Leggy seedlings: the cause, and how to save them
Long, pale, floppy stems: seedlings draw up almost always for want of light, sometimes made worse by warmth and by sowing too thickly. What can still be saved, and what cannot.
In this guide
A seedling that draws up stretches into a thin, pale, floppy stem, with wide gaps between the leaves, and ends up lying down. The cause is nearly always the same: not enough light for the warmth it is getting. A stem that has already stretched will not shorten again, but the seedling can still make a good plant if the light and the temperature are put right over the next few days.
Recognising a leggy seedling
Etiolation — stretching for want of light — shows first in the proportions. A healthy seedling holds its seed leaves close to the compost, on a short, firm stem of a clear green. A drawn-up seedling shows a long bare stem below the seed leaves, very pale green and sometimes almost translucent, and the leaves that follow space out instead of sitting close together.
Which way they lean says a great deal. If every seedling leans the same way, they are reaching for the window: the light comes from one direction only and there is not enough of it. If they stretch straight up, the shortage is general, with no favoured direction, which happens under a lamp set too high or in a dark room.
Do not confuse it with damping off. In that disease the stem browns and pinches in at compost level, and the seedling drops all at once after looking normal the day before. A drawn-up seedling sags gradually under its own weight, with no rot at the base.
Why the seedling stretches
A seedling has a reserve in its seed and one aim at the start: to reach the light. As long as it does not get enough, it spends that reserve on height rather than on thickening its stem and making leaves. It is a normal response, useful when a seed has germinated under a dead leaf, but ruinous in a living room in February.
Three factors usually combine.
- Light, first of all. Behind glass the intensity falls away very fast within a few tens of centimetres, and a winter day is short. A grow lamp run for only a few hours does not make up for it.
- Warmth, second. A warm room speeds up growth without adding a scrap of light. The imbalance between the two is exactly what makes seedlings draw up.
- Sowing density. Crowded seedlings shade each other and compete from the moment they emerge.
Two mistakes of timing add to this: sowing too early, which forces you to keep plants indoors for weeks, and leaving the clear lid on after emergence, which keeps the air warm and stagnant.
| What you see | Most likely cause | What to do |
|---|---|---|
| Every stem leans towards the window | Light from one side only, too weak | Turn the pots each day, add a lamp |
| Stems upright but long and pale | Lamp too high or run too briefly | Lower the lamp, aim for 14 to 16 hours a day |
| Drawing up although the light looks right | Room too warm after emergence | Drop to 16 to 18 °C once the seedlings are up |
| Drawing up only in the middle of the tray | Sown too thickly | Thin out, then prick out into individual pots |
| Drawing up within days under a lid | Lid left on after emergence | Take the lid off as soon as the seedlings appear |
What can be saved, and what cannot
Let us be clear on this, because it saves a lot of disappointment. The length of stem already stretched stays stretched. No adjustment will shorten it. What can change is what comes next: the internodes formed after the correction will be short, and the plant will end up with a normal shape above its long base.
The rescue is worth it, then, as long as the seedling is still green and its stem is neither brown nor soft. A whole tray of seedlings lying down, tangled and pale for a fortnight, on the other hand, rarely deserves the effort: sowing again takes ten days and gives better plants. Sowing indoors is quick enough to allow a fresh start.
The five rescue moves
- Give more light, straight away. It is the only correction that acts on the cause. Bring the lamp closer to the seedlings, without letting it touch the foliage, and lengthen the daily run. For seedlings, the University of Illinois advises 16 hours of lighting, without going beyond 18 hours, and a domestic LED strip usually sits fifteen to thirty centimetres above the seedlings.
- Bring the temperature down. Once the seedlings are up, germination warmth is no longer any use. The Royal Horticultural Society keeps pepper and aubergine seedlings at around 16 to 18 °C in full light: that figure suits most indoor sowings. A cool, bright room beats a warm, dark one.
- Thin out without waiting. Keep the strongest seedling in each pot and cut the others off at compost level with scissors rather than pulling them out: the neighbouring roots stay intact.
- Prick out into individual pots as soon as the seedlings can be handled, generally at the first two true leaves. Hold them by a leaf or by the root ball, never by the stem. Tomatoes can be buried up to the seed leaves: the bare stem puts out roots along its whole buried length. Peppers, aubergines and herbs are not set any deeper than they were: just firm a little compost around the base.
- Keep the air moving. A gentle draught, or a hand brushed over the foliage once or twice a day, thickens the stems and limits stretching. A desk fan on its lowest setting, aimed past the plants rather than at them, is enough; it also helps avoid stagnant air.
Stopping it happening again
Sow at the right time. This is the most effective prevention, and the cheapest. Plants sown weeks before they can go outside spend that time stretching indoors. Our indoor sowing calendar gives the periods species by species.
Plan the lit space before you sow. Count how many pots your windowsill or your shelf can really light. Sowing three trays for one lit space inevitably leaves part of the batch drawn up.
Sow thinly. A dense layer suits microgreens, which are cut at the seed-leaf stage, but not plants meant to grow on. For microgreens, in fact, stretching is deliberately encouraged during the dark phase: the subject is covered in our beginner's guide to microgreens.
Set the lamp up on day one. A timer prevents forgotten hours, and an adjustable fixing lets you raise the lamp as the plants grow. The distance and duration figures are in our guide to indoor grow lamps.
Take the lid off at emergence. It serves no purpose once the seedlings are through, and it creates exactly the warm, humid climate that makes them draw up.
A word on compost and watering
A fine, light, low-nutrient seed compost does not make seedlings draw up, but it does affect how sturdy they are. Waterlogged compost starves the roots of air, and a poorly rooted seedling holds itself badly. Water from below where you can, standing the pots in a saucer of water for a few minutes, then let them drain.
Too much nitrogen fertiliser also produces soft, elongated growth. For the first few weeks seed compost is enough on its own: it supplies what is needed without pushing the seedling to grow faster than the light allows.
Pots, composts and small tools are gathered in the Pots & compost section.
Sources
- How to sow seeds indoors — Royal Horticultural Society
- How to grow sweet peppers — Royal Horticultural Society
- How to grow tomatoes — Royal Horticultural Society
- A Primer to Supplemental Lighting for Indoor Seed Starting — University of Illinois Extension
- Starting Vegetable Seeds Indoors: II. Materials — Utah State University Extension
Frequently asked questions
Should leggy seedlings be thrown away and sown again?
Not necessarily. As long as the seedling is green and its stem is neither brown nor soft, it is worth saving: the part already stretched will stay that way, but the growth made after the correction will be compact. A whole tray of seedlings lying down and pale for a fortnight, on the other hand, rarely deserves the effort: sowing again takes about ten days and gives better plants.
Should a leggy seedling's stem be buried when pricking out?
Only for tomatoes, whose stem puts out roots along its whole buried length: they can be set down to the seed leaves. For peppers, aubergines, basil and most herbs, a buried stem rots instead of rooting. Better to firm a little compost around the base and improve the light.
My seedlings are drawing up on a south-facing windowsill: is that possible?
Yes, especially in late winter. The light falls away very fast as soon as you move back from the glass, the days are still short, and the room is usually heated. The imbalance between high warmth and weak light is enough to make them stretch. Move the pots closer to the glass, turn them each day and add a grow lamp.
Does a fan really help to toughen seedlings up?
A gentle movement of air thickens the stems and limits stretching, as well as freshening the atmosphere around the seedlings. Set the fan to its lowest speed and aim it past the plants, not straight at them, so the compost does not dry out. Running a hand gently over the foliage once or twice a day has much the same effect.
