The sudden yellowing of cannabis leaves, a symptom known as chlorosis, is a common sign of plant stress. This discoloration occurs because the plant is no longer producing sufficient chlorophyll, the green pigment responsible for photosynthesis. Chlorosis is not a disease itself but rather a visual indicator of an underlying problem. Identifying the exact cause requires a systematic diagnostic approach, distinguishing the pattern, location, and speed of the yellowing to determine if the issue is environmental, nutritional, or biological.
Environmental Stressors: Water and Light Problems
Watering practices are often the first source of issues, as both extremes quickly lead to leaf yellowing. Overwatering saturates the growing medium, eliminating oxygen pockets around the roots and causing them to suffocate. This lack of oxygen leads to a generalized, dull yellowing and drooping of the leaves, which feel heavy and turgid. The remedy involves allowing the medium to dry out thoroughly and adjusting the watering frequency to prevent root rot.
Underwatering causes the plant to lose turgidity as it struggles to transport water. This leads to yellowing accompanied by a flimsy, papery texture in the leaves. The leaves may also become brittle and curl, and the soil will feel dry. This issue is solved by thoroughly rehydrating the growing medium. Differentiating the two often comes down to checking the weight and moisture level of the pot before watering.
Light intensity also contributes to chlorosis, most commonly manifesting as light burn. This occurs when the light source is too close or too powerful, causing the leaves at the top of the canopy to turn pale yellow or white. Unlike nutrient deficiencies, these light-burned leaves remain firm, with the yellowing concentrated only on the leaves closest to the light. Lower light levels, or light deficiency, can cause the opposite problem, resulting in a generalized pale yellow color and weak growth, particularly on the lower foliage shaded by the upper canopy.
Macronutrient Deficiencies: The Primary Causes
The most frequent cause of chlorosis stems from a lack of macronutrients, specifically elements required in large quantities like Nitrogen, Magnesium, and Potassium.
Nitrogen (N) Deficiency
Nitrogen (N) deficiency is arguably the most common and presents a distinct pattern because nitrogen is a highly mobile nutrient. The plant relocates nitrogen from older, lower leaves to support new growth at the top. This causes the older foliage to turn a uniform pale yellow before eventually falling off. The solution is to increase the nitrogen content in the feed solution to meet the plant’s high demand during the vegetative stage.
Magnesium (Mg) Deficiency
Magnesium (Mg) deficiency is characterized by interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain distinctly green. As a mobile nutrient, magnesium is relocated from older leaves to new growth, meaning symptoms appear first on the mid-to-lower sections of the plant. This distinct pattern relates to magnesium being the central atom in the chlorophyll molecule; without it, chlorophyll cannot be synthesized.
Potassium (K) Deficiency
Potassium deficiency typically begins on the older, lower leaves with yellowing or browning along the leaf margins and tips, often described as a scorched appearance. Potassium regulates water and nutrient transport and is highly mobile. The plant sacrifices the potassium in the oldest leaves to support the most active growth. This peripheral scorching differentiates it from the generalized yellowing of a nitrogen deficiency.
pH Imbalance and Micronutrient Lockout
Nutrient availability is governed by the pH level of the root environment, a factor that can cause deficiencies even if the nutrient is present in the medium. When the pH is too high or too low, the roots cannot absorb certain elements, a phenomenon known as nutrient lockout. For cannabis grown in soil, the optimal pH range is generally between 6.0 and 7.0. Soilless media like hydro or coco coir require a more acidic range, typically 5.5 to 6.5. Testing the pH of the feed water and the runoff is therefore a mandatory step in diagnosis.
A distinct category of deficiencies involves micronutrients, which are required in small amounts and are often immobile within the plant.
Iron (Fe) Deficiency
Iron deficiency is a primary example, presenting as interveinal chlorosis, much like magnesium deficiency, but it appears on the newest growth at the top of the plant. Because iron cannot be moved from old leaves, the new leaves suffer first, often turning extremely pale yellow or white while the veins remain green. This symptom on new growth is a strong indicator of a pH issue, as iron is easily locked out in alkaline conditions.
Other Immobile Micronutrients
Other immobile micronutrients, such as Zinc (Zn) and Manganese (Mn), also cause chlorosis that begins on new growth. Zinc deficiency causes interveinal yellowing and can lead to new leaves becoming distorted or wrinkled. Manganese deficiency also causes interveinal chlorosis on new growth, sometimes accompanied by small brown, necrotic spots. Since these micronutrients are susceptible to pH-related lockout, correcting the pH is the most effective solution to restore their uptake.
Biological Factors: Pests, Disease, and Natural Aging
Yellowing is not always a nutritional or environmental issue; it can also be a sign of biological stress or the plant’s life cycle.
Pests
Common pests, such as spider mites and thrips, cause yellowing by physically damaging the leaf tissue and extracting chlorophyll and sap. Spider mites leave behind a pattern of tiny, distinct yellow or white speckles, known as stippling, across the leaves, and in severe cases, fine webbing will be visible. Thrips also cause stippling and silvery patches, often accompanied by small black dots of fecal matter on the leaf surface.
Disease
Certain root diseases, like root rot, can also cause generalized yellowing and wilting that often mimics the symptoms of overwatering. The disease destroys the roots’ ability to absorb water and nutrients, leading to a sudden decline in plant health. Proper sanitation and ensuring excellent drainage are the best preventative measures against such pathogens.
Natural Aging (Senescence)
Yellowing can simply be a sign of the plant reaching the end of its life cycle, a process called senescence. During the late flowering stage, it is normal for older fan leaves to turn yellow and drop off. The plant naturally draws mobile nutrients, such as nitrogen, from this foliage to send to the developing flowers. This uniform, late-stage yellowing is not a sign of distress and is an intended part of the final maturation process.