Yellow leaves on a cannabis plant are a symptom, not a diagnosis. The fastest way to name the cause is to answer two questions: where did the yellow start (old lower growth or new upper growth), and what shape does it take (an even fade across the leaf, yellow between green veins, or yellow edges that turn crisp). Mobile nutrients leave the old leaves first. Immobile nutrients hit the new leaves first. Water and root problems affect the whole plant but show low. Light and heat damage shows up only on the canopy nearest the lamp. Work in that order and you can name the problem in minutes instead of adding another bottle to the feed.
Nitrogen Deficiency
Nitrogen moves through the plant, so when supply runs short the plant pulls it from the oldest leaves to feed new growth. Lower leaves fade to a flat, even yellow, then to pale cream. The fade starts at the leaf tip and moves inward, and the leaf often drops on its own.
Signs that fit
- Uniform yellow on lower and middle leaves, not a pattern of stripes
- New growth stays green and looks healthy
- Plant is in mid veg or the first weeks of flower
- Medium has been flushed or watered with plain water for a stretch
Signs that point elsewhere
- Green veins running through yellow tissue
- Brown, crisp edges on the yellow leaves
- Yellow limited to the top of the canopy
Use this read when the plant has sat in the same pot for weeks and the lower fans are draining of color while the top looks fine.
Overwatering and Root Problems
Roots that stay wet cannot take up oxygen, and uptake of everything slows at once. Leaves yellow low on the plant and take on a soft, drooping look rather than a dry one. The medium stays damp for days after a watering.
Signs that fit
- Pot feels heavy days after watering
- Leaves curl down and feel limp, not papery
- Yellow creeps up from the bottom of the plant
- Musty smell near the drainage holes
Signs that point elsewhere
- Medium is dry and light
- Yellow sits only at the leaf margins
- Leaves are crisp and snap when bent
Use this read when you water by the calendar instead of by pot weight. Let the container dry back before the next feeding, and check that drainage holes are clear.
Magnesium Deficiency
Magnesium shortage shows as interveinal chlorosis: yellow sits between the veins while the veins stay green. It starts on middle and older leaves and works outward. Growers in coco and in filtered or reverse osmosis water see it most, and it often appears alongside a pH drift.
Signs that fit
- Green veins against yellow leaf tissue
- Middle and older leaves affected before the top
- Leaf edges cup upward and dry out
Signs that point elsewhere
- The whole leaf fades to one flat color
- Only the newest growth is yellow
Use this read when you grow in coco or soft water and the yellow arrives as a stripe pattern rather than a full fade.
pH Drift and Nutrient Lockout
When root zone pH sits outside the window where nutrients stay available, several shortages appear together. Yellow shows in patches, different leaves show different patterns, and adding more fertilizer makes the plant look worse rather than better.
Signs that fit
- More than one deficiency pattern at the same time
- Yellow on both old and new leaves
- You have not measured the pH of what goes in and what drains out
- Recent change in water source or nutrient line
Signs that point elsewhere
- One clean pattern that matches a single nutrient
- Symptoms started right after a change in light distance or room temperature
Use this read when the plant shows a mixed set of symptoms that no single deficiency chart explains.
Potassium Deficiency
Potassium is mobile, so yellow starts at the leaf edges on older leaves. The margins then turn brown and crisp and curl inward. Plants in heavy flower have the highest demand.
Signs that fit
- Yellow margins with brown, burnt edges
- Older leaves affected first
- Plant is in mid to late flower
Signs that point elsewhere
- Yellow between veins with green veins
- Whole leaf pale and even
- Only new leaves yellow
Use this read when a flowering plant shows scorched leaf edges while the veins and centers stay green.
Light and Heat Stress
Damage from a lamp stays where the light is strongest. Leaves closest to the fixture yellow first, and the rest of the plant looks normal.
Signs that fit
- Yellow limited to the top of the canopy
- Leaves taco or curl up at the edges
- Light was moved closer or the fixture was turned up
- Canopy temperature runs hot at leaf level
Signs that point elsewhere
- Yellow on the bottom of the plant
- Even fade across every leaf
Use this read when the yellow sits directly under the light and the shaded leaves stay green.
Iron and Sulfur: Immobile Nutrients
Iron and sulfur do not move through the plant, so new growth yellows first. Iron shortage shows yellow between the veins on new leaves while the veins stay green. Sulfur shortage turns the whole new leaf pale and can lighten the entire top of the plant. Both point back to pH.
Signs that fit
- Newest leaves yellow while old leaves stay green
- Green veins on new growth (iron)
- Whole new leaf pale and even (sulfur)
Signs that point elsewhere
- Lower leaves yellow first
- Yellow follows a watering change
Use this read when the top of the plant is the only part that changed color.
Late Flower Fade Is Normal
In the final weeks of flower, the plant pulls stored nutrients from the fan leaves to finish the buds. Leaves fade from the bottom up and can turn yellow, purple, or both. This is senescence, not a shortage you need to correct.
Signs that fit
- Plant is in the last few weeks before harvest
- Buds are swelling and filling in
- Fade moves upward in an even wave
Signs that point elsewhere
- Plant is in veg or early flower
- Sudden yellow plus spots, wilting, or stalled growth
Use this read when a healthy, ripening plant loses lower fan leaves on schedule.
A Quick Order of Operations
- Check medium moisture by pot weight before you feed anything.
- Measure the pH of your input water and the runoff.
- Write down where the yellow started and its shape.
- Change one variable and give the plant several days before judging the result.
- Remove leaves that are fully brown or crisp so you can spot new damage.
If You Came Here for CBD, Not Growing
A yellowing plant says nothing about the quality of a CBD product. Those are separate questions. If your interest is what you buy rather than what you grow, the useful check is a third-party lab report for cannabinoid content and contaminant screening from the seller you are considering.