Chlorosis is the term for yellow foliage on a hydrangea, indicating the plant is struggling to produce chlorophyll. Chlorophyll is the green pigment necessary for photosynthesis; its absence means the plant cannot convert sunlight into energy. Chlorosis is a symptom, not a disease, requiring investigation into the root cause, usually starting with the growing environment. Since hydrangeas need consistent moisture and are sensitive to soil conditions, chlorosis is typically related to water management or nutrient availability.
Issues Related to Soil Moisture and Drainage
Improper watering is a frequent cause of yellowing leaves, as hydrangeas require consistently moist but not saturated soil. Overwatering leads to waterlogged soil and a lack of oxygen around the roots. This root suffocation prevents the plant from absorbing necessary nutrients and water, sometimes mimicking drought stress symptoms. Overwatered leaves often turn yellow, appear limp, and may wilt even when the soil is wet.
An underwatered hydrangea also exhibits yellowing, but the leaf texture provides a differentiating clue. Drought-stressed foliage typically becomes dry and crisp at the edges, and the entire plant will wilt severely. To determine the issue, check the soil moisture about four inches deep; soggy soil indicates overwatering, while dry soil indicates underwatering. Improving drainage by incorporating organic matter like compost is often necessary in heavy clay soils.
Nutrient Deficiencies and Soil Acidity
Beyond water management, chlorosis often signals an inability to absorb micronutrients, which is frequently mediated by soil pH. Hydrangeas thrive in slightly acidic soil, ideally with a pH between 5.5 and 6.5. When the soil becomes too alkaline (typically pH 7.0 or above), certain minerals become chemically locked up and unavailable. This is the most common cause of iron chlorosis.
Iron deficiency presents as interveinal chlorosis on the newest growth, where the tissue between the veins turns yellow while the veins remain dark green. Because iron is immobile within the plant, newer leaves show symptoms first as they cannot draw the nutrient from older foliage. Long-term correction involves lowering the soil pH using acidifying amendments such as elemental sulfur or iron sulfate. A temporary solution is a foliar spray of chelated iron, though this does not address the underlying soil chemistry.
Magnesium deficiency is another common nutrient problem, exhibiting a different yellowing pattern than iron. Magnesium is a component of the chlorophyll molecule and is mobile within the plant, so symptoms first appear on the older, lower leaves. The yellowing often begins at the leaf margins and moves inward, sometimes leaving a distinct green arrowhead pattern near the midrib. Applying magnesium sulfate (Epsom salts) can quickly remedy this deficiency. Avoid over-fertilizing with high-potassium products, as excessive potassium can interfere with magnesium absorption.
Environmental Stressors and Sun Exposure
Yellowing can arise from sudden environmental changes that stress the plant, independent of soil chemistry or watering habits. Hydrangeas receiving intense, direct afternoon sun, especially in hot climates, can suffer from sun scorch. This damage manifests as yellow, tan, or white splotches concentrated on the most exposed parts of the leaves. Providing afternoon shade or relocating the plant may be necessary to prevent solar damage.
Temperature extremes, particularly high heat, can disrupt the plant’s physiological processes, leading to yellowing or distortion of the foliage. Newly planted or moved hydrangeas may experience transplant shock, causing temporary yellowing and leaf drop as the root system struggles to establish. A few yellowing leaves at the base of the plant can also be a normal sign of natural leaf senescence as the plant sheds its oldest foliage or prepares for dormancy.
When Pests or Disease Are the Cause
While cultural issues are the most frequent culprits, pests and diseases can also result in leaf chlorosis. Spider mites thrive in hot, dry conditions and feed by sucking sap from plant cells. This feeding causes characteristic stippling—fine, yellow or white speckles—on the leaves, often accompanied by fine webbing underneath. A strong blast of water can dislodge the mites, and horticultural oil or insecticidal soap can treat severe infestations.
Fungal diseases, such as Cercospora leaf spot, typically present as distinct yellow or tan spots with darker borders, rather than uniform yellowing. Root rot, caused by persistent overwatering, is a fungal infection that leads to widespread yellowing and wilting. Consistent monitoring and ensuring good air circulation are important preventative measures against these pathogens.