Damping Off: Why Seedlings Collapse

If your seedlings have suddenly flopped over and gone thin and brown right at the base of the stem, that's damping off. It's a soil-borne fungus that strikes young seedlings in cold, wet, overcrowded conditions — and it can clear a whole tray in a day. The good news: it's almost entirely preventable with a bit of cleanliness and care.

How to recognise it

Damping off attacks the stem at soil level. The seedling doesn't wilt slowly from thirst — it keels over because the stem has rotted through. Look for:

• A pinched, water-soaked, dark patch right where the stem meets the compost.
• Seedlings toppling over while the leaves still look green and healthy.
• A fuzzy white or grey mould on the compost surface or around the collapsed stems.
• Whole patches going down together, often spreading outward across the tray.

It can also strike before seedlings even emerge (pre-emergence), which looks like poor germination — your seeds simply never come up.

Why it happens

Damping off is caused by several fungi (often Pythium or Rhizoctonia) that live in soil, dirty trays and old compost. They're always around — they only become a problem when conditions favour them. The usual culprits:

• Cold, wet compost. Slow-growing seedlings in chilly, soggy compost are sitting ducks. Warmth and steady growth are your best defence.
• Overwatering and poor drainage. Waterlogged compost starves roots of air and gives the fungus exactly the damp it needs.
• Overcrowding. Seedlings packed shoulder to shoulder trap moisture and stagnant air around their stems.
• Dirty trays and reused compost. Last year's pots and old, used compost carry the spores straight to your new sowings.
• Stagnant, still air. No airflow means the compost surface stays permanently damp.

How to prevent it

Prevention is the whole game here. Get these basics right and damping off rarely appears.

Start clean. Wash old seed trays and pots in hot, soapy water before reusing them. Always sow into fresh, peat-free seed compost — never the leftover bag from last year or garden soil. If you make your own, a well-rotted, home-made compost is fine for potting on, but use bought seed compost for the actual sowing.

Sow thinly. Resist the urge to empty the packet. Space seeds out so each seedling has room and air around it. Crowded trays are where damping off thrives — and thin sowing saves you fiddly thinning later.

Water from below. Stand trays in a shallow tray of water and let the compost draw moisture up, then lift them out to drain. This keeps the stems and surface drier than watering from above. Use clean tap water rather than water-butt water for seedlings, as butt water can harbour the fungus.

Give it air. Don't seal seedlings under a propagator lid for longer than they need it. Once germinated, crack the lid or remove it, and keep the air gently moving — a windowsill with the odd open window, or a small fan on a low setting, makes a real difference.

Keep them warm and growing. Most seedlings want 15–21°C to grow steadily. A warm, bright spot indoors keeps them moving and tough. Avoid sowing too early in a cold spring — there's no prize for the first tray, and chilly February seedlings are far more vulnerable.

What to do once it starts

You can't cure a collapsed seedling — but you can stop the rot spreading.

1. Remove the affected seedlings straight away, plus a little of the compost around them. Don't compost the casualties; bin them.
2. Stop overhead watering. Let the compost surface dry out a touch before you water again, and switch to watering from below.
3. Improve the airflow. Take off any cover, space the tray out, and get gentle air moving across the surface.
4. Move them somewhere warmer and brighter if they've been sitting cold and damp.
5. If a tray is badly hit, it's often quickest to start again with clean trays and fresh compost rather than nurse the survivors.

There's no domestic chemical fix worth bothering with — clean conditions are the only reliable answer. Get the cleanliness, spacing, airflow and warmth right and you'll rarely see it again.

Sorting out damping off is one of the small skills that makes seed-sowing reliable. For the full beginner picture, head back to our guide to starting a vegetable garden, and browse the problem-solving hub for the next thing that goes wrong.

PS — if your seedlings never came up at all, it might not be damping off; see why seeds don't germinate.