Short answer
Yellow or pale cannabis seedlings are usually overwatered, under-lit, or hungry in an inert medium. Cotyledons yellowing at days 10–14 is often normal. Check pot weight and light (150–300 µmol/m²/s) before reaching for nutrients. Supported.
Why
Wet roots can't breathe. A soggy plug starves roots of oxygen, so uptake stops and leaves pale even in a rich mix. Constantly wet media also favour Pythium root rot [3] and damping-off [1].
Weak light makes pale, stretched seedlings. Young plants below about 8 mol/m²/day of light grow inferior roots and mass in other crops [2].
Inert media carry no food. Coco, rockwool and hydro seedlings run out once the cotyledons' reserves are spent, so they yellow from the bottom up.
Cotyledons are temporary. As true leaves take over, the seed leaves often yellow and drop, which is not a problem by itself.
Over-feeding causes damage of its own. Extension advice is to hold fertiliser until true leaves appear [1].
The numbers
| Symptom |
Likely cause |
Check |
| Whole seedling pale, wet mix, droop |
Overwatering |
Pot still heavy days later |
| Pale and stretched |
Low light |
PPFD under 150 µmol/m²/s |
| Cotyledons yellow, day 10–14, true leaves green |
Normal |
Nothing to fix |
| Lower leaves yellowing in coco/rockwool |
Hunger |
No feed yet; start EC 0.4–0.8 |
| Bleached top, crisp edges |
Too much light or heat |
Over 300 µmol/m²/s or hot lamp |
What to do
- Lift the pot. If it's heavy, stop watering until it lightens.
- Measure PPFD at the leaves and adjust to 150–300 µmol/m²/s.
- In coco, rockwool or hydro, feed at EC 0.4–0.8 mS/cm, pH 5.8–6.2.
- In soil, wait for 2–3 sets of true leaves before feeding.
- Check the stem base: pinched and brown means damping-off.
Common mistakes
- Feeding full strength to "green it up".
- Calling normal cotyledon yellowing a deficiency.
- Plain water only in coco for three weeks.
- Watering daily on a schedule.
A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.A soilless medium contains no mineral soil, such as peat, coco, rockwool or perlite blends, and relies mostly on the grower to supply nutrients.One of the two rounded seed leaves packed in the embryo that open first after germination and feed the seedling until the true leaves take over.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.The collapse of seedlings or cuttings when soil-borne pathogens such as Pythium, Rhizoctonia solani and Fusarium rot the stem at the soil line.Daily light integral: the total number of PAR photons a square metre of canopy receives in one day, in mol/m²/day; PPFD multiplied by hours of light.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.The first leaves produced by the shoot tip after the cotyledons, with the plant's characteristic serrated leaflets.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.
