compost within the conditions recommended on the seed packet.
Table of Contents
Before the seeds germinate
A clear cover slows moisture loss from the compost. It can also trap considerable heat in sunshine, even on an unheated propagator. Check the unit regularly rather than sealing it and leaving it unattended on a sunny windowsill.
Follow the seed packet’s temperature and light instructions. If the enclosure is too warm, move it away from direct sun and increase ventilation. A thermostat can stop an electric heater adding more heat; it cannot actively cool an enclosure heated by the sun.
What to do when shoots appear
The RHS advises removing glass or plastic covers as soon as seedlings emerge. Move the uncovered tray into bright conditions at a temperature suitable for that crop. Keep tender seedlings protected from cold draughts and frost.
If separate pots germinate at different speeds, take the emerged pots out individually where the setup allows. This is a useful reason to sow different crops in separate containers. Do not keep a whole batch enclosed for weeks while waiting for the last few seeds.
When a packet gives a specialist method, follow that method. Rooting leafy cuttings is also different from germinating seed: cuttings may need a humid enclosure for longer while they develop roots. Seedling lid advice should not be applied automatically to them.
Read the conditions, not just the condensation
Some droplets on a cool lid are normal and are not a precise humidity reading. Persistent heavy condensation, saturated compost or mould are reasons to inspect watering, temperature, cleanliness and ventilation together. Opening a vent cannot correct a tray that is permanently standing in water.
Check temperature against the plant’s requirements. There is no universal 27°C damage threshold for all seeds and seedlings. Similarly, pale or stretched seedlings do not establish carbon dioxide deficiency: inadequate light, unsuitable temperatures and other growing problems need consideration.
Watering after the lid comes off
Uncovered trays can dry faster. Check the compost gently and water when needed, keeping it moist without leaving it saturated. Drain water that remains in a holding tray after watering. Avoid restoring a sealed, very humid enclosure simply because the surface has dried a little.
Use a stable cover with designed vents rather than balancing heavy glass on loose objects. Once a cover is no longer needed, clean it and store it safely for the next sowing.
Prepare for the growing-on stage
Removing a lid is not the same as hardening plants off for outdoor life. Seedlings usually need more time to grow, and they may need pricking out or potting on first. Harden them off gradually when they are ready and outdoor conditions suit the crop. Keep track of sowing dates and varieties so batches with different requirements stay separate.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Vented seed propagator: Allows the gardener to incrementally adjust airflow as seedlings develop, as described in the ventilation sections. Ensure the lid has adjustable sliders or clips rather than a fixed seal. Search Amazon for Vented seed propagator.
- Maximum-minimum thermometer: Shows temperature changes inside the propagator so they can be compared with the particular seed packet’s requirements. Check that it can be placed inside a small propagator and has a clear digital or analog read-out. Search Amazon for Maximum-minimum thermometer.
Related garden guides
- Greenhouse condensation: reducing persistent damp
- Greenhouse thermometers: tracking minimum and maximum temperatures
- More greenhouse & growing guides
Sources and further guidance
- How Plants Breathe and Exchange Gases | RHS Advice
- How to Sow Seeds Indoors | RHS Advice
- Greenhouse: Ventilation and Shading Tips | RHS Advice
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.