Leaf yellowing, known as chlorosis, is the most frequently observed symptom of distress in Cannabis plants. This discoloration occurs when the plant cannot produce enough chlorophyll, the pigment responsible for capturing light energy and giving leaves their green color. Diagnosing the cause requires a systematic approach, as the location and pattern of the yellowing offer distinct clues about the underlying problem. The following guide provides a framework for identifying the root cause of this common issue.
Nutrient Deficiencies and pH Imbalances
The most frequent reason for leaves turning yellow is a lack of available nutrients, stemming from an actual deficiency or an inability to absorb existing nutrients. Nitrogen (N) deficiency is a common culprit because it is a mobile nutrient, meaning the plant can relocate it from older tissues to support new growth. Consequently, the yellowing begins on the oldest, lowest fan leaves, which turn pale green, then yellow, as the plant strips them of nitrogen reserves.
Yellowing patterns help distinguish other deficiencies. Magnesium (Mg) deficiency also presents on older leaves, but the yellowing appears between the leaf veins, a pattern called interveinal chlorosis, while the veins themselves remain green. This distinctive marbled look results from Mg’s role as the central atom in the chlorophyll molecule. Conversely, Iron (Fe) is an immobile nutrient, so its deficiency is first visible on the newest growth tips, which display interveinal chlorosis because the plant cannot relocate iron from older leaves.
A common issue that mimics a true lack of nutrients is pH lockout, where nutrients are present but unavailable to the roots. Cannabis plants thrive within a narrow pH range—typically 6.0 to 7.0 in soil or 5.5 to 6.5 in hydroponics. If the growing medium’s pH moves outside this optimal zone, chemical reactions prevent the roots from absorbing specific elements. For example, a high pH can impede the uptake of iron, leading to deficiency symptoms even when iron is abundant.
Issues Related to Watering and Root Health
Improper watering practices are a leading cause of chlorosis because they directly compromise the root system’s ability to function. Overwatering occurs when the growing medium remains saturated for too long, displacing the oxygen roots require for respiration. This lack of oxygen leads to root suffocation, causing generalized yellowing and a droopy appearance as nutrient and water transport becomes impaired. Prolonged overwatering can encourage root rot, where dead roots are colonized by pathogens, leading to a foul odor and widespread plant decay.
In contrast, underwatering causes the leaves to first wilt dramatically before they start to yellow and dry out. The lack of moisture causes the leaves to become limp and brittle, with yellowing often starting at the leaf edges and tips. While overwatering is a chronic problem that develops over days, underwatering can cause visible stress and yellowing quickly due to the rapid loss of turgor pressure.
A plant that has outgrown its container is considered rootbound, which restricts the root system’s ability to expand and absorb nutrients effectively. When roots are severely constricted, they cannot maintain optimal health, leading to symptoms that closely resemble nutrient deficiencies, including generalized chlorosis and stunted growth. Transplanting into a larger container is the only way to resolve this physical barrier.
Environmental Stressors and Light Damage
External environmental factors can also trigger leaf yellowing. Light burn occurs when light intensity is too high, often from grow lights placed too close to the plant canopy. This stress reaction causes the leaves closest to the light source, typically the newest growth at the top, to turn yellow or even white due to chlorophyll bleaching. This damage is distinct from a mobile nutrient deficiency because the yellowing is localized to the upper part of the plant.
Temperature extremes can similarly disrupt metabolic processes. Cold stress, such as a sudden drop in ambient temperature, slows down essential enzymatic activity, resulting in discoloration and yellowing as the plant struggles to photosynthesize efficiently. Excessive heat also stresses the plant, leading to accelerated water loss and metabolic slowdown, which manifests as yellowing or browning on the leaves.
Strong, continuous air movement, sometimes called wind burn, can physically damage the leaf surface and cause excessive transpiration. This constant movement can lead to patches of yellowing or browning where the tissue has been physically stressed or dried out. Maintaining a gentle but consistent airflow is important, as too much direct wind can be detrimental.
Pests, Diseases, and Natural Leaf Loss
Yellowing can also be a symptom of biological attack. Certain pests feed on the plant’s tissues, causing localized yellowing patterns. For instance, spider mites use piercing mouthparts to suck the contents out of individual plant cells, resulting in tiny yellow or white spots called stippling across the leaf surface. Thrips also cause discoloration and yellowing patches as they feed, often leaving behind silvery streaks on the leaves.
Fungal and bacterial diseases can cause widespread yellowing, often alongside other signs of decay. Leaf Septoria, a common fungal infection, causes yellow spots that expand into brown lesions, leading to premature yellowing and dropping of affected leaves. Root rot, a disease of the root system, ultimately causes systemic yellowing because the damaged roots cannot supply the plant with water and nutrients.
Finally, some yellowing is a normal part of the plant’s life cycle known as senescence. This natural aging process occurs late in the flowering stage, where the plant redirects mobile nutrients, like nitrogen, from the oldest fan leaves to the developing flowers. The lowest leaves will yellow and eventually drop. This is a sign of successful harvest preparation, not a deficiency, as long as it only affects the oldest foliage.