What to do now
Mildest step first.
- Take the humidity dome off once most seeds have sprouted.
- Let the top of the mix dry before watering again, and water from the bottom.
- Run a small fan on low near the trays for a few hours a day.
- If tiny dark flies hover over the trays, add yellow sticky cards and keep the top inch drier.
Is it harmful?
The white, cottony or fuzzy growth that appears on top of seed-starting mix is usually a saprophytic mold, a fungus that lives on the in the mix. University of Maine Extension calls it a common occurrence when starting seeds and typically a sign of overly wet growing medium; in itself it isn't necessarily a problem. Iowa State says surface molds are unsightly but won't harm healthy seedlings. What matters is what the mold is telling you: conditions that wet also favor damping-off, the disease that kills young seedlings.
How it differs from damping-off
Damping-off looks different. The seedling's stem turns dark and right at the soil line, and the seedling shrivels and falls over, often with no visible growth on the surface. Seedlings that have damped off won't recover; pull them and the mix around them. White fuzz on the mix while the seedlings stand upright and green is the harmless kind.
What to do now
The fix is drier, airier conditions. Remove clear covers or domes once most seeds have germinated. Water thoroughly, then wait until the mix is nearly dry on top before watering again, rather than sprinkling a little every day. Bottom-watering helps: set trays in about half an inch of water for 20 minutes, then let them drain. A small fan blowing gently over the trays dries the surface and also makes stems sturdier. You can scrape off heavy patches of mold, but they'll come back if the mix stays wet. Maryland's Ask Extension experts note that home remedies such as cinnamon haven't been shown to do much against soil fungi, and there are no effective fungicides labeled for this use indoors. Start next year's seeds in a sterile, soilless mix and clean trays.
The tiny flies: fungus gnats
Fungus gnats often show up in the same wet trays. The adults are delicate, dark flies about 1/8 inch long that run across the mix and flutter up when disturbed. They're a nuisance, but their larvae, tiny clear worms with shiny black heads, live in the top layer of wet mix and feed on organic matter and fungi. UC notes that in large numbers they can damage roots and stunt growth, particularly in seedlings and young plants. To check for larvae, push a few slices of raw potato into the mix and look underneath after 3 or 4 days.
Getting rid of fungus gnats
Drying the surface is the main control, since the larvae need moist mix. University of Wisconsin–Madison Extension suggests letting the top inch dry out before you water. Yellow sticky cards set right above the mix catch adults and show how many there are. If larvae persist, a drench of Bti (Bacillus thuringiensis israelensis), sold as Gnatrol or as Mosquito Bits soaked in water, kills them without harming plants; Wisconsin suggests several applications 5 to 7 days apart. The life cycle takes about 3 to 4 weeks at room temperature, so expect a few weeks of drying and trapping before the gnats are gone.
Read the whole label first: it must list your crop, and its directions (amount, timing, gear, bees, days before harvest) are the law. If this page and the label differ, follow the label.
- TomatoThe warm-season anchor of the summer garden.
- PepperA tropical perennial grown as an annual — patient, slow, and particular about warmth.
- BasilThe summer companion — to tomatoes, to pasta, and to the gardener with a south-facing window.
- LettuceA cool-season leaf crop that thrives in spring and fall, sulks in summer heat.
- KaleThe cold-weather workhorse that improves when everything else quits.
- Damping OffSeedlings collapse at soil level with a pinched, rotted stem — happens quickly in wet, cool conditions.
- Gray Mold (Botrytis)Gray-brown fuzzy mold on fruit, flowers, or stems — soft, collapsing tissue beneath the coating in cool, wet conditions.
- Downy MildewAngular yellow patches on leaf tops with gray-purple fuzzy growth beneath; worse in cool, humid conditions.
- Late BlightLarge water-soaked leaf lesions with white fuzzy growth beneath; plants can collapse within days.
- Why are my seedlings falling over?Seedlings collapsing at the soil line is usually damping off, a disease caused by soilborne fungi and fungus-like water molds that attack stems in wet, poorly ventilated conditions.
- Why are my seedlings tall, pale, and floppy?Seedlings stretch toward inadequate light — the usual fix is moving the light closer, to about 2–6 inches above the leaves, and running it 14–16 hours a day.
- Why aren't my seeds germinating?Cold soil is the most common culprit — most vegetable seeds stall below 60°F, and old or improperly stored seed may not be viable at all.