Why are my seedlings tall, thin and falling over?

The commonest cause is sowing too early. Sown in February on a warm windowsill, a tomato seedling has heat driving growth and almost no light to support it, so it stretches. The same seed sown in late March grows short and stocky.

What you'll need

• Bright windowsill, cold frame or grow light
• Small fan (optional)
• Fresh compost for potting on

Step by step

1. Give them much more light.A south-facing windowsill is the domestic minimum, and even that is marginal in midwinter. A cold frame or unheated greenhouse from March gives far better light than any indoor position.
2. Turn the trays daily.Seedlings on a windowsill lean towards the glass. Turning the tray each day keeps them upright and even.
3. Add reflection.White card or foil on the room side of the tray bounces light back and noticeably reduces leaning and stretch.
4. Lower the temperature after germination.Seeds need warmth to germinate. Seedlings need light more than warmth. Move them off the heat mat and into a cooler, brighter place the moment they emerge.
5. Sow later.Almost everything sown in February in the UK would do better sown in late March or April, when light levels have risen. Later sowings routinely overtake earlier ones.
6. Give them air movement.A gentle breeze from an open window or a small fan makes stems thicker and stronger. Still air produces weak, soft growth.
7. Brush the tops daily.Running a hand lightly over seedlings once or twice a day produces measurably sturdier plants. It is free and it works.
8. Rescue tomatoes and brassicas by burying the stem.Tomatoes root from the buried stem, so pot them on deeply, up to the seed leaves. Brassicas can also be planted deeper. Most other seedlings cannot be rescued this way.
9. Space them out.Overcrowded seedlings shade each other and stretch. Prick out into individual cells or pots as soon as the first true leaves appear.
10. Harden off before planting out.Indoor-grown seedlings collapse outdoors without a week or two of gradual acclimatisation.

Tips

• Warmth without light is what causes legginess. Germinate warm, grow cool and bright.
• Brushing your hand over seedlings daily genuinely thickens the stems.
• A March sowing usually catches and passes a February one.

Common mistakes

• Leaving seedlings on the propagator after they germinate — Bottom heat drives fast soft growth that the available light cannot support. Move them off heat as soon as they are up.
• Sowing everything as early as possible — Light, not date, limits growth in a UK spring. Early sowings produce leggy plants that are overtaken by later, stockier ones.

If it doesn't work

Questions people ask