Why are my seedlings pale and yellow?

Three different problems produce a pale seedling and the fixes contradict each other, so guessing costs you the tray. Two questions separate them in seconds: is the stem stretched or stocky, and did the yellowing start at the bottom or the top of the plant.

What you'll need

• A very bright windowsill or a grow light
• Weak liquid feed (optional)
• Fresh potting compost (optional)

Step by step

1. Look at the stem first.Long, thin and pale between each pair of leaves means the problem is light. Short and thick means light is adequate and something else is wrong.
2. Ask which leaves yellowed first.Oldest and lowest first is nitrogen shortage: the plant is moving what it has to the new growth. Newest leaves first points to roots that cannot take nutrients up — cold, waterlogged, or damaged.
3. Check the age against the compost.Seed compost is deliberately low in nutrients so it does not scorch germinating roots. It carries a seedling for two or three weeks and then it is empty. Yellowing at about three weeks in seed compost is normal and expected.
4. Feed at quarter strength, or pot on.Potting on into ordinary multipurpose is the better fix because it gives root room as well as nutrition. If they are not ready to move, a balanced liquid feed at a quarter of the label rate once a week will hold them.
5. Lift the tray and check the weight.Cold saturated compost stops roots working, and the plant goes yellow with plenty of nitrogen present. If it is heavy and the surface is green with algae or liverwort, it is too wet.
6. Get it warmer at the roots, not warmer in the air.A tray on a cold windowsill or a stone floor in March sits several degrees below room temperature. A sheet of polystyrene or a folded towel under it makes a real difference; more air heat only makes the stretching worse.
7. Improve the light before anything else.In a British February and March the limit is almost always light. The brightest south-facing sill you have, a sheet of white card or foil behind the tray to bounce light back, and the tray turned daily.
8. Add a light if you sow early.If you want to sow in February, a simple LED grow light 100 to 300mm above the seedlings for fourteen to sixteen hours a day changes the outcome completely. Without one, sowing in March or April gives better plants than sowing in February does.
9. Check for the obvious wrong compost.Ericaceous compost used for ordinary seedlings, or ordinary compost for blueberries and other lime-haters, produces yellowing between green veins on the new leaves. It is uncommon but instantly diagnostic.
10. Consider starting again.A stretched, starved seedling never becomes a good plant. Seed is cheap, and a sowing made three weeks later in better light routinely overtakes the rescued one.

Tips

• Yellow from the bottom up: feed it. Yellow from the top down: it is cold and wet.
• White card behind a windowsill tray is free and works better than most people expect.
• Watering from below with tepid rather than cold water avoids chilling the roots every time you water.

Common mistakes

• Feeding a seedling that is yellow because it is waterlogged — There is already nitrogen in the compost that the roots cannot take up. More feed makes the situation worse and can burn the roots that are left.
• Sowing into rich multipurpose to avoid the hunger problem — The nutrient level scorches germinating roots and produces soft, disease-prone growth. Sow in seed compost and pot on instead.
• Adding heat instead of light — Warmth without light is precisely the combination that produces long, weak, pale seedlings. If anything, cooler and brighter is the answer.

If it doesn't work

Questions people ask