Every seed-starter meets damping off sooner or later: one evening your seedlings stand proud, the next morning they lie flat, pinched and brown at soil level, dead beyond saving. It feels like sabotage — but it is a preventable fungal disease, and a few simple habits nearly eliminate it.
This Plant Doctor guide explains what damping off is, why wet, still, airless conditions cause it, the prevention habits that stop it (fresh compost, watering from below, airflow, thin sowing), and what to do when it strikes. Written for beginners starting seeds indoors.
| Question | What to know |
|---|---|
| What is it? | Fungal disease that rots seedling stems at soil level |
| Signs? | Seedlings topple overnight, stems pinched and brown at the base |
| When? | Usually 1–3 weeks after germination |
| Causes? | Wet, cool, still, airless conditions + contaminated compost |
| Cure? | None for affected seedlings — start over |
| Prevention? | Fresh compost, bottom-watering, airflow, thin sowing |
Step 1: Recognise it — the overnight collapse
Damping off has a signature: healthy-looking seedlings collapse at the base, the stem pinched to a thread and browned right at soil level. It strikes fast — often overnight — and spreads through a tray within days. Seeds may also simply fail to emerge, rotting underground before you ever see them.
Do not confuse it with legginess: leggy seedlings are tall, pale and floppy but alive; damped-off seedlings are dead at the base. If the stem is pinched and brown at soil level, it is damping off.
Step 2: Understand the cause — wet, still and airless
Damping-off fungi live in soil and compost, waiting for the right conditions: constantly wet compost, cool temperatures, still air and crowded seedlings. Overwatered trays on a cool windowsill with no air movement are the classic set-up — which is why it strikes beginners most.
Contaminated compost or dirty reused trays add the fungi in the first place; the wet, airless conditions let them attack. Every prevention step below removes one of these conditions.
Step 3: Start clean — fresh compost, clean trays
Use fresh seed compost from a sealed bag — not last year's opened bag, not garden soil. Wash reused trays and pots in hot soapy water before sowing. This removes the fungi before they meet your seedlings.
Our seed-starting guide covers the full clean set-up. Cleanliness sounds fussy, but it is the cheapest insurance in gardening.
Step 4: Water from below — keep the surface dry
The single most effective habit: water trays from below by standing them in a shallow tray of water for 10–15 minutes, then draining. The surface stays relatively dry while roots get moisture — and dry surfaces starve the fungi.
Never leave trays standing in water, and let the compost surface dry slightly between waterings. See our overwatering guide — the same wetness that drowns roots feeds damping off.
Step 5: Give them air and space
Airflow is the great preventer: a gentle fan on low near the seedlings, or simply opening the propagator vents and the room window daily, keeps air moving and surfaces dry. Sow thinly — crowded seedlings trap humidity and spread infection plant to plant.
Remove propagator lids once seeds germinate: the humid dome that helped germination becomes a disease incubator afterwards. Light and air from day one grow stocky, resistant seedlings.
Step 6: Keep them warm enough — cool and wet is the danger zone
Most seedlings want 15–20°C after germination — cool rooms slow growth and keep compost wet longer, the exact conditions fungi love. A warm windowsill or heated propagator base speeds seedlings past the vulnerable stage.
This pairs with our leggy seedlings guide: warmth plus maximum light grows fast, stocky seedlings that outrun disease.
Step 7: When it strikes — remove, isolate, start over
Found it? Act fast: remove and bin every collapsed seedling plus the compost immediately around them — do not compost them. Move healthy-looking survivors to fresh compost in clean trays, and improve airflow at once.
Be honest with yourself: sowing again is usually better than nursing survivors. Damping off spreads invisibly, and a fresh sowing in clean compost with better habits overtakes struggling survivors within weeks. Most sowings have time for a second chance.
Troubleshooting: seedling emergencies
Whole tray collapsed overnight: Classic damping off in wet, airless conditions. Bin the lot, wash the tray, and re-sow in fresh compost with bottom-watering and airflow.
Seeds never emerged at all: May be damping off before emergence (rotted underground) — or old seed, or too cold. Test with fresh seed in warm, clean compost.
Seedlings fine, then flopping at the base one by one: Early-stage spread — remove affected seedlings immediately, thin the rest for airflow, and let surfaces dry. Often you can save the tray if you catch the first cases.
White mould on the compost surface: Surface mould from constant wetness — not damping off itself, but the same conditions cause it. Let the surface dry, improve airflow, and water from below.
What to do this October
October is clean-up for next spring's sowings: wash all seed trays, pots and propagator lids now while the memory of the season is fresh, and store them dry. Buy fresh seed compost — do not rely on the opened bag from March.
Plan next year's sowing station: a warm, bright spot with room for airflow. See the full October checklist for the month's other jobs.
Frequently asked questions
Can I save damped-off seedlings?
No — once the stem is pinched and brown at the base, the seedling is dead. Remove it and focus on the healthy ones, or re-sow.
Does cinnamon or chamomile tea prevent it?
These folk remedies have thin evidence. Clean compost, bottom-watering and airflow have strong evidence — spend your effort there.
Is garden soil safe for seed sowing?
No — garden soil carries damping-off fungi and is too heavy for seedlings. Always use fresh seed compost from a sealed bag.
Will it affect my outdoor sowings?
Rarely — outdoor soil drains and breathes, and sun and wind keep surfaces dry. Damping off is overwhelmingly an indoor seed-tray disease.
Related guides
- Starting Seeds Indoors: A Beginner's Guide — the clean set-up that prevents it
- Leggy Seedlings? How to Fix Them — the other classic seedling problem
- Overwatering? How to Fix Soggy Plants — wetness causes both problems
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