Why do my seedlings wilt or collapse inside a propagator?

A propagator does one job well: it holds moisture around seed while it germinates. Once the seedling is up, everything about that environment turns against it — too warm, too humid, too still, and often not bright enough. The single most common propagator mistake is leaving the lid on out of caution.

What you'll need

• A propagator with adjustable vents, or a clear bag
• A bright cool position
• A max-min thermometer (optional)

Step by step

1. Take the lid off when the first seedlings appear.Not when they are all up, not in a few days. The first ones through are the signal. If the tray holds a mix of species, move the germinated ones out.
2. Never leave a covered propagator in direct sun.It behaves like a small greenhouse with no ventilation. On a bright March day the temperature inside can pass 40°C within the hour and the seedlings are lost before you notice.
3. Tell heat wilt from damping off.Heat wilt affects whole leaves, is worst in the afternoon, and recovers by morning. Damping off collapses individual seedlings at soil level, the stem looking pinched, dark and water-soaked, often with white fuzz, and it spreads outward through the tray.
4. Open the vents rather than removing the lid, if you must keep it.Most propagator lids have sliding vents. Open them fully, and prop the lid on a block if the seedlings are close to the top.
5. Move it out of the propagator once germinated.The brightest, coolest position you have. Seedlings want much more light and much less heat than the seeds did — those two requirements are genuinely opposite.
6. Water from below and let the surface dry.A constantly wet surface is what damping off needs. Standing the tray in shallow water for a few minutes, then draining it fully, keeps the compost damp and the surface dry.
7. Get air moving.Still humid air is the problem, not humidity itself. A window cracked open, or a small fan running gently in the room for a few hours a day, transforms the disease situation and produces stockier seedlings as well.
8. Turn off a heat mat once things germinate.Bottom heat gets seed up; it then drives soft stretched growth and makes the compost dry unevenly. Most seedlings do far better cool.
9. Sow thinly so there is space between plants.Crowded seedlings hold humidity between them and damping off runs through the tray. Thin sowing is a disease control measure as much as a space one.
10. Use clean trays and fresh seed compost.Damping-off organisms carry over in dirty trays and in reused compost. Washing trays and starting fresh removes most of the risk.

Tips

• Lid on for germination, lid off the moment they appear. That is nearly the whole subject.
• A cheap max-min thermometer left inside a propagator on a sunny day is genuinely alarming and worth doing once.
• Tap water rather than water butt water for seed trays — butt water carries the organisms that cause damping off.

Common mistakes

• Leaving the lid on until all the seed is up — The first seedlings are stretching and rotting while you wait for the last ones. Move them or take the lid off and accept lower germination on the stragglers.
• Standing a covered propagator on a sunny windowsill — It reaches temperatures that kill seedlings within an hour. Bright shade, or lid off, or both.
• Misting seedlings to help them — Wet leaves and still air is exactly the combination damping off needs. Water the compost from below, not the plants.

If it doesn't work

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