Why Your Cannabis Leaves Turning Yellow and How to Fix It

The occurrence of yellow leaves in commercial cannabis cultivation in Thailand is a common but highly concerning issue. It is often a distress signal from the plant, indicating problems with the growth environment or internal physiology. Systematic diagnosis and resolution are required, with the following key steps and response plans:

 

Quick diagnosis: Look at the type and location of yellow leaves

Yellowing old leaves (bottom leaves):

High probability of nitrogen deficiency (N): Nitrogen is a mobile element, and when lacking, the nitrogen from old leaves will be transferred to new leaves, resulting in uniform yellowing of old leaves.

Solution: Supplement nitrogen-containing fertilizers (such as ammonium calcium nitrate, fish manure) and pay attention to balancing the ratio of nitrogen, phosphorus, and potassium.

 

New leaves turn yellow (top tender leaves):

Iron (Fe), zinc (Zn), or sulfur (S) deficiency: yellowing between new leaf veins (veins still green) is common due to iron deficiency; The overall uniform yellowing may be sulfur deficient.

Solution: Spray chelated iron (EDTA Fe) or foliar fertilizer containing trace elements; Check the soil pH value (see below).

 

Local yellow spots/edges on the leaves:

Fungal/bacterial diseases (such as leaf spot disease and downy mildew): prone to occur in humid environments, with yellow spots accompanied by brown spots and mold layers.

Pest infestations (such as red spider mites and thrips): Insect bodies or spider webs can be seen on the back of the leaves, and the leaves show spotted chlorosis.

Solution: Isolate the diseased plants and spray fungicides (such as Bacillus subtilis, copper agents) or insecticides (such as azadirachtin, pyrethroids).

 

Core environmental factor investigation

1. Imbalance of soil/substrate pH (the most common cause!)

Question: The optimal pH for cannabis cultivation in soil is 6.0-7.0. Soilless cultivation (coconut coir/hydroponics): 5.5-6.5. Exceeding the range will lock in nutrients (such as iron and manganese that are difficult to absorb when pH>7).

Solution:

Immediately use a pH pen to detect irrigation water and substrate exudate.

Adjust pH: If it is too high, use citric acid/phosphoric acid to lower it; Adjust to a higher pH with potassium hydroxide/lime if it is too low.

Follow up: Adjust the water pH before each irrigation, and regularly test the substrate.

 

2. Improper watering

Overwatering: The Root system lacks oxygen, old leaves turn yellow and wilt, and roots turn brown and rot.

Drought stress: Leaves turn yellow and droop overall.

Solution:

Following the principle of 'see dry, see wet', insert your fingers into the soil 2-3 centimeters until it is completely dry before watering.

Improve drainage (add perlite/vermiculite) to avoid water accumulation at the bottom of the basin.

 

3. Lighting issue:

Excessive light: Leaves turn yellow and dry (especially at the top), often seen when LED lights are too close or when exposed to strong tropical light.

Insufficient light: The lower old leaves turn yellow and fall off due to the inability to carry out photosynthesis.

Solution:

Adjust the height of the lighting fixture (keep the LED at 30-60 centimeters); Sunshade net for high temperature days (30% -40% shading rate).

Regularly rotate the plants to ensure even light exposure.

 

Key points of nutrition management

Scientific Fertilization:

Avoid excessive fertilization (salt damage): The leaf tips are burnt yellow and curled, and there are white salt crystals on the surface of the substrate.

First aid: Rinse the substrate with a large amount of pH 6.0 water (2-3 times the volume of the basin).

Regular measurement of EC value (conductivity): Soil EC should be maintained at 1.2-2.0 mS/cm, and 1.0-1.8 mS/cm for soilless cultivation.

Increase application of phosphorus and potassium during the flowering period: yellowing of old leaves during the flowering period may be a normal metabolism, but it is necessary to ensure sufficient phosphorus and potassium (such as the 0-10-10 formula).

 

Trace element supplementation:

Thailand is prone to magnesium (Mg) and calcium (Ca) deficiency during the high temperature and rainy season, manifested as yellowing between leaf veins.

Plan: Regularly spray calcium magnesium foliar fertilizer (such as 1g/L magnesium sulfate+chelated calcium), or soil apply dolomite powder.

Environmental optimization and prevention

 

Temperature and humidity control:

Ideal temperature range: 24-28 ° C during the day and 18-22 ° C at night.

Ideal humidity range: 40-70% during growth and 40-50% during flowering. Using a good dehumidifier for grow room will do you a favor. Preair is a reliable dehumidifier manufacturer that sells various dehumidifier of good quality.

High temperature countermeasures: ventilation+sunshade+spray cooling (avoid spraying water during flowering to prevent mold).

 

Root health maintenance:

Regularly adding microbial agents (such as arbuscular mycorrhizal fungi and Bacillus subtilis) to enhance stress resistance.

Avoid substrate compaction, rotate crops every season, or disinfect soil (steam/solar disinfection).

 

In general

Starting with pH and watering, 80% of yellow leaf problems can be easily solved. If accompanied by disease spots and insect traces, quickly isolate the diseased plant to avoid complete annihilation!