Why are my cannabis leaves turning yellow? Causes and fixes

Yellowing leaves, or chlorosis, in cannabis plants can be worrying. Understanding the underlying causes is crucial for effective treatment and prevention. This guide explores the most common reasons for yellowing cannabis leaves and offers practical solutions to restore your plant’s health. If you’re just getting started, check out our feminised cannabis seeds or high-yielding strains to grow strong, resilient plants.

Common causes of yellowing cannabis leaves

Nutrient deficiencies

Nutrient imbalances are a leading cause of discolouration. Deficiencies in macro and micronutrients impact the plant’s ability to photosynthesise and grow effectively.

Nitrogen deficiency: Older leaves turn yellow and fall off. This is the most common deficiency and happens when nitrogen is redirected to support new growth.
Iron deficiency: New leaves turn yellow while veins remain green (interveinal chlorosis). Usually caused by pH issues rather than an actual lack of iron.
Magnesium deficiency: Begins in lower, older leaves. You may see yellowing between veins, leaf curling and brown spots.
Calcium deficiency: Results in dead patches (necrosis), curled leaf tips and weak root systems.

Solution: Apply a full-spectrum cannabis fertiliser. Supplement with products like Cal-Mag if needed. Adjust your feeding schedule to match your plant’s growth stage.

Improper watering

Cannabis plants need a balanced watering routine. Both overwatering and underwatering can lead to yellow leaves.

Overwatering: Drenches the roots, reducing oxygen intake. Causes limp, yellow leaves and root rot.
Underwatering: Leaves become dry, brittle, and yellow due to dehydration.

Solution: Only water when the top inch of soil is dry. Ensure your containers have drainage holes. Use moisture metres if needed.

pH imbalance

Even with adequate nutrients, an incorrect pH can cause lockout and yellowing.

Soil pH: Should stay between 6.0 and 7.0.
Hydroponic pH: Should remain between 5.5 and 6.5.

Solution: Regularly test your runoff or reservoir pH. Adjust using pH up/down solutions. Buffered soils help maintain a stable pH.

Light stress

Improper lighting can harm your cannabis plants.

Light burn: Happens when lights are too close, bleaching or curling top leaves.
Light deficiency: Lower leaves yellow due to insufficient exposure.

Solution: Set proper light distance based on your light type (LED, HID, CFL). Rotate plants and trim lower growth for better light penetration.

Pests and diseases

Various pests and pathogens can cause yellowing in cannabis.

Spider mites: Leave tiny yellow dots on leaves.
Fungus gnats: Larvae damage roots, stunting growth.
Root rot: Caused by fungi like Pythium, leading to dark, soft roots.
Powdery mildew: Starts as yellow spots and turns into a white, powdery fungus.

Solution: Use organic pest control like neem oil or predatory mites. Add beneficial microbes to your root zone. Control humidity and airflow to prevent fungal outbreaks.

Environmental stress

Unstable growing conditions can quickly lead to chlorosis.

High temperatures: Over 29°C (85°F) disrupt nutrient absorption.
Low temperatures: Below 15°C (60°F) slow plant metabolism.
Humidity swings: Inconsistent VPD disrupts transpiration and health.

Solution: Keep temperatures between 21–27°C (70–80°F). Use a grow room controller to maintain proper RH and VPD levels.

Diagnosing based on growth stage

Seedling stage

Yellowing here often results from overwatering, inadequate lighting, or poor-quality soil.

Vegetative stage

As the plant grows rapidly, deficiencies in nitrogen or magnesium become more noticeable.

Flowering stage

Some yellowing of lower leaves is normal, but widespread discolouration may suggest nutrient issues or stress.

Pro tip: Always start diagnosing from the roots. Check growing media, pH, and environment before adjusting nutrients.

When yellowing is normal

Yellowing isn’t always a cause for concern:

• During late flowering, plants naturally reabsorb nutrients from old leaves.

• A couple of yellow leaves isn’t unusual.

• However, fast-spreading yellowing, especially in new growth, needs immediate attention.

Preventative measures

• Routine checks: Inspect plants daily. Look beneath leaves, check soil moisture and stem health.

• Integrated pest management: Introduce companion plants, neem oil or beneficial insects.

• Flush schedule: Flush the grow medium every few weeks to avoid nutrient buildup.

• Strong genetics: Choose resilient strains. Our easy-to-grow range is ideal for growers looking for robust, forgiving plants.

Case study: Indoor vs outdoor yellowing

Indoor growers may see yellowing from excessive light or dry air. Outdoor growers often face pests and weather stress.

Example: A grower in Manchester resolved nitrogen deficiency by switching to a balanced vegetative fertiliser with added iron chelate.
Another grower in Cornwall stopped yellowing by repotting root-bound plants into larger containers and using compost tea.

These real-world cases show how common problems can be solved with practical steps.

Advanced troubleshooting tips

• Leaf tissue analysis: Send leaves to a lab to identify specific nutrient shortages.

• Soil testing: Evaluate nutrient content, pH and salt levels.

• Microbial products: Use beneficial fungi like mycorrhizae and bacteria like Bacillus subtilis to enhance nutrient uptake.

• Foliar sprays: Apply magnesium sulphate (Epsom salt) or kelp extract when early signs of yellowing appear.

Final thoughts

Yellow cannabis leaves are an early sign that something needs attention. Whether the cause is nutritional, environmental or biological, spotting it early makes all the difference. With proper monitoring, preventive care and quality genetics, you’ll keep your crop healthy.

Want to get started with stronger plants from day one? Explore our collection at Mavericks Genetics UK and grow with confidence.