Seedlings collapsing at the stem is about air, not water
What it looks like
Seedlings collapsing at soil level with a thin dark constriction on the stem. Sudden, and it spreads across a tray in a day.
The actual cause
Still, humid, warm air. Overwatering contributes but ventilation is the controlling factor and is what people never change.
What to do
Take the propagator lid off during the day, space the seedlings, and water from below in the morning rather than the evening.
