Why Are My Seedlings Leggy?

Seedlings get leggy because they are not getting enough light, and they stretch toward what light there is. The fix is to move the light much closer -- a few inches above the leaves for an LED strip, raised as the plants grow -- or to add a light if you are relying on a windowsill. Warmth without enough light makes it worse, so if your seedlings are on a heat mat under a dim window, that combination is the cause.

A leggy seedling has a long, thin, pale stem between the soil and the first leaves, and it often leans or falls over. It is not a disease and it is not a nutrient problem. It is a plant spending its limited stored energy on reaching upward instead of thickening up, which is a sensible response to being in the dark and a disastrous one in a seed tray.

The stem that stretched will never thicken to what it should have been. That is why this is worth fixing immediately rather than hoping -- a week of stretching produces a plant that is permanently weaker at the base.

Much closer than most people start with. An LED strip or bar of the kind sold for seed starting runs cool enough to sit a few inches above the leaf canopy, and that is where it belongs -- then it gets raised every few days as the seedlings grow. If the light is hanging a foot or two above the tray, the seedlings are effectively in shade.

Linkable strips are useful here precisely because you can run several close together across a tray rather than one point source high above it. University of Minnesota Extension's guidance on lighting for seed starting is worth reading on intensity and duration; the short version is that intensity falls away fast with distance.

On duration: 14 to 16 hours a day on a timer is the standard, with a genuine dark period. Running lights 24 hours does not help and plants do need the night.

Heat mats are excellent for germination and they are the wrong thing to leave running afterwards. Warmth drives growth; light supports it. Heat with inadequate light produces exactly the fast, soft, stretched growth you are trying to avoid.

So: mat on until the seeds are up, then off. After germination, seedlings do better slightly cool and very bright than warm and dim. Our seedling heat mats page covers which crops genuinely need bottom heat -- peppers and tomatoes do, lettuce and brassicas do not.

Sometimes, and it depends entirely on the crop. Tomatoes and anything else that roots from the stem can be rescued: pot them on deeper, burying the stretched stem up to just below the first leaves, and they will root along the buried section and end up stronger than before. This genuinely works and is the standard remedy.

Peppers, brassicas, cucumbers, squash and most other things cannot be buried that way -- burying their stems causes rot rather than roots. The best you can do is fix the light, pot them on into a deeper cell so more of the stem is supported, and accept a weaker plant. If they are badly stretched and it is still early, resowing is faster than nursing them.

1. Buy the light before the seeds. It is the cheapest piece of seed starting equipment to under-spend on and the one that decides the outcome.
2. Set the light a few inches above the trays from day one and put it on a timer. Raise it as the plants grow.
3. Take the heat mat away once they germinate. Cool and bright, not warm and dim.
4. Sow one seed per cell, or thin ruthlessly as soon as the first true leaves show.
5. Sow on time rather than early. Enthusiasm in February produces leggy plants in March. When to start seeds indoors works back from your own frost date.

If you are deciding whether to buy a light at all, grow lights vs a windowsill is the direct comparison, and the best grow lights for seedlings covers the specific options.

Legginess is the most common complaint but not the only one. Seedlings collapsing at soil level with a pinched, dark stem is damping-off, a fungal problem driven by overwatering and poor air movement -- water less, from below where possible, and get some air across them. Pale yellowing leaves on seedlings that have been in the same tray for weeks is usually nutrient exhaustion, because a seed starting mix is deliberately low in nutrients; pot them on.

White fuzz on the surface of the mix is usually harmless saprophytic mould indicating the surface is staying too wet. Green algae on the mix means the same thing. Neither kills seedlings directly, but both are a warning that damping-off conditions are present.

Reusable heavy-gauge trays are worth mentioning here for a practical reason: flimsy trays flex, which means uneven watering and uneven drainage, and uneven moisture is behind most of the problems above.