Damping Off in Seedlings, and How to Stop It

Damping off is a fungal disease of seeds and young seedlings, caused by soil organisms including Pythium, Rhizoctonia and Fusarium. The classic sign is a seedling that was fine yesterday lying flat today with a thin, dark, water-soaked pinch at soil level. There is no cure once a seedling has collapsed -- the stem is destroyed -- so the whole of the answer is prevention: clean containers, a sterile seed starting mix, warmth to speed germination, good air movement, bottom watering, and taking the humidity dome off the moment seedlings emerge.

That last row is the useful early warning. Fuzzy white growth on the surface of a tray is not the pathogen, but it tells you the tray is too wet, too still and too warm -- which is exactly the combination damping off needs. Improve the airflow and ease off the watering before anything collapses.

• Persistently wet medium. Not damp -- wet. The pathogens are water molds; they move through films of water in the mix.
• Still, saturated air. A humidity dome left on after germination is the single most common cause.
• Cool soil and slow germination. The longer a seed sits in the mix before emerging, the longer it is vulnerable. Warmth shortens that window.
• Contaminated containers or medium. Garden soil, reused unwashed trays, last year's mix left open in the shed.
• Overcrowded sowing. Dense seedlings hold moisture between them and block airflow at the base.
• Too little light. Weak, slow-growing seedlings stay susceptible for longer than vigorous ones.

1. 1

   Start with a clean, sterile seed starting mix

   A bagged seed mix is processed and clean. Garden compost and garden soil both carry the pathogens, and a tray of seedlings is the most vulnerable thing in the garden to them.

2. 2

   Wash reused trays

   Hot soapy water and a scrub, then a rinse, before each season. If a tray had damping off last year, wash it properly or do not reuse it.

3. 3

   Sow thinly and at the right depth

   Crowded seedlings trap moisture at the base. Sowing too deep extends the time underground, which is exactly the vulnerable window.

4. 4

   Use bottom heat to get them up fast

   A heat mat does not prevent the fungus directly; it shortens germination time, which is one of the few variables you fully control. Take the tray off the mat once seedlings are up.

5. 5

   Take the dome off the moment seedlings emerge

   The dome is for germination only. Left on, it holds the still, saturated air the fungus needs, and a tray can collapse overnight.

6. 6

   Water from below, and let the surface dry

   Bottom watering keeps the medium evenly damp and the surface drier, which is the opposite of what the pathogen wants. Empty any standing water from the tray afterwards.

7. 7

   Run a small fan on low

   Gentle constant air movement dries the surface, strengthens stems and is probably the highest-value single change most people can make.

You cannot save a collapsed seedling. What you can do is stop it spreading through the rest of the tray, and that is worth acting on the same day you see it.

1. Remove the affected seedlings and the mix immediately around them, and throw them out rather than composting them.
2. Take the dome off permanently and move the tray somewhere with better air movement.
3. Stop watering until the surface is genuinely dry. Seedlings tolerate a dry surface far better than they tolerate the fungus.
4. Separate the tray from your other trays. Splash and shared water carry it.
5. If more than a third of the tray has gone, re-sow. Survivors from a badly affected tray are usually checked enough that a fresh sowing catches them up.
6. Sterilize the tray before reusing it, and start the new sowing in fresh mix.

Cinnamon and chamomile tea are widely recommended as home remedies. Evidence for them is thin, and neither addresses the water-and-air conditions that caused the problem. Fix the conditions first; anything else is at best a small extra.

In practice, a tray that never gets damping off has a fairly specific setup: clean heavy-gauge trays, a bagged seed starting mix, bottom heat until emergence, a dome removed the day seedlings appear, a light close over the canopy, a fan on low, and water from below rather than above. That is not a long list, and nothing on it is expensive.

The one thing that is easy to get wrong afterwards is feeding. Seedlings exhaust the seed's reserves once the first true leaves appear, and a stalled, hungry seedling stays vulnerable for longer. The best seed starting mix covers the handover from starting mix to a richer medium.