Plants communicate their well-being through various signs, and yellowing leaves are a common indicator of distress. This condition, known as chlorosis, occurs when leaves produce insufficient amounts of chlorophyll, the green pigment responsible for photosynthesis. Chlorophyll absorbs sunlight to convert carbon dioxide and water into energy, so a reduction in its production directly impacts a plant’s ability to thrive.
Watering Imbalances
Improper watering practices are a frequent reason for leaves turning yellow. Both excessive watering and insufficient watering can lead to chlorosis.
Overwatering often results in waterlogged soil, depriving roots of oxygen. When roots cannot “breathe,” they suffocate and lose their ability to absorb water and nutrients, leading to root damage or rot. Symptoms typically include mature leaves turning brownish-yellow, appearing wilted or limp, and feeling mushy. Fungal growth or fungus gnats can also indicate persistently wet soil.
Conversely, underwatering causes dehydration, as plants cannot take up essential nutrients without adequate moisture. Leaves on underwatered plants may curl inwards, and the soil can appear dry or pull away from the pot edges. The newest, youngest leaves might wilt and turn yellow first.
Nutrient Shortages
A lack of specific nutrients can also lead to chlorosis, as many elements are necessary for chlorophyll synthesis and overall plant function. The pattern and location of yellowing often provide clues about which nutrient is deficient. Soil pH plays a significant role in nutrient availability; if the pH is too high or too low, certain nutrients become unavailable for absorption by the roots, even if present in the soil. Most nutrients are optimally available when soil pH is between 6.0 and 7.5.
Nitrogen deficiency typically causes a general yellowing, beginning with older leaves closest to the stem. This occurs because nitrogen is a mobile nutrient, and the plant relocates it from older foliage to support new growth. Affected leaves may also appear pale green before turning yellow, and plant growth can be stunted.
Iron deficiency, also known as “lime-induced chlorosis,” often presents as interveinal yellowing where the leaf tissue turns yellow but the veins remain green. This symptom usually appears first on younger, newly emerging leaves because iron is not a mobile element within the plant. High soil pH (above 6.5) can render iron unavailable for absorption, even if it is present in the soil.
Magnesium deficiency similarly causes interveinal yellowing, but it typically starts on older leaves. Magnesium is a mobile nutrient, so the plant moves it to support newer growth, leaving older leaves with a deficit.
Environmental Stress Factors
External environmental conditions can significantly impact a plant’s health, leading to yellow leaves as a sign of stress. Light is a primary factor, as both too little and too much can cause chlorosis. Insufficient light limits photosynthesis, reducing chlorophyll production and often resulting in pale, yellow leaves. Excessive direct sunlight can lead to sun scorch, where leaves may yellow, develop white spots, or have crispy brown edges.
Temperature extremes also contribute to plant stress and yellowing. Cold temperatures, especially sudden drafts, can cause leaves to turn yellow, particularly in sensitive tropical plants. Prolonged cold exposure might result in brown leaves rather than yellow. High temperatures and heat stress can also cause foliage to yellow and wilt, as plants struggle to pull water and nutrients through their roots in dry conditions.
Sudden changes in a plant’s environment, such as relocation or repotting, can induce stress. This “transplant shock” can manifest as wilting and yellowing leaves as the plant struggles to adapt to new light, temperature, or soil conditions. The disruption to the root system during transplanting reduces the plant’s ability to absorb water and nutrients, contributing to chlorosis.
Pests and Pathogens
Biological threats like pests and pathogens can also cause plant leaves to yellow, indicating damage or disease. Pests such as spider mites, aphids, and whiteflies feed on plant sap, directly harming the leaves and interfering with their ability to photosynthesize. This feeding activity can result in yellow spots or widespread chlorosis on the affected foliage.
Plant diseases, including fungal, bacterial, and viral infections, can manifest with yellowing as a symptom. Fungal diseases like root rot, often exacerbated by overwatering, can cause leaves to yellow and wilt as the diseased roots lose their function. Viruses can also lead to chlorosis by disrupting the plant’s normal functions and chlorophyll development. Identifying pests or disease often involves looking for physical signs on the leaves or examining the roots.
Natural Leaf Cycle and Other Causes
Beyond specific stresses, some yellowing is a natural part of a plant’s life cycle. As plants mature, older leaves naturally yellow and drop off, allowing the plant to redirect energy and resources to newer growth. This process is particularly noticeable if the yellowing starts on the lowest, oldest leaves and progresses upwards without other signs of distress.
Other causes for yellowing include chemical exposure and poor drainage. Herbicide drift or over-fertilization can cause leaves to turn yellow or brown due to chemical toxicity. Poor drainage, even without overwatering, can lead to root issues and yellowing by creating anaerobic conditions that inhibit root function.