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Yellow Leaves on Cannabis: a 6-Question Decision Tree
By the Cultivar grow team · published 2026-07-26 · updated 2026-07-26 · how we source these numbers
Yellow leaves are the single most common thing growers panic about, and the panic is usually the problem. Six causes explain nearly every case, they look similar in a photo, and the wrong fix — almost always "add more nutrients" — makes four of the six worse.
Work through the questions in order. Stop at the first one that matches.
Question 1: Where on the plant did it start?
This one question eliminates most of the field, because it separates mobile nutrients from immobile ones.
Mobile nutrients (nitrogen, magnesium, potassium, phosphorus) can be moved by the plant. When the plant runs short, it strips them out of old growth and sends them to new growth — so damage shows on the bottom leaves first. Immobile nutrients (calcium, iron, sulphur, most micros) cannot be moved, so damage shows on new growth at the top.
| Where it started | What it points to | Where to go next |
|---|---|---|
| Bottom leaves, oldest growth | Nitrogen, magnesium, overwatering, or natural late-flower fade | Question 2 |
| New top growth, tips and youngest leaves | Iron or a pH lockout affecting immobile nutrients | Question 4 |
| Tops of the plant nearest the light | Light burn or heat stress | Question 5 |
| Everywhere at once, quickly | Root-zone problem: overwatering, root rot, or severe pH swing | Question 3 |
Question 2: What week are you in?
If you are in the last two or three weeks of flower and the yellowing started low and is creeping up while the buds swell, this is very likely natural nitrogen mobilisation, not a deficiency. The plant is cannibalising its own fan leaves to finish. It is supposed to happen, and feeding it away costs you nothing but money.
The tell: the plant looks healthy apart from colour. Buds are still fattening, trichomes are turning cloudy, and there is no clawing, spotting or crisping.
If you are in seedling, veg, or early flower, keep going.
Question 3: How heavy is the pot?
Pick it up. This takes two seconds and settles the most misdiagnosed cause on this list.
Overwatering yellows leaves because saturated medium starves roots of oxygen, and roots that cannot breathe cannot take up nutrients — so the plant shows a nutrient deficiency while sitting in a pot full of nutrients. Classic pattern: droopy and yellow together, leaves that feel thick or "puffy", slow growth, and a pot that still feels heavy days after watering.
The fix is not a nutrient. It is:
- Let the medium dry until the pot is noticeably light. Lift it right after watering and again a few days later so you learn the difference.
- Water in a smaller circle around a small plant rather than soaking the whole pot.
- If it recurs, get more air to the root zone: fabric pots, more perlite, or a smaller pot for the plant's size.
Underwatering also yellows, but the plant will be limp and light, and it recovers within hours of a proper water.
Question 4: What is your runoff pH?
If you have not measured it, this is the step to stop and do. A very large share of what growers call a deficiency is pH lockout — the nutrient is present in the medium, and the roots simply cannot pull it in at that pH.
The windows are narrow:
| Medium | Target pH |
|---|---|
| Soil | 6.0 – 6.8 |
| Coco | 5.5 – 6.2 |
| Hydro / DWC | 5.5 – 6.0 |
Drift outside that range and specific nutrients fall out of availability at specific points — iron goes first in high-pH soil (yellow new growth, green veins), magnesium and calcium suffer at the low end. The reason "add more nutrients" so often fails is that more of an unavailable nutrient is still unavailable, and the extra salts push pH further out.
Fix the pH of what goes in, flush with correctly pH'd water if runoff is far out, then re-measure. Full ranges and the lockout logic are on the pH by growing medium page.
Question 5: How close is the light, and how hot is it?
Light burn yellows — then bleaches — the leaves closest to the lamp, in a rough circle under the fixture, while the rest of the plant looks fine. Veins often stay green longest. Heat stress does something similar but adds curled or "taco" edges.
Check your hang height against the manufacturer's chart for your stage, and check leaf-level temperature rather than the number on a hygrometer sitting on the floor. If the top colas are yellowing and the lower canopy is green, the light is the suspect, not the feed.
Environment targets by stage are on the VPD, temperature and humidity chart.
Question 6: What is the pattern within the leaf?
Only now, once weeks, water, pH and light are ruled out, does it make sense to read the leaf itself.
| Pattern | Most likely | Note |
|---|---|---|
| Whole leaf pales evenly, oldest first, moving up | Nitrogen deficiency | Feed at normal strength; over-correcting causes clawed, dark, brittle growth |
| Yellow between the veins on older leaves, veins stay green | Magnesium deficiency | Very common in coco and under LEDs; cal-mag is the usual fix |
| Yellow between veins on new growth, veins green | Iron — nearly always a pH problem | Fix pH before adding iron |
| Yellow with brown rusty spots | Calcium, or potassium if edges burn | Check pH first; see the deficiency chart |
| Yellowing with fine webbing or stippled dots | Spider mites, not a deficiency | Look under the leaves with a loupe |
What to do after you have your answer
Change one thing. Growers who fix three things at once never learn which one worked, and half the time one of the three made it worse.
Then judge the result correctly:
- New yellowing should stop within 3 to 7 days.
- Already-yellow leaves will not turn green again. That is not failure; leaves do not recover, plants do.
- Photograph the same leaf and the same growth tip daily. Memory is a bad instrument, and yellowing is gradual enough to fool anyone.
This is the actual reason logging beats guessing: the grower who can look back at a pH reading from four days ago and a photo from a week ago diagnoses in a minute. The grower relying on memory diagnoses in a forum thread, three days later.
Still not sure?
Take a photo in white light — turn the grow lights off or step outside the tent, because purple LED light hides every colour cue this page depends on — get close enough to see the affected leaves clearly, and run it through the free plant diagnosis. No account, three checks a day, and it tells you what it sees rather than what it hopes.
- Digital pH pen + calibration solution — measures the thing that causes most yellowing before you spend money on nutrients
- Cal-mag supplement — the standard fix for magnesium-pattern yellowing in coco and under LEDs
- Thermometer + hygrometer — rules out heat and VPD stress in one glance
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FAQ
Why are my cannabis leaves turning yellow?
Six causes explain almost every case: natural nitrogen mobilisation in late flower, nitrogen deficiency, pH lockout, overwatering, light burn, and magnesium deficiency. Which one you have is decided by WHERE the yellowing starts (old growth vs new growth), how fast it spread, and what your runoff pH is.
Should I add nutrients as soon as leaves go yellow?
No. Feeding a pH-locked or overwatered plant makes it worse. Measure runoff pH first, check the pot's weight, and only then consider feeding. Roughly speaking, most "deficiencies" growers report are lockout or root-zone problems, not a lack of nutrients in the medium.
Is yellowing normal in late flower?
Yes. In the last two to three weeks a photoperiod plant pulls nitrogen out of its fan leaves to finish buds. Yellowing that starts low, moves up slowly, and coincides with swelling buds and mostly cloudy trichomes is the plant working as designed.
How fast should yellow leaves stop spreading after a fix?
New yellowing should stop within 3 to 7 days. Already-yellow leaves do not turn green again — judge the fix on new growth, not on the damaged leaves.
Can I just cut the yellow leaves off?
Only if they are more than about half dead or actually mouldy. A yellow leaf is still exporting nutrients to the rest of the plant, and removing leaves does not fix the cause. Fix the cause, then tidy up.
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