Damping Off

You had a seedling. It came up, it opened its first leaves, it looked fine yesterday. This morning it is lying flat with its stem collapsed where it meets the soil.

That is damping off, and it is the most common way a first grow ends before it starts. It is also almost entirely preventable, which is worth knowing because once it happens the seedling is gone.

What you are seeing#

The defining sign is at the soil line, not the leaves.

Look at the point where the stem enters the medium. In damping off it goes thin, dark, water soaked and pinched, as if squeezed. The stem above that point may still look green and healthy, which is what makes it so confusing: the plant looks fine and is lying down.

The collapse is fast. A seedling can be upright in the evening and flat by morning. Once the stem has pinched, the plant does not recover, because the tissue that carried water has been destroyed.

Sometimes you see it before the fall: a seedling that seems to stall, with a stem that looks slightly dark or glassy at the base.

In trays, it spreads. One seedling goes, then its neighbors over the following days.

How to tell it apart#

From overwatering: an overwatered seedling droops with its leaves curled down, but the stem stays firm and normal colored at the base, and it slowly recovers as the medium dries. Damping off pinches the stem and does not recover.

From underwatering: a dry seedling wilts limply all over and perks up within hours of water. If watering does nothing, look at the stem base.

From cold shock: cold slows a seedling and may purple the stem, but it does not create a water soaked pinch at the soil line.

From stretching: a seedling that is simply too tall and falls over has a thin stem along its whole length, not a dark constriction at one point. That is a light problem and the seedling can be saved by burying the stem.

Why it happens#

Damping off is caused by several common soil-borne fungi, mainly Pythium and Fusarium, which are present in most growing media most of the time and are harmless until conditions let them take over.

The conditions are always some combination of these:

• Medium kept too wet, particularly a saturated surface
• Poor air movement, especially under a humidity dome left closed too long
• Cool temperatures, which slow the seedling without slowing the fungus
• Too deep a pot for a tiny plant, so the medium stays wet for days
• Reused or unsterile medium, which carries a larger spore load

Nearly every case traces back to a surface that stayed wet and still.

How to prevent it#

1. Water in a small ring, not over the whole surface. A seedling's roots occupy a tiny volume. Wetting a ring an inch or two from the stem encourages roots outward and leaves the stem base dry, which is the single most effective thing on this list.
2. Let the surface dry between waterings. The top half inch should go dry to the touch. The medium below stays moist and the fungus loses its best habitat.
3. Get air moving. A gentle fan, not pointed at the seedling, across the surface. Air movement at the soil line matters more than humidity in the room.
4. Open the dome progressively. A humidity dome is for the first few days. Leaving it sealed for two weeks is how this happens.
5. Keep the medium warm. Around 70 to 75F at the medium. A heat mat in a cool room does more for seedling survival than anything else you can buy.
6. Start in a small container, or plant into the final pot and only water the small ring. A seedling in a full three gallon pot watered thoroughly is sitting in days of wet medium.
7. Use fresh medium. Not reused soil, not a bag that has been open in a damp garage since last season.

If it is spreading in a tray#

Remove the collapsed seedlings and the medium immediately around them. Back off watering sharply across the whole tray. Open the dome fully, add airflow, and raise the temperature if you can.

Some growers use a dilute hydrogen peroxide drench or a cinnamon dusting on the surface at this point. Both are plausible and neither reliably rescues a tray that is already going. The reliable move is to change the conditions.

And start replacement seeds now rather than waiting to see. A week lost at the start is a week, and a tray that is damping off usually keeps going.