The appearance of yellow leaves on your geranium is a sign of chlorosis. This condition occurs when a plant leaf produces insufficient chlorophyll, the green pigment necessary for photosynthesis. Chlorosis is not a disease but a symptom, indicating an underlying problem with the plant’s environment, care routine, or health. Determining the exact cause requires careful observation of where the yellowing is occurring and the conditions the plant is experiencing.
Moisture Levels and Root Health
Improper watering is the most frequent reason geranium leaves begin to yellow, as these plants prefer slightly drier conditions. Overwatering suffocates the roots by displacing oxygen in the soil, leading to root rot. Compromised roots cannot effectively absorb water or nutrients, causing the older, lower leaves to turn yellow and sometimes feel mushy.
Lack of proper drainage contributes to overwatering, so containers must have functional drainage holes. Conversely, severe underwatering also causes yellowing as the plant stresses to conserve moisture. This yellowing often begins at the leaf edges and tips, which may become dry and crispy.
To diagnose a moisture issue, check the soil one inch below the surface before watering. Geraniums should only receive water when the soil at this depth feels dry. If the soil is consistently soggy, allow it to dry out and reduce the watering frequency.
Nutrient Shortages
If watering and drainage are ruled out, a shortage of necessary minerals may be the culprit. The most common deficiency is nitrogen, a mobile nutrient that the plant moves from older leaves to support new growth. This causes the older, lower leaves to turn uniformly pale green or yellow first.
Deficiencies in immobile nutrients like iron or magnesium present a different visual pattern, typically affecting newer foliage first. Iron deficiency results in interveinal chlorosis: the tissue between the veins turns yellow while the veins remain green. Magnesium deficiency also causes interveinal yellowing, but it usually starts on older leaves and progresses inward from the margins.
The soil’s pH level can prevent nutrient uptake, even if nutrients are present. If the soil is too alkaline, iron can become insoluble and unavailable to the roots. A soil test confirms whether the issue is a true lack of nutrients or an accessibility problem caused by soil chemistry.
Environmental Stressors
Geraniums require adequate light, and insufficient sunlight can cause the leaves to turn yellow and the plant to become spindly. If a plant receives too much shade, the leaves may become uniformly pale as chlorophyll production is reduced. Relocating the plant to an area with at least partial sunshine, or full sun in cooler climates, often corrects this issue.
Sudden temperature fluctuations can also induce yellowing, often seen after a cold snap or when plants are moved outdoors too early. Geraniums do not tolerate cold temperatures well, leading to stress and leaf discoloration. This physiological response is a sign of shock, not a disease or nutrient issue.
Natural senescence causes yellowing, usually seen on the lowest leaves. As the geranium grows, the oldest leaves naturally yellow and drop off as the plant redirects energy to new growth and flowers. If yellowing is limited to a few of the oldest leaves, it is a normal part of the plant’s life cycle.
Identifying Pests and Pathogens
Yellowing can result from a biological attack by insects or disease-causing organisms. Common pests like spider mites, whiteflies, or aphids feed on the plant sap, damaging the leaf tissue. This feeding causes stippling or a mottled yellow pattern on the leaves. Inspecting the undersides of the leaves confirms the presence of these insects or their sticky honeydew residue.
Fungal and bacterial diseases also manifest as yellowing, often with other distinct visual cues. Some fungal leaf spot diseases begin with small, discolored lesions surrounded by a yellow halo. Rust is another fungal pathogen that causes yellow spots on the upper leaf surface with corresponding rusty-brown pustules underneath.
Good air circulation and avoiding overhead watering help mitigate conditions favorable for fungal growth. If a disease is suspected, promptly removing and destroying the affected leaves prevents the pathogen from spreading to the rest of the plant.
Summary and Next Steps
The systematic diagnosis of geranium chlorosis begins with assessing moisture levels, as water issues are the most frequent cause. If the soil is appropriately moist, the next step is to examine the pattern of yellowing to differentiate between nutrient deficiencies and environmental stresses. Finally, inspect closely for insects or signs of disease to complete the diagnostic picture. Addressing the root cause is necessary to halt the progression of yellowing and restore the plant’s natural green color.