Even a careful grower gets warning signs from a tray of seedlings, and the skill that separates the confident from the anxious is reading those signs correctly and acting on the right cause. Seedlings cannot talk, but they are surprisingly legible once you know the vocabulary. This guide is a field manual to the common problems - what each symptom actually means, the most likely cause first, and the fix. Diagnose before you act: the wrong remedy (usually “more water” or “more fertiliser”) often makes things worse.
Leggy: tall, thin, stretched, leaning
By far the most common complaint, and the one with the clearest cause. Leggy seedlings are too tall for their age, with a long stretch of bare stem between the seed leaves and the first true leaves, stems thin enough to flop, often leaning hard in one direction. The plant looks like it is reaching - because it is.
The cause is almost always insufficient light, in intensity, closeness or duration. A seedling starved of light pours its resources into stem length trying to reach the sun. A distant windowsill, a light hung too high, or too few hours all produce it. Occasionally excess heat contributes (a seedling left on the heat mat after germination stretches in the warmth).
The fix: get the light closer and stronger, now. Lower the fixture to two to four inches above the seedlings, run it 14-16 hours a day, and get seedlings off any heat mat. You cannot reverse the stretch already there, but you can stop it worsening. For crops that can be planted deeper - notably tomatoes - a leggy seedling can be rescued at transplant by burying the long stem, which roots along its length; for most other crops, a leggy seedling is simply weaker and you prevent it next time. If you take one lesson from this guide: leggy means light, not water. Reaching for the watering can here is the classic wrong move.
Yellow leaves
Yellowing has a few possible causes, so read the details. If the seed leaves (cotyledons) yellow and drop while the true leaves look fine, that is often normal - the seed leaves are temporary, and their job ends once true leaves take over. No action needed.
If the true leaves yellow, the likely causes in order are: overwatering (the most common - waterlogged mix suffocates roots, which then cannot take up nutrients, so the plant yellows despite plenty of water; check whether the mix is constantly soggy and, if so, let it dry and improve airflow); nutrient depletion (once the seed’s packed lunch runs out, usually around the first or second set of true leaves, a seedling in nutrient-poor starting mix will pale and yellow from hunger - the cue to begin a dilute liquid feed or pot up into a richer mix); or, less often, cold stress or root damage.
The fix depends on the diagnosis, which is why “just fertilise it” is a coin flip. If the mix is soggy, the problem is too much water, and adding fertiliser to a drowning plant does nothing. If the mix is fine and the seedling has several true leaves and has simply outgrown its starter mix, a weak feed (quarter to half strength) is exactly right. Check the mix first, then decide.
Purple leaves or stems
Seedlings sometimes flush purple or reddish, especially on the undersides of leaves and on stems. This usually signals cold and, relatedly, phosphorus that the plant cannot take up because the roots are too cold to absorb it. It is common on tomatoes and brassicas started in a chilly room or set too close to a cold window at night.
The fix: warm them up. Move seedlings off cold surfaces and away from icy glass overnight, raise the ambient temperature a little, and the colour usually corrects itself as the roots warm and resume feeding. It is rarely serious and rarely needs fertiliser - it is a temperature message more than a nutrient one.
White fuzz, or seedlings collapsing at the base
This is damping-off, covered in the last guide, and it is the emergency of the list. Fuzzy white or grey mould on the mix surface, or seedlings that topple with a pinched, rotted, thread-thin stem right at soil level, mean the fungus has arrived. There is no cure for affected seedlings.
The fix is damage control and prevention of spread: remove collapsed seedlings immediately, stop overwatering, maximise airflow with a fan and by uncovering everything, let the surface dry between waterings, and thin any crowding. You may save the unaffected seedlings; you will not save the fallen. Then fix the root causes for next time - clean mix, bottom watering, no dome after germination, airflow, no overwatering. Damping-off is a verdict on the previous week’s conditions; treat its appearance as feedback, not just misfortune.
Fungus gnats: little black flies
Small dark flies drifting up from the trays when disturbed are fungus gnats. The adults are a nuisance rather than a killer, but their larvae live in the top layer of constantly moist mix and can nibble tender roots in heavy infestations. Their presence is also a signal in itself: they breed in soggy, organic-rich, always-wet surfaces, so a gnat problem usually means the mix is being kept too wet.
The fix: let the surface dry between waterings (bottom watering helps enormously, since it keeps the top dry), which breaks the larvae’s habitat. Yellow sticky traps catch the adults and monitor numbers. A layer of dry material or fine grit on the surface discourages egg-laying. Mostly, though, drier surface management makes them fade - another reason bottom watering is the default.
Patchy or no germination
Sometimes the problem is that not much came up. Diagnose by cause. Nothing at all, or very little: likely old or dead seed (run the rag-doll viability test on the batch), seed sown too deep (especially small seed that exhausted itself in the dark), mix that dried out mid-germination (fatal, and easy to do), or, for warm-crops, a room too cold to trigger them. Patchy - some up, some not: often uneven moisture or temperature across the tray, or naturally variable seed. Slow but coming: many crops are just slow (peppers and parsley are famously unhurried), so check the expected days-to-germinate on the seed chart before concluding failure - impatience diagnoses a lot of “failures” that were simply going to take another week.
The fix is usually next time, not this time: fresher seed, shallower sowing for small seed, never letting the mix dry during germination, and heat for warm-crops. If a tray is a genuine bust, re-sow promptly rather than waiting - a week’s delay compounds down the line.
Seedlings that simply stall
Occasionally seedlings germinate, make their first true leaves, and then just sit - not dying, not growing, stuck for weeks. The usual culprits are cold (growth slows to a crawl in a chilly room - warm them up), hunger (they have used the packed lunch and are in nutrient-poor mix with nothing to grow on - start a dilute feed), or root-bound stall (they have filled a small cell and have nowhere to go - time to pot up, the subject of the next guide). Check temperature, feed, and root space in that order.
The meta-lesson: diagnose, then act
Notice the pattern across every entry: the correct fix is rarely the reflexive one. The reflexes are “more water” and “more fertiliser,” and both are wrong at least as often as they are right - a leggy seedling wants light not water, a yellow one is often over-watered not under-fed, a purple one is cold not hungry. The whole skill is pausing to read the plant and the mix before you reach for a remedy. Lift the tray to feel its weight, look at whether the mix is soggy or dry, count the true leaves to judge whether hunger is plausible, notice the temperature and the light. Then act on the actual cause. Do that, and your seedlings will tell you what they need clearly enough - and most of them will make it to the day you start moving them into bigger quarters, which is where the masterclass turns next.