Cannabis yellow leaves diagnosis starts with one question: are the yellow leaves old or new? The top pick for yellowing that begins on the lowest fan leaves and moves up the plant is nitrogen deficiency. That covers most cases in soil and coco where a plant has sat in the same pot for weeks. The criteria used here are the age of the affected leaves, the pattern inside the leaf, the speed of the color change, and the companion symptoms such as droop, clawed tips, spots, or crisp edges.

why are my cannabis leaves yellow

Criteria: read the leaf before you change the feed

  • Leaf age. Mobile nutrients leave the old growth first. Immobile nutrients hit the new growth first.
  • Pattern inside the leaf. A full fade points to nitrogen. Green veins with yellow tissue between them point to magnesium or iron.
  • Speed. A change over two or three days suggests a root or pH event. A change over two weeks suggests a feed that runs lean.
  • Companion symptoms. Droop, soft stems, and a heavy pot point to the root zone. Burnt tips and curled edges point to salt buildup, heat, or light stress.

Nitrogen deficiency: the top pick

Nitrogen is mobile inside the plant. When supply runs short, the plant pulls nitrogen from the lower fan leaves and sends it to the growing tips, so the bottom of the canopy turns first while the top stays green.

why are my cannabis leaves yellow

Signs that fit:

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  • Lower leaves fade to pale yellow, starting at the tip and margins and moving toward the center.
  • New growth holds a healthy green after the bottom has turned.
  • The fade climbs the plant over one to two weeks.
  • Plants in late veg or early flower that have been in the same medium for a month or more.

Signs that point elsewhere:

Cannabis Yellow Leaf Disease: What You Need to Know

  • Yellow appears on the newest leaves first, which rules out a mobile nutrient.
  • Margins are crisp and brown, which looks more like salt buildup or a potassium issue.
  • Leaves droop while the pot stays wet, which points at the roots before the feed.

Recommendation: if the pattern fits, measure runoff pH and EC first, then raise nitrogen with a mild veg feed. Do not double the dose in one shot.

Magnesium deficiency

Magnesium sits at the center of the chlorophyll molecule, so a shortage shows as pale tissue while the veins stay green. It starts on older and middle leaves and spreads as interveinal chlorosis.

Signs that fit:

  • Yellow between the veins on old and middle leaves, with the veins still dark green.
  • Leaf edges curl upward as the shortage continues.
  • Common in coco, in soft water, and under strong LED light where demand runs high.

Signs that point elsewhere:

  • Veins turn yellow along with the tissue, which fits nitrogen instead.
  • Yellow on the newest leaves, which fits iron.
  • pH outside the working range, meaning magnesium is present but locked out.

Recommendation: correct pH first. In soil, keep the root zone near 6.0 to 7.0. A magnesium source such as Epsom salt can help at a mild rate, but not until pH is in range.

Overwatering and root problems

A wet root zone starves roots of oxygen, and the first sign is yellow that mimics a nutrient shortage. Leaves may droop, feel soft, and the pot stays heavy for days.

Signs that fit:

  • The pot stays heavy well past the normal dry back window.
  • Droop that does not lift after watering.
  • Yellow on lower and middle leaves with slow overall growth.
  • Sour smell at the drainage holes or fungus gnats around the surface.

Signs that point elsewhere:

  • The pot runs light and dry between feeds.
  • Yellow sits at the top of the plant with crisp edges.

Recommendation: let the medium dry back, improve drainage and airflow, and inspect for root rot before adding any nutrient.

pH drift and nutrient lockout

Nutrients can sit in the pot and still stay out of reach. In soil, a range near 6.0 to 7.0 keeps most nutrients available. In coco and hydro, aim near 5.5 to 6.5.

Signs that fit:

  • Several deficiency patterns show at once, on old and new leaves.
  • Feed strength has not changed, but the plant declines.
  • Rusty spots appear alongside pale leaves.
  • Your pH meter has not been calibrated in weeks.

Signs that point elsewhere:

  • A single pattern that obeys the mobile and immobile rule.
  • A recent repot into fresh, charged soil.

Recommendation: calibrate the meter, check runoff, and shift pH in small steps rather than one large swing.

Nutrient burn, heat, and light stress

Too much salt or too much light can produce yellow that reads like a shortage. Burnt leaf tips and curled or taco-shaped leaves are the tell. Under strong fixtures, top leaves may bleach toward white while lower leaves stay green.

  • Signs that fit burn: crisp brown tips, dark waxy leaves, rising EC in runoff.
  • Signs that fit heat or light: yellow or bleached top canopy, leaf edges that curl up, damage worst at the center of the light footprint.
  • Signs that point elsewhere: yellow on lower leaves only, with soft and drooping stems.

Recommendation: reduce feed strength, flush the medium, and raise the light or dim it before you treat the plant for a deficiency it does not have.

Yellow new growth: iron, sulfur, and zinc

These nutrients move less freely in the plant, so the top changes first while the bottom holds green. Iron shows as yellow new leaves with sharp green veins. Sulfur shows as a flat pale yellow across new growth. Zinc shows as narrow, short new leaves with yellow between the veins.

Signs that fit: yellow at the crown, green lower canopy, pH that runs high in soil or cold and wet roots.

Signs that point elsewhere: yellow that starts low and travels up, which fits a mobile nutrient.

Recommendation: check pH and root temperature. Iron uptake drops in cold, wet, or alkaline media, so the fix is often the root zone rather than more iron.

Late-flower fade is normal

In the final two or three weeks, plants pull mobile nutrients out of the fan leaves and the canopy turns yellow, then purple. That fade tracks the harvest window and needs no fix beyond a plain water finish if you run one. The difference between fade and a real deficiency is timing: fade arrives at the end of flower, while a deficiency arrives mid cycle and spreads fast.

Use-case recommendation: match the fix to the pattern

  • Bottom leaves fade, top stays green: check root zone and runoff pH, then raise nitrogen.
  • Green veins with yellow between them: treat as magnesium after pH is verified.
  • Whole plant pale with burnt tips: reduce feed, flush, and measure runoff EC.
  • New leaves yellow first: treat as iron or sulfur and fix pH and root temperature.
  • Yellow only in the last weeks of flower: hold the course and let the plant finish.

The same rules apply to high-CBD hemp cultivars grown indoors or in the field, since the plant species and its nutrient mobility are the same. Field hemp adds waterlogging and soil compaction to the list of causes, and both produce yellowing that looks like a feed problem.