Leggy seedling with a long, thin stem
First 1-2 weeks; the hypocotyl stretches under weak light or too much heat..
- Also called
- stretched seedling
- seedling too tall
- seedling falling over thin stem
Likely causes, most likely first
Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.
LikelyNot enough light
Fix: Bring PPFD to stage targets: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s. Yield rose linearly with PPFD from 120 to 1,800 in a controlled trial. Lower canopy buds under ~300-400 µmol/m²/s make larf: thin or lollipop the shaded bottom, or train flat.
Recovery: New internodes shorten within a week; denser buds on the following weeks of growth.
How to confirm it
- Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
Look for
Seedling side-on with the light distance..
The full guide: Why is my seedling drooping?
Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.
