Chlorosis, the yellowing of plant leaves, is a common symptom of distress in cannabis plants. This discoloration signifies a reduction in chlorophyll, the pigment responsible for capturing light. Yellow leaves are a diagnostic clue pointing to an underlying issue requiring investigation. The location of the yellowing—whether it appears on old or new leaves—is the first piece of information needed to identify the root cause.
Nutrient Deficiency and Excess
The most frequent cause of chlorosis relates to the plant’s nutrient intake, involving either deficiency or over-fertilization. Diagnosis requires understanding how nutrients move within the plant, classifying them as either mobile or immobile. Mobile nutrients, such as Nitrogen (N), Phosphorus (P), and Potassium (K), can be relocated from older leaves to support newer growth when supplies are low.
A Nitrogen deficiency is a classic example, manifesting as progressive yellowing that begins on the oldest, lowest leaves and moves upward. The entire leaf, including the veins, turns pale and may eventually wilt and drop off. Magnesium (Mg), a mobile secondary nutrient, presents differently, causing interveinal chlorosis where the tissue between the veins turns yellow while the veins remain dark green, also starting on older leaves.
In contrast, immobile nutrients like Iron (Fe) and Calcium (Ca) cannot be moved from where they are initially deposited. A deficiency in these elements will therefore appear first on the newest, upper leaves. Iron deficiency results in interveinal yellowing on the new growth, often giving the leaves a very pale or white appearance.
Excessive nutrient levels can also cause leaf discoloration, often referred to as “nutrient burn.” This toxicity usually presents as yellow or brown discoloration and crisping that starts at the tips of the leaves. If severe, this burning can spread inward from the edges and is frequently accompanied by a dark green coloration due to excess nitrogen.
The Critical Role of pH
Even if the correct nutrients are present, they may not be available if the pH level is outside the optimal range, a phenomenon known as nutrient lockout. This chemical imbalance prevents the root system from absorbing specific elements, leading to symptoms that mimic a deficiency even when fertilizer is available.
The ideal pH range varies depending on the growing medium used. In soil, roots best absorb nutrients within a range of approximately 6.0 to 7.0, which is naturally buffered. For soilless mediums like coco coir or hydroponic systems, a slightly more acidic pH range of 5.5 to 6.5 is preferred for optimal nutrient uptake.
When the pH deviates too far, certain nutrients become chemically unavailable. Iron and Manganese become less soluble and can be locked out when the pH is too high (alkaline). Conversely, a low pH (acidic) can prevent the uptake of elements like Magnesium and Phosphorus. Growers should regularly test the pH of their runoff water or nutrient solution and use pH up or pH down solutions to make precise adjustments.
Incorrect Watering and Root Zone Stress
Improper irrigation practices are common causes of yellowing because they directly impact root zone health. Overwatering is a frequent error that saturates the growing medium, driving out oxygen and causing roots to suffocate. This lack of oxygen inhibits root function, preventing the plant from absorbing water and nutrients, resulting in general, widespread yellowing, often starting in the lower leaves.
Overwatered plants typically display firm but droopy leaves, with the medium remaining soggy. Underwatering causes the plant to wilt, leading to leaves becoming dry, brittle, and yellow as the plant is starved of moisture. Implementing a proper “wet/dry cycle” is necessary, involving allowing the top layer of the medium to dry out before watering again to ensure roots receive adequate oxygen.
Environmental Factors, Pests, and Disease
External environmental stressors can also cause leaves to turn yellow, often presenting patterns distinct from nutrient or watering issues. Light burn, common with high-intensity LED lights, occurs when the light source is too close or too strong. This stress typically affects the topmost leaves directly beneath the light, causing them to turn yellow or white, sometimes with green veins.
Temperature stress, such as cold shock, can induce yellowing or purpling as the plant’s metabolic processes slow down. Pests and diseases also contribute to chlorosis, but often with a localized or speckled pattern. Spider mites feed on plant cells, creating tiny yellow or white speckles that can eventually cause the entire leaf to yellow. Fungal infections like root rot, often a consequence of chronic overwatering, cause a rapid decline and yellowing of the foliage due to root system destruction.