Why are my seedlings collapsing at soil level?

Damping off spreads through a tray in days, so speed matters. It is almost entirely preventable through hygiene, and almost entirely untreatable once it has taken hold.

What you'll need

• Fresh seed compost
• Clean modules or pots
• Vermiculite (optional)

Step by step

1. Identify it properly.The stem at soil level goes thin, dark and water-soaked, and the seedling topples with the leaves still green. That is damping off. Seedlings that dry, yellow and shrivel from the top are a different problem.
2. Remove affected seedlings at once.Take them out with the compost around them and bin them. Do not leave them in the tray hoping they recover; they will not, and they will infect neighbours.
3. Improve ventilation immediately.Take the propagator lid off, open a window or add a fan. Still, humid air over the compost surface is the condition the fungus needs.
4. Water from below and let the surface dry.Stand trays in shallow water until the surface darkens, then remove them. A constantly wet surface is where the infection lives.
5. Use mains water, not a butt.Water butts carry the fungi that cause damping off. For seedlings specifically, use tap water; rainwater is fine for established plants.
6. Sow thinly.Crowded seedlings hold moisture around each other's stems and stop air moving. Thin sowing prevents more damping off than anything else.
7. Cover the surface with vermiculite.A fine layer over the compost keeps the stem base drier while still letting water through. It measurably reduces losses.
8. Use fresh compost and clean containers.Reused compost and unwashed trays carry the spores. Wash pots and modules in hot soapy water, and buy fresh seed compost each year.
9. Do not overheat.Excessive bottom heat combined with high humidity is the classic combination. Reduce heat once germination has happened.
10. Resow rather than persist.Once a tray is affected, the remaining seedlings usually go too. Resowing into fresh compost costs a fortnight and saves the effort of nursing a doomed tray.

Tips

• Green leaves with a collapsed stem is the signature. Yellow shrivelled tops are something else.
• Tap water for seedlings, rainwater for everything else.
• A thin layer of vermiculite over the compost surface is a cheap and effective preventative.

Common mistakes

• Keeping the propagator lid on after germination — The lid holds saturated air over the compost surface, which is exactly what the fungus needs. Ventilate as soon as seedlings emerge.
• Reusing last year's compost for seed sowing — It carries spores and it is nutritionally exhausted. Fresh seed compost each spring is one of the cheapest insurance policies in gardening.

If it doesn't work

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