The appearance of yellow leaves on a hydrangea, known as chlorosis, is a common signal that the plant is under stress. Chlorosis is not a disease itself, but rather a symptom indicating an underlying issue that interrupts the production of chlorophyll, the pigment responsible for the green color of the foliage. While a few yellowing leaves on the lower, older parts of the plant can be normal as the plant sheds them, widespread or patterned yellowing on new growth suggests an environmental or nutritional problem. Understanding the specific pattern of this discoloration is the first step toward diagnosing and resolving the root cause.
Nutrient Availability and Soil pH
The most frequent culprit for chronic leaf yellowing relates to the soil chemistry, specifically the pH level, which dictates nutrient accessibility. Hydrangeas prefer a slightly acidic soil, ideally with a pH between 5.0 and 6.5, to properly absorb essential micronutrients like iron. When the soil becomes too alkaline, typically above a pH of 7.0, a condition known as lime-induced chlorosis occurs. Iron is present in the soil but is chemically locked into a form the plant cannot take up through its roots.
This iron deficiency is characterized by interveinal chlorosis, where the newest leaves turn bright yellow or pale green while the veins remain distinctly dark green. Since iron is an immobile nutrient, the plant cannot relocate it from old growth to new growth, so the symptoms appear first and most severely on the youngest leaves. A simple soil pH test is required to confirm this diagnosis.
A different pattern of yellowing may point toward a mobile nutrient deficiency, such as nitrogen or magnesium. These nutrients can be moved by the plant from older leaves to support new growth, meaning the symptoms appear first on the older, lower foliage. Nitrogen deficiency causes a more generalized, uniform pale yellowing across the entire older leaf. Magnesium deficiency also starts on older leaves, presenting as yellowing between the veins, sometimes showing a green “arrowhead” shape near the midrib.
Improper Watering and Drainage
Consistently saturated soil is a primary cause of stress that leads to yellowing leaves. Overwatering or poor soil drainage suffocates the fine root hairs, preventing them from absorbing oxygen and nutrients. This root stress quickly leads to a general yellowing and wilting of the leaves. The foliage may also feel limp, and the yellowing can progress to brown, mushy edges as root rot sets in.
Conversely, underwatering causes the plant to conserve moisture by sacrificing its leaves. This typically manifests as wilting, followed by yellowing that often includes dry, crispy, or brown edges and tips. The key distinction is the soil condition: an underwatered plant will perk up quickly after a deep watering, while an overwatered plant remains wilted in soggy soil. Improving the soil structure with organic matter or adjusting the irrigation schedule is necessary to resolve these moisture-related issues.
Light Stress and Biological Factors
External environmental conditions and biological threats can also trigger chlorosis in hydrangeas. Light stress, particularly excessive exposure to intense afternoon sun, can cause a condition called leaf scorch. This stress results in yellowing or bleached-looking patches, often accompanied by irregular dry, brown blotches or crispy margins, typically appearing on the leaves most exposed to the sun. Moving the plant to a location that receives dappled shade, especially during the hottest part of the day, can prevent this damage.
A closer inspection of the leaves may reveal biological factors such as pests or fungi. Pests like spider mites feed on the chlorophyll, causing a fine, stippled yellowing that progresses to a bronze or overall yellow appearance. Fungal issues, such as leaf spot diseases, usually present as distinct yellow or brown spots on the leaves, which can eventually lead to broader chlorosis and leaf drop. Addressing these issues often involves pruning the affected foliage and, if necessary, applying an appropriate treatment.
Immediate Steps for Diagnosis and Recovery
The most effective approach to recovery begins with accurately identifying the cause, which often requires a simple soil pH test. If the soil pH is found to be above 6.5, the yellowing is likely iron chlorosis. For a fast, temporary fix, a foliar spray of chelated iron can be applied directly to the leaves, providing an immediate dose of the micronutrient for quick green-up.
For a long-term solution to high soil pH, amendments like elemental sulfur or aluminum sulfate should be added to the soil to gradually increase the acidity. If the diagnosis points to a moisture issue, checking the soil saturation by hand is the fastest diagnostic tool. If the soil is heavy and saturated, improving drainage with organic matter is necessary, and watering should be withheld until the top inch of soil is dry. Removing severely yellowed or damaged leaves will improve the plant’s appearance and focus its energy on producing new, healthy growth.