Hydrangeas are beloved shrubs known for their lush foliage and spectacular, long-lasting blooms. When the normally vibrant green leaves begin to turn yellow (chlorosis) or develop brown, necrotic patches, it signals that the plant is under stress. These symptoms are the most frequent issues gardeners encounter, indicating that a fundamental need is not being met. Pinpointing the cause requires careful observation, as the location and nature of the discoloration often reveal the underlying problem, whether it stems from improper care, soil chemistry, or external threats.
The Critical Balance of Soil Moisture and Drainage
The most frequent cause of hydrangea distress is an imbalance in water management, leading to both yellowing and browning. Hydrangeas require consistently moist but not saturated soil, a balance often disrupted by overwatering or underwatering.
Overwatering leads to a lack of oxygen in the root zone, causing root suffocation and root rot, which prevents nutrient absorption. Symptoms include overall yellowing, often starting with older, lower foliage, combined with a limp, drooping appearance. Browning often develops along the edges, sometimes accompanied by mold growth on the soil surface.
In contrast, underwatering causes leaves to lose moisture faster than the roots can supply it, leading to desiccation. This stress presents as dry, brittle, and crispy leaves, especially along the edges and tips. The plant may wilt in the heat of the day, but the soil will be completely dry several inches down. A deep, infrequent watering schedule is better than frequent, shallow watering.
Diagnosing Nutrient Deficiencies and Soil pH
If proper watering is established, yellowing leaves may point to a chemical imbalance within the soil, specifically related to nutrient availability. Hydrangeas are acid-loving plants, and the soil’s pH level governs which nutrients the plant can absorb.
When the soil pH is too high (alkaline), iron becomes unavailable to the plant. This results in interveinal chlorosis, where the newest leaves turn bright yellow or pale green, but the network of veins remains distinctly green. This pattern reliably indicates an iron deficiency, common for hydrangeas planted in alkaline soil.
A nitrogen deficiency affects the oldest leaves first, causing them to turn a uniform pale yellow before eventually dropping. To correct pH issues, a soil test can confirm alkalinity. Amendments like elemental sulfur or aluminum sulfate can be added to lower the pH, making micronutrients accessible to the roots. A foliar application of chelated iron can provide a temporary boost to the leaves.
Environmental Stressors and Leaf Scorch
External atmospheric conditions, particularly sun intensity and temperature, can directly cause leaf browning. Leaf scorch is an abiotic disorder that occurs when the rate of water loss through the leaves exceeds the roots’ capacity to supply water.
This issue presents as browning, crisping, and necrosis along the leaf edges and tips, often on the side most exposed to the sun. Intense, direct afternoon sun drives this rapid water loss, causing the leaf tissue to burn and dry out. Hydrangeas perform best when they receive morning sun and are protected from the afternoon sun.
Sudden temperature drops, such as a late spring frost, cause immediate and localized damage. Frost injury results in blackened, withered leaf margins or entire leaves, particularly on new growth. Providing afternoon shade or relocating the plant away from reflective surfaces can mitigate leaf scorch. Applying organic mulch helps regulate soil temperature and conserve moisture, reducing environmental stress.
Identifying Damage from Pests and Diseases
When yellowing or browning appears in distinct, patterned spots rather than a general discoloration, a biological agent like a pest or disease is the likely culprit. Fungal diseases are common in hydrangeas, especially in warm, humid conditions where air circulation is poor and leaves stay wet.
Cercospora leaf spot, a prevalent fungal issue, begins as small, circular spots that are purple or reddish-brown and may develop a tan or gray center. These spots have a yellow halo, and as the infection progresses, the leaves will yellow and drop prematurely, usually starting with the lower foliage. Removing infected leaves and avoiding overhead watering helps reduce the spread of fungal spores.
Pests like spider mites and aphids feed by piercing the leaf tissue. Spider mites thrive in hot, dry conditions and cause a speckled yellowing called stippling, confirmed by fine webbing on the undersides of the leaves. Aphids cluster on new, tender growth; their feeding causes leaf distortion and yellowing, leaving behind a sticky residue called honeydew. These pests can be managed by a strong spray of water or an application of horticultural oil or insecticidal soap.