Damping Off: Prevent Seedling Collapse

Damping off is the name for several problems caused by soil-borne fungi and water molds. Some Pythium species produce swimming spores that move easily through saturated growing media. Other pathogens can cause a similar collapse under different conditions.

Pre-emergence damping off happens before a seedling breaks the surface. The seed decays before it sprouts, or the first young root dies as it cracks the seed coat. If an evenly sown tray has isolated bare patches, probe into the mix with a toothpick. A soft, mushy seed points to pre-emergence rot rather than a simple failure to germinate.

Post-emergence damping off happens after the seedling emerges. The usual sign is a pinched, water-soaked or brown stem at the soil line. Growers often call the thin, constricted form wirestem. A leggy seedling can lean, but its stem stays firm.

Saturated mix and stagnant air favor damping-off pathogens. Clear plastic humidity domes help hold moisture during germination, but take the dome off when the first green shoot appears. Leaving it in place traps stale, humid air against tender stems.

This is also a good time to look across the whole flat. A few slow cells may still be normal for the crop. A patch of seedlings failing together in wet mix needs attention sooner.

Instead of testing individual cells with your finger, use the whole-flat heft test. Pick up the seed tray with both hands. A saturated flat feels dense and heavy. A tray that needs water feels noticeably lighter. Water before seedlings wilt or the mix pulls away from the sides of the cells.

Bottom watering can hydrate seedlings without disturbing fragile stems, but trays should never sit continuously in water. Let the mix wick up moisture, then remove the tray and allow excess water to drain.

Reusing cell packs and flats is standard practice, but used containers can carry pathogens from one season into fresh mix. Wash away soil and old roots with warm soapy water before sanitizing. Bleach is quickly neutralized by organic matter.

Submerge washed containers in a 10% household bleach solution, using one part bleach to nine parts water, for 30 minutes. Rinse and air-dry each container before filling it with fresh soilless seed-starting mix. Garden soil and unfinished home compost are too dense and can harbor damping-off organisms.

Seedlings are most vulnerable while their stems are soft and tender. The faster they germinate and put on true leaves, the less opportunity pathogens have to infect them.

Use a heat mat to keep the mix at the germination temperature listed for your crop. Once seeds sprout, switch the mat off or adjust it if the crop prefers cooler growing conditions.

Put the flat under strong grow lights right away so stems stay compact. A small fan on a low setting helps surface moisture evaporate and keeps air from becoming stale.

Cinnamon and diluted hydrogen peroxide are widely shared home remedies, but they cannot rescue a collapsed seedling. Post-emergence damping off destroys stem tissue at the soil line. Once the stem is girdled, it can no longer move water from the roots to the leaves. No topical powder or rinse can reconnect it.

Discard a collapsed seedling with the surrounding plug of mix and inspect the remaining cells. If the tray has stayed wet or the air around it is still, correct that condition before watering again.