Why Did My Lettuce Seedlings Collapse at the Base?

You raise healthy-looking lettuce seedlings, and then they suddenly keel over, pinched and thin at the soil line, and die — often several at once, spreading across the tray. This is damping off, a fungal disease that attacks seedlings at the base. There is no cure once a seedling is struck, so this is firmly a problem you prevent. The good news is that prevention is simple and reliable. Let me explain.

What damping off looks like

The classic sign is a seedling that flops over because the stem has gone thin, brown, water-soaked and pinched right at the soil line, as if nipped. The top may still look green briefly, but the base has rotted through and the seedling cannot stand. It often spreads outward in a patch, taking neighbouring seedlings. Sometimes it strikes before seedlings even emerge, rotting the seed or sprout underground so they never appear. On lettuce, which is sown thickly and shallow, it can sweep through a tray fast.

What causes it

Damping off is caused by several soil-borne fungi and fungus-like organisms that flourish in cool, wet, stagnant conditions — exactly the damp, crowded environment seedlings are often raised in. The classic recipe is overwatered, soggy seed-starting mix, poor air circulation, sowing too thickly, and using unsterile soil or dirty reused containers. Garden soil brought indoors is a common source of the spores. The wetter, more crowded and more airless the setup, the more it invites the disease.

How to prevent it

Since you cannot cure it, prevention is everything. Always start seed in fresh, sterile seed-starting mix, never garden soil, and use clean, sanitised containers. Do not overwater — keep the mix moist but never soggy, and let the surface dry slightly between waterings; watering from the bottom keeps the surface drier. Provide good air circulation: a small fan gently moving air over the seedlings dramatically reduces damping off and toughens the stems. Sow thinly so seedlings are not crowded, and give them good light and warmth (within lettuce's cool preference) so they grow sturdily rather than soft.

If it starts

Act fast to protect survivors: remove the collapsed seedlings and the soil immediately around them, stop overwatering, increase airflow with a fan, and move the tray somewhere brighter and better ventilated. A dilute chamomile tea watering can slow the fungus for some growers, but the seedlings already pinched at the base are lost. Because lettuce is so quick and easy to resow, it is often simplest to start a fresh batch with a clean, airy, well-drained setup. Get the prevention right — sterile mix, clean containers, careful watering, airflow and thin sowing — and damping off rarely troubles your lettuce again.