Damping Off – Why Seedlings Collapse at the Soil Line
Damping off is the collapse that happens overnight: healthy tray, then flat stems by morning. It is almost always a water-and-air problem, and it is preventable.
TL;DR
- Pinched brown stem at the soil line, plant flat, no recovery — discard it with the mix.
- Cause: wet surface + still air + cool + non-sterile mix or dirty tray.
- Prevention is mechanical: dome off at first sprout, water from below, move air, clean trays.
The conditions it wants
The organisms behind damping off are present almost everywhere and only become a problem when the seedling is kept in the conditions they like: a surface that never dries, air that does not move, a temperature cool enough to slow the plant but not the fungus, and a container or mix that was not clean to begin with.
Every one of those is something the grower controls. That is why the honest advice is prevention rather than treatment: by the time a stem is pinched, there is nothing to apply that will bring the plant back.
The five habits that prevent it
1. Dome off at the first sprout. Humidity helps germination and harms seedlings. The moment green shows, the cover comes off for good.
2. Water from below, and let the surface dry. Fill the base tray, let it soak up, pour off the excess. A surface that dries between waterings is a hostile place for the fungus.
3. Move the air. A small fan on low, or even a doorway's draught, cuts surface moisture. This is the cheapest control there is and the one most often skipped in a closed room.
4. Use fresh sterile mix and clean trays. Garden soil in a seed tray is the classic route in. Trays from last year get scrubbed and soaked — see the cleaning note in the tray guide.
5. Do not over-sow. A dense cell keeps its own humidity, blocks air and makes a single infection spread through twenty seedlings. Thin to one per cell and the tray breathes.
When it happens anyway
Remove the affected cell's contents — plant and mix — and bin them, not the compost heap. Let the rest of the tray dry back a little further than usual, get air moving, and stop watering the surface entirely for a few days.
If more than half the tray has gone, start again with fresh mix and clean containers rather than nursing the survivors; seedlings that stall this early rarely make good plants, and the calendar still has room in the season for a second sowing.
Tools for this job
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Seed starting mix (sterile)
Using fresh sterile mix is the single biggest reduction in risk.
Small oscillating fan
Moving air dries the surface and is half the prevention.
Squeeze bottle for bottom watering
Water into the base tray, never over the leaves.
Related guides
Frequently asked questions
What does damping off look like?
A seedling that was upright yesterday is lying on the mix today, with a pinched, water-soaked or brown constriction at the soil line. Often the whole cell of seedlings goes at once, and a light grey mould may be visible on the surface.
Can damping off be cured?
Not in an affected seedling — the stem tissue is destroyed and the plant will not recover. Remove and discard it with the mix around it, because the pathogen spreads through the tray from plant to plant.
What causes damping off?
A group of soil-borne fungi and water moulds (Pythium, Rhizoctonia, Fusarium among them) that thrive in wet, cool, still conditions. Overwatering, a dome left on after sprouting, dirty trays and non-sterile garden soil are the usual routes in.
Does cinnamon or chamomile tea prevent it?
There is no reliable evidence that home remedies stop it. The things that demonstrably work are physical: clean containers, sterile mix, bottom watering, air movement and taking the dome off at the first sprout.
Why did it happen after the seedlings looked fine for a week?
Because the conditions were right the whole time and the fungus needed a few days to build up. A dome left on, or a tray sitting in its drainage water, gives it exactly that window.
Is it the same as root rot?
Related but not identical. Damping off attacks the stem at the soil line and usually kills a seedling within days; root rots can afflict older plants and show up as wilting and yellowing with brown roots. Both trace back to a mix that stayed too wet.