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Seedlings fall over for two very different reasons, and the stem base tells you which. A dark, pinched, water-soaked constriction right at the soil line is damping off — a soil-borne fungal disease, and those seedlings will not recover. A stem that is uniformly pale, thin and elongated, with the seedling flopping but staying green and firm, is stretching from too little light. Damping off calls for immediate sanitation and drier, better-ventilated conditions; stretching calls for brighter light held closer to the canopy.
How do I tell damping off from leggy stretch?
Look at the seedling under good light, ideally with a hand lens. The difference is unambiguous once you know what to compare.
| Sign | Damping off | Leggy from low light |
|---|---|---|
| Stem base | Pinched, brown or water-soaked | Normal color and diameter |
| Stem overall | Normal length | Long, thin, pale |
| Leaves | Still green when it topples | Green but widely spaced |
| Recovery | None | Stands back up, keeps growing |
| Spread | Moves outward in a patch | Affects the whole tray evenly |
| Speed | Overnight | Over days to weeks |
| Surface of medium | Often has white fuzz or algae | Normal |
A patch that expands outward from one point over a day or two is the strongest evidence of damping off, because the pathogens spread through the medium and the film of water on the surface. Even, tray-wide flopping is almost always light.
What causes damping off, and how do I stop it spreading?
Damping off is caused by several soil-borne organisms — most commonly species of Pythium, Rhizoctonia and Fusarium — that thrive in cool, wet, poorly ventilated conditions. They attack at or just below the soil line, and once the stem tissue collapses the seedling has no way to move water. Extension services from land-grant universities are consistent that management is preventive: sterile medium, clean containers, good airflow and moisture control.
- Remove affected seedlings and the medium around them at once. Do not compost it near your seed-starting area.
- Increase air movement. A small oscillating fan on low, running several hours a day, dries the surface and also thickens stems.
- Water from below through the tray, and let the surface dry slightly between waterings.
- Uncover. Humidity domes come off as soon as germination is complete — they are for germination, not for growing.
- Thin crowded cells so air can move between plants.
- Sanitize containers before reuse. A ten percent bleach solution soak, then a full rinse and dry, is the standard method.
- Use fresh sterile seed-starting mix rather than garden soil or reused potting mix.
Bottom heat helps in cool rooms because faster germination shortens the window in which seedlings are vulnerable, but heat combined with a closed dome and saturated medium is the exact condition the pathogens want. Heat and ventilate together.
Why are the stems stretching, and can I fix it?
Stretch is a light problem in almost every case. A seedling that is not receiving enough light elongates its stem searching for more, producing a tall, thin plant with widely spaced leaves. A bright south-facing window in February provides far less usable light than the same window in June, and far less than most gardeners assume, which is why windowsill trays stretch so reliably.
Three fixes, in order of effect. First, get the light closer and longer: 12 to 16 hours a day with the fixture 2 to 4 inches above the canopy, raised as the plants grow. Second, drop the temperature a few degrees after germination — warm nights promote stem elongation, and cooler growing conditions produce stockier plants. Third, add air movement, since the mechanical stress of a gentle breeze thickens stems measurably. Whether a grow light is genuinely necessary for your setup is worked through in do you need grow lights to start seeds, and the full rescue procedure for stems that have already stretched is in how to fix leggy seedlings.
What else makes seedlings collapse?
Less common, but worth ruling out when the two main causes do not fit.
- Fertilizer burn. Full-strength feed on young seedlings scorches roots and the stem base. Feed at quarter strength once true leaves appear, not before.
- Drying out completely. A tray that went bone dry for a few hours can wilt seedlings past recovery, and they do not always stand back up after rewatering.
- Fungus gnat larvae. They feed on roots and the stem base in persistently wet medium. Yellow sticky cards confirm the adults; letting the surface dry breaks the cycle.
- Cold shock. A tray left against a cold window overnight, or moved outside too abruptly, can collapse warm-season seedlings — which is what hardening off exists to prevent, as set out in hardening off seedlings.
- Planted too deep. Seeds sown deeper than the recommended depth exhaust their reserves before reaching light and emerge weak. Depths by crop are in how deep to plant vegetable seeds.
Getting the fundamentals right at sowing prevents most of this — the full sequence is in how to start seeds indoors, watering practice is in how often to water seed starting trays, and equipment choices in best seed starting kits.
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