Damping Off: Bin the Mix and Start Clean

Damping off is a seedling collapse caused by several soil fungi and water molds — Pythium, Rhizoctonia, Fusarium — that pinch stems at the soil line or rot seed before it emerges. It is not the adult one-sided wilt of Fusarium wilt and not the leathery fruit bottom of blossom-end rot. Collapsed seedlings cannot be saved.

Do this first: bin the fallen plants and that mix, wash the tray, and sow again in a fresh bag on warmth with air moving.

Pinched at the soil line, or a wilted adult

What you see	Likely	Do this first
Seedlings topple; stem thread-thin at the soil line; top still green	Damping off	Bin them; start the tray over
Seed never came up; little rotted sprouts under the mix	Pre-emergence damping off	Fresh mix; more warmth
Fruiting tomato yellow on one side; brown inside a cut stem	Fusarium wilt	See Fusarium wilt
Leathery scar on the bottom of fruit	Blossom-end rot	See blossom-end rot
Whole tray evenly burned or crusted white	Salts or drought	Check watering and feed strength

Gray mold on spent blooms is a different week. If you need a pinched soil line on a seedling, stay here.

Is this Fusarium wilt on my tomato?

Not if it is a seedling that pinched at the soil line. Adult Fusarium yellows one side of a fruiting plant and browns a cut stem. See Fusarium wilt for that job. This tray is damping off.

Is this just dry wilt?

Dry cells go limp and the stem stays round. Damping off pinches the stem at the soil line until it cannot stand. Watering a pinched stem will not stand it back up.

They topple while the top is still green

After emergence the stem constricts at the soil line until it cannot hold the seedling. The top can still look green. Stems may go mushy or gray-brown. Roots can be missing or stubby. Under a humidity dome you may see a thin cobweb growth. Whole patches of a tray go at once. Before emergence it looks like bad seed — lift a pinch of mix and you may find rotted sprouts that never saw light. Check the tray in the morning after a cool wet night — a patch that was upright at dusk can be down by breakfast. Do not assume the rest of the flat is safe because two cells still look fine.

What is damping off?

Several soil fungi and water molds — Pythium, Rhizoctonia, Fusarium — that rot seeds before they emerge or pinch seedling stems at the soil line until they topple.

Why did the whole tray die?

These organisms move through wet mix and shared water. A patch or a whole flat can go in a day. That is the tell versus one dry cell you forgot.

They cannot be saved — start the tray over

There is no spray that stands a pinched stem back up. Bin the collapsed seedlings and the mix they sat in. Do not transplant neighbors from the same wet block. Wait is for cells that are still upright with a round stem. Fail if you dust cinnamon and hope. Skip the brand aisle. Replant — most springs still have time. A second sowing the same week is honest; waiting on a pinched stem is not.

Can I save seedlings with damping off?

No. Once the stem is thread-thin at the soil line, they cannot be saved. Bin them. Wash the tray. Sow again in fresh mix. Fail if you wait for cinnamon to reverse it.

Is there a spray for damping off?

Not a home bottle that finishes a collapsed stem. Culture is the job: clean, warm, airy, not soggy. Skip the brand aisle. Fail if a spray replaces a wash.

Fresh mix, never garden soil

Garden soil and last year’s open bag carry these organisms into a warm wet tray. Use a fresh seed-starting mix that drains. Do not reuse mix from a lost tray. Compost in a cell pack is the same mistake. Dump infected mix in the bin, not back on the vegetable bed as if it were finished.

Should I use garden soil to start seeds?

No. Garden soil carries these organisms into a warm wet tray. Use a fresh bag of seed-starting mix. Do not reuse last year’s damp leftover.

Can I replant the same cells?

Not in the same mix. Dump it. Wash or replace the tray. Fresh mix, then sow. You still have time for a second start most springs.

Wash and soak the trays

Scrub old mix off. Soak about thirty minutes in a ten percent bleach solution, then rinse well. Dirty hose ends and tools seed the next sowing. If a plastic pack already lost a crop, recycle it. Fail if last year’s unwashed tray is the “clean” start.

How do I wash the trays?

Scrub off old mix. Soak about thirty minutes in a ten percent bleach solution, then rinse. If a tray already lost a crop, recycle it rather than gamble.

Warm the mix to seventy to seventy-five

Cool wet mix slows the seedling and speeds Pythium. A heat mat at about seventy to seventy-five degrees helps indoor trays. Water at room temperature, not from a fifty-degree tap. Outdoors, wait until that crop’s soil is warm enough — cucumbers want more heat than peas. Deep sowing that delays emergence is extra time in the danger window.

How warm should the mix be?

About seventy to seventy-five degrees for indoor trays. A heat mat helps. Ice-cold water slows growth and invites rot. Use room-temperature water.

Fan and light beat cinnamon

After sprouts show, take the dome off. A small fan on low dries the soil line. Give twelve to sixteen hours of grow-light. A windowsill is not enough in March. Space cells so leaves do not make a wet tent. Cinnamon on the surface is a leftover myth, not a method. Fail if the dome stays on a wet crowded tray.

Do I need a fan?

Moving air dries the soil line. A small fan on low after germination beats a humidity dome left on too long. Light from a window is not enough — give twelve to sixteen hours.

Cool wet garden soil is the same job

Seed sown too early into cold mud can damp off in the row. Wait. Crowded outdoor seed lines stay wet longer. This is not a second disease and not adult Fusarium on a fruiting tomato. Same organisms, same rule: warm enough, not soggy, then they outgrow the window.

Can damping off happen in the garden?

Yes. Seed sown into soil that is too cool and wet can rot before or just after it emerges. Wait until that crop’s soil is warm enough. Peas tolerate cooler ground than cucumbers.

Bin the tray — then start clean and warm

Confirm the pinch at the soil line. A fruiting wilt is Fusarium. A leathery fruit bottom is blossom-end rot.
Bin collapsed seedlings and that mix. They cannot be saved.
Wash trays about thirty minutes in ten percent bleach, or replace them.
Sow in fresh mix on a heat mat about seventy to seventy-five degrees. Room-temperature water from below.
Fan and twelve to sixteen hours of light after sprouts. Take the dome off.
No cinnamon-as-cure. Skip the brand aisle. Fail if last year’s mix goes back in the cells.

How do I prevent damping off?

Fresh sterile mix, washed trays, warmth about seventy to seventy-five degrees, twelve to sixteen hours of light, air moving, water from below. Never garden soil in a tray.