Hydrangeas are popular garden shrubs known for their lush foliage and large, colorful blooms. When leaves and flowers turn brown, it signals that the plant is under significant stress. Browning is a symptom of distress, not a disease, making diagnosis the first step toward recovery. Identifying the specific cause, which can range from environmental factors to biological threats, is necessary for effective treatment.
Browning Caused by Water Imbalance and Heat Stress
The most frequent causes of browning relate directly to how the plant manages water. Under-watering, or drought stress, causes the plant to sacrifice its farthest tissues to conserve moisture. This manifests as crispy, dry, brown edges and tips on the leaves, which can quickly progress to wilting if the soil remains dry. Hydrangeas have shallow root systems and require consistent moisture, especially during peak summer temperatures.
Conversely, overwatering causes root rot, where roots are deprived of oxygen in saturated soil and begin to die. Dead roots cannot absorb water, resulting in the paradoxical symptom of a wilting plant in wet soil. Root rot browning often appears as sudden collapse or dark, mushy brown spots. Ensuring proper soil drainage is necessary because the plant cannot survive if its roots are constantly submerged.
Extreme heat and direct sunlight can lead to leaf scorch, another form of dehydration-related browning. Sun scorch appears as large, dry, brown or bleached patches, often on the leaves at the top or the sunniest side of the shrub. Cultivars like mophead hydrangeas are sensitive and may develop brown edges when exposed to harsh afternoon sun. High heat increases water loss faster than the roots can supply it, causing the tissue to die.
Browning Caused by Fungal Infections and Pests
Browning can also be triggered by living organisms, primarily fungi that thrive in warm, humid conditions. Fungal leaf spot, often caused by the Cercospora fungus, presents as small, distinct brown or purplish spots. These spots typically begin on the older, lower leaves and spread upwards, causing the infected foliage to yellow and drop prematurely. Anthracnose is another common fungal disease that creates large, irregular brown spots, sometimes affecting the flowers as well as the leaves.
If the browning appears as a fuzzy gray mold on the flowers or irregular brown blotches on the leaves, the cause may be Botrytis blight. This fungus is exacerbated by poor air circulation and excessive moisture on the foliage, often appearing after heavy rain or overhead watering. The presence of tiny, sap-sucking pests like spider mites can also lead to browning. Mite damage begins as fine stippling, but the collective cell damage causes the leaves to take on a bronzed or brownish appearance.
Accidental Chemical and Physical Damage
Sometimes, browning is external and introduced through gardening practices or chemical exposure. Fertilizer burn occurs when excessive fertilizer salts accumulate in the soil, drawing moisture away from the roots and leaves through osmosis. This chemical dehydration results in brown, scorched margins and tips on the foliage, which can be severe enough to injure the roots. Using a slow-release formula or limiting applications prevents this type of damage.
Chemical overspray, specifically from herbicides used nearby, can cause significant leaf distortion and browning. Herbicide drift symptoms may include shriveled or cupped leaves, stunted growth, and irregular, scorched brown patches. Physical trauma, such as transplant shock, can also lead to localized browning or wilting as the plant struggles to establish a new root system while supporting its foliage.
How to Diagnose the Issue and Promote Recovery
Determining the exact cause requires careful inspection of the plant and its environment, starting with the soil. Insert your finger a few inches deep to check moisture levels; bone-dry soil suggests drought stress, while saturated soil points toward root rot. Examine the location and pattern of the damage, as crispy edges are typical of water stress, while distinct spots usually indicate a fungal infection. Look closely under the leaves and on the stems for signs of pests, such as fine webbing or sticky residue.
Once the probable cause is identified, immediate action is necessary to stabilize the shrub. If the plant is dehydrated, provide a deep, slow watering and apply organic mulch to retain soil moisture. For suspected root rot, cease watering and improve drainage by amending the soil or relocating the plant. If fungal spots are present, prune and discard the affected leaves to reduce spore spread, and water the plant at the base to keep the foliage dry. Avoid applying additional fertilizer until the plant shows signs of recovery.