Flowers And Fungus In My Closet
It was early September, and I was starting seeds for overwintering — delphiniums, foxgloves, snapdragons.
The delphiniums needed darkness to germinate, so I put those trays in a closet. No light, no air circulation, warm and humid.
I didn't check on them for two days.
When I came back, the trays were covered in white fuzz. Not just on top. Everywhere. On the paper pots, on the soil, running along the sides where the pots touched. Thick. Fluffy. It looked like it could get up and walk away.
The trays I'd left out on the shelf, under the grow lights — the foxgloves and snapdragons — had a little fuzz too. A polite dusting. Barely worth noticing.
Same seeds. Same paper pots. Same soil. The only difference was light and air.
I was sure I'd ruined them. I stood there trying to decide whether to throw the trays out.
What's actually growing
That white fuzz is usually a saprophytic fungus — a fungus that feeds on dead organic matter. In this case, it may be feeding on the paper, peat, coir, or other organic material in and around the pot. It's not feeding on your seedlings.
It's the same kind of organism that breaks down fallen leaves on a forest floor. It's a decomposer. It's just doing its job somewhere you didn't ask it to.
Sometimes that white fuzz turns green as the fungus matures and produces spores. The green color comes from the spores themselves, or from pigments associated with them. It looks worse. It usually isn't.
When it shows up, and when it stops
Mold is worst during germination.
That's when the dome is on, the tray is warm, and everything stays damp. It's the environment seeds need. It's also the environment fungus needs. The dome traps moisture and still air.
Once the seedlings germinate and the dome comes off, the mold usually recedes. Sometimes it stops entirely. Sometimes it doesn't go green. The change isn't mysterious. The conditions changed. Air started moving. The surface began to dry.
The thing you actually need to worry about
Mold on the pot is not the same thing as damping off. They look different, and they mean different things.
Damping off is a syndrome caused by several soil-borne pathogens, including Pythium, Rhizoctonia, Fusarium, and Phytophthora. These organisms can be present in soil and growing media without causing problems for mature plants. They become a problem when young seedlings are exposed to the right combination of moisture and other environmental conditions.
Damping off can happen before or after emergence.
Pre-emergence. The seed rots before it breaks the surface. You see a gap in the tray where a seedling should be. It looks like poor germination, but the seed may have been killed by infection.
Post-emergence. The seedling emerges, looks fine for a few days, then collapses. The stem at the soil line is pinched, dark, and soft. The seedling falls over. It doesn't recover.
If your seedlings are upright and green, the surface mold itself is usually not a reason to panic. But the damp, stagnant conditions that encourage it can also favor damping off.
Preventing damping off
Once damping off takes hold, affected seedlings usually can't be saved. Prevention is your best defense.
Water from below. Set pots in a shallow tray of water, let them wick up what they need, then remove them. Top-watering keeps the surface and stems wet, which can create conditions favorable to infection.
Let the surface dry between waterings.
Keep the air moving. A small fan on low nearby. Remove the dome once seedlings emerge.
Sow at the right density. Crowded seedlings stay wet and compete for light.
Use clean mix and clean trays. Use fresh or properly prepared seed-starting mix, and disinfect reused trays before using them again.
Give seedlings enough light and keep them at an appropriate temperature for the crop. Cool, low-light conditions slow growth and can favor disease.
Disease needs three things at once: a susceptible host, a pathogen, and the right environment. You can't remove the seedlings. You can't sterilize the world. But you can change the environment.
What to do if damping off starts
Stop watering. Let the surface dry.
Remove affected seedlings and pots to prevent spread.
Increase airflow and light.
If it spreads, consider removing the whole tray.
The reframe
A pot that grows mold is a pot that will break down in the soil. The mold is a preview of the decomposition that's the whole point.
I've grown in plastic, peat, coir, and paper. The organic ones grow fuzz on the pot itself. The plastic ones grow it on the soil surface, sometimes greening the whole tray. Either way, the mold is telling you the same thing: too wet, not enough air.
It stopped bothering me once I understood what it was.
The closet trays went back on the shelf. The fuzz dried up.
The germination rate was good. And it was consistent — the closet trays came up at the same rate as the ones that had stayed under the grow lights.
No damping off.
To think I'd almost thrown them out.
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