Disease
Seedlings collapsing at the soil line: damping off
Last reviewed September 2026 · written from Cooperative Extension and university horticulture references — see how we source data
What you’re seeing
Seedlings emerge normally, then within days the stem develops a thin, dark, water-soaked pinch right at the soil surface and the seedling topples over and dies; sometimes seeds rot before ever emerging at all.
Could it be something else?
Likely causes
Ranked most common first.
- Overly wet, poorly drained starting mix. Fungi that cause damping off (Pythium, Rhizoctonia and others) thrive in saturated, oxygen-poor soil around the seedling stem.
- Poor air circulation. Still, humid air around trays - common under a clear dome left on too long - keeps the stem base constantly damp.
- Overcrowded seedlings. Sowing too thickly keeps humidity high right at the soil surface between crowded stems.
- Reused, unsterilized containers or old potting mix. Fungal spores persist on dirty trays and in old, previously-used soil.
How to fix it
- Remove and discard any affected seedlings immediately along with the soil directly around them to stop the fungus from spreading to healthy neighbors.
- Cut back watering and let the surface dry slightly between waterings; water from the bottom (setting trays in a shallow tray of water) rather than overhead where possible.
- Increase air circulation with a small fan on low, run a few hours a day, and remove any humidity dome as soon as most seeds have germinated.
- Thin overcrowded seedlings so remaining ones have breathing room, even though it feels wasteful to cut healthy ones.
- Start over with fresh, sterile seed-starting mix (not garden soil) and clean or new containers if damping off keeps recurring in the same trays.
Common mistakes
- Keeping the humidity dome on after germination. Take it off as soon as most seeds have sprouted.
- Using garden soil or old potting mix. Start seeds in fresh, sterile seed-starting mix.
- Watering on a schedule instead of by feel. Let the surface dry slightly between waterings, and water from below where you can.
- Sowing thickly and not thinning. Crowded seedlings keep the air still and humid right at the stem base - see thinning and spacing seedlings.
When and where it shows up
Damping off strikes in the first one to three weeks after germination, almost always indoors in trays or pots, and mostly in cool, constantly wet mix. Any seedling can get it, but tomatoes, peppers, brassicas like cabbage, and cucumbers are commonly hit. Once seedlings grow their first true leaves and the stem firms up, the risk drops sharply.
Preventing it next time
Use a sterile, well-draining seed-starting mix, avoid overwatering, keep good airflow around trays, and don't rush the humidity dome removal past the point most seeds have sprouted.
Questions gardeners ask
Can seedlings with damping off be saved?
No - once the stem has pinched and collapsed, the seedling won't recover. Remove it and the soil around it, fix the conditions, and resow.
Can I reuse the tray?
Yes, after cleaning. Scrub off old soil, wash with soapy water, then soak for 10 minutes in a 1-part-bleach to 9-parts-water solution and rinse.
Does sprinkling cinnamon prevent damping off?
Cinnamon has mild antifungal properties and some gardeners swear by it, but it isn't a reliable fix. Airflow, sterile mix and careful watering matter far more.
Does a heat mat cause damping off?
No. Bottom heat speeds germination, which shortens the time seedlings are most vulnerable. The problem is warmth combined with soggy mix and still air.