Weed leaves turning yellow almost always come down to one of five causes: a nutrient deficiency, pH out of range, overwatering, too much light or heat, or normal ageing at the end of flowering. The main clue is where the yellowing starts: on the lower leaves it usually points to a mobile nutrient such as nitrogen, magnesium or potassium, on the new leaves at the top to an immobile one such as iron, sulphur or calcium, often locked out by the wrong pH. Measure pH before adding any fertiliser: 6.0–7.0 in soil, 5.5–6.5 in coco and hydro.
In brief
- Yellowing from the bottom up: nitrogen, magnesium or potassium. Yellowing on new growth: iron, sulphur, calcium or pH.
- In the last 2–3 weeks of flowering, yellow fan leaves are normal and should not be corrected.
- Wrong pH is one of the most common causes: soil 6.0–7.0, coco and hydro 5.5–6.5.
- Pot always heavy, leaves yellow and droopy: overwatering and roots short of oxygen.
- Leaves that are already yellow do not turn green again: judge any fix by the new leaves after 5–10 days.
Where the yellowing starts: lower or new leaves
Nutrients are either mobile or immobile. When a mobile nutrient (nitrogen, phosphorus, potassium, magnesium) runs short, the plant moves it from old leaves to new ones, so the deficiency shows up at the bottom first. Immobile nutrients (calcium, iron, sulphur, manganese, boron) stay where they were laid down, so a shortage hits the young leaves at the top first.
| What you see | When | What to check first | Most likely cause |
|---|---|---|---|
| Lower leaves evenly yellow, later dropping | Veg or early flowering | Feeding dose and start date, pH | Mobile nutrients used up, usually nitrogen |
| Large fan leaves yellowing from the bottom while buds keep swelling | Last 2–3 weeks of flowering | Nothing to fix | Normal ageing |
| Yellow between the veins of middle leaves, veins green | Any stage, often in flower or in coco | pH, water hardness, Cal-Mag | Deficiency or lockout of a mobile nutrient |
| Yellowing on new leaves at the top | Any stage | pH of the solution and the runoff | pH lockout of an immobile nutrient |
| Yellowing or bleaching only near the light | Mostly in flower, under powerful LEDs | Distance, dimmer, canopy temperature | Too much light or heat |
| Whole plant yellow, leaves heavy and drooping | Any stage, seedlings in big pots | Pot weight, saucer, smell of the medium | Overwatering, roots short of oxygen |
| Only a few shaded lower leaves, the rest green | Any stage | Nothing, if new leaves are healthy | Leaves no longer getting light |
The table starts from what you see and the stage, so you know whether to act and what to measure. The element-by-element table, covering nitrogen, phosphorus, potassium, magnesium, calcium, iron and sulphur, their excesses and fixes, is in the guide to cannabis nutrient deficiencies.
Normal fading in late flower or nitrogen deficiency?
In the last weeks of flowering the plant pulls nitrogen and other mobile nutrients out of the big leaves to finish the buds: fan leaves yellow from the bottom up and drop. That is normal as long as the buds keep swelling and the small leaves around them stay mostly green. The same sign in veg or in the first weeks of flowering points to a nitrogen deficiency instead.
| Normal fading | Nitrogen deficiency | |
|---|---|---|
| When | Last 2–3 weeks of flowering | Veg or first weeks of flowering |
| Where | Big fan leaves, from the bottom | Lower leaves, then the whole plant turns pale green |
| Speed | Gradual, a few leaves a week | Fast, with slower growth |
| What to do | Nothing: do not add nitrogen | Check pH, then grow fertiliser at the label dose |
A lightly pre-fertilised soil runs out after 2–4 weeks: if you have not started feeding yet, nitrogen deficiency in veg is often the first sign. Raise the dose step by step without going above the label; too much nitrogen in flower makes leaves dark green and glossy and slows ripening.
pH out of range: nutrient lockout
With the wrong pH the nutrients are there, but the plant struggles to take them up. The typical sign is symptoms of several deficiencies at once that do not improve with more fertiliser. With pH too high, iron and manganese become poorly available and new leaves turn yellow; with pH too low, calcium and magnesium uptake suffers and spots and yellowing appear on the middle leaves. Check it like this:
- Measure pH and EC of the solution you give, after adding fertiliser: soil 6.0–7.0, coco and hydro 5.5–6.5.
- Water until 10–20% runs out of the bottom and measure pH and EC of the collected runoff.
- If the runoff pH stays well away from the solution pH (as a rough guide, more than half a point) or the EC is clearly higher, the pH in the medium has drifted or salts have built up.
- Correct the solution with small doses of pH down or pH up and, if runoff EC is high, give one watering with pH-corrected water and reduced fertiliser.
To measure you need a calibrated pH meter and an EC meter, found among the pH and EC meters, plus pH adjusters. Values and the full method are in the guide to cannabis pH water.
Overwatering: roots without oxygen
A medium that is always wet starves the roots of oxygen: the plant takes up little and the leaves turn yellow, heavy and droopy, often across the whole plant. Typical signs: pot still heavy 2–3 days after watering, a stale smell, brown and soft roots instead of white ones, fungus gnats. The opposite, pots drying out completely again and again, also makes the lower leaves yellow and crispy.
- Water only when the pot feels light and the top 2–3 cm of medium are dry.
- Give enough to get some runoff, then empty the saucer within 30 minutes.
- Use pots with good drainage holes or fabric pots sized to the plant: a seedling in a huge pot stays wet for a long time.
- Keep the solution at 18–22 °C; in hydro, oxygenate it with an air pump.
Amounts and frequency for each pot size are in the guide to watering cannabis.
Light too close and heat: yellowing at the top
If only the leaves closest to the light turn yellow or bleach while those below stay green, the cause is too much light or heat, not a deficiency. Under powerful LEDs the yellowing can appear between the veins and look like iron deficiency; leaves often fold upwards like a canoe as well. In flower the canopy usually needs no more than 600–900 µmol/m²/s and 20–26 °C with the lights on (20–24 °C in the last 2–3 weeks): raise the light or dim it. Other causes of misshapen leaves are in the guide to curling leaves.
Step by step: what to do when you see yellow leaves
- Look at where the yellowing starts (bottom, top, near the light) and what stage the plant is in.
- Check watering: weight of the pot, saucer, smell of the medium.
- Measure pH and EC of the solution and of the runoff.
- Check the canopy temperature and the distance to the light.
- Change one thing at a time and write down what you did.
- For 5–10 days, watch only the new leaves: if they grow in green, the fix is working.
Frequently asked questions
Is it normal for leaves to turn yellow during flowering?
Yes, in the last 2–3 weeks it is normal for the big leaves to yellow from the bottom while the buds ripen. If the yellowing starts in the first weeks of flowering or the whole plant turns pale at once, it is a deficiency to fix.
Do yellow leaves turn green again?
A fully yellow leaf will not turn green again. A slightly yellowed leaf can partly recover after a pH problem, but you judge the fix by the new leaves.
Should you remove yellow leaves?
Remove leaves that are dry or come away with a gentle tug, as they can encourage mould. Leaves that are still partly green keep producing energy and pass nutrients back to the plant: leave them on.
Do yellow lower leaves mean nitrogen deficiency?
Often, yes, in veg and when the yellowing is even and moves upwards. But shaded lower leaves also yellow on their own, and in late flower it is normal: check pH and watering first.
Grow and bloom nutrients are in the Gro.w.LaB fertilisers collection.
Updated on 13 October 2026.