Hydrangeas are popular garden shrubs, prized for their lush foliage and impressive blooms. When the vibrant green leaves begin to turn red, purple, or maroon, it can signify a range of issues, from a natural seasonal process to a deficiency or environmental stressor. Understanding the appearance and timing of the color change is the first step in accurately diagnosing the plant’s health.
Seasonal and Genetic Color Changes
The appearance of red or purple foliage in hydrangeas is often a benign process related to natural chemistry. This color change is typically a sign of leaf senescence, the natural aging and preparation for dormancy. As temperatures cool in the late summer and autumn, the plant stops producing chlorophyll, allowing other pigments to become visible.
The red and purple hues are caused by the accumulation of anthocyanins, pigments the plant synthesizes as a form of natural sunscreen. These compounds protect the leaf’s photosynthetic machinery while the plant reabsorbs valuable nutrients before the leaves drop. Cool night temperatures, especially those above freezing, coupled with bright, sunny days, accelerate this production, leading to the most vibrant displays of autumn color.
Some hydrangea varieties, particularly certain Hydrangea macrophylla (Bigleaf) and Hydrangea quercifolia (Oakleaf) cultivars, are genetically predisposed to develop red or burgundy foliage regardless of the season. For these plants, the red color is simply a characteristic of the cultivar and not an indicator of stress. If the plant is otherwise healthy and the color change occurs uniformly in the late season, no intervention is necessary.
Addressing Nutrient Deficiencies
A persistent red or purple discoloration appearing earlier in the growing season, especially on older leaves, often points toward a struggle with nutrient uptake. The most common cause of this color change is a deficiency in phosphorus (P), a macronutrient needed for root growth and energy transfer. When phosphorus is limited, a purple discoloration, which can look distinctly reddish, appears on the lower, older leaves as the plant mobilizes the limited nutrient to new growth.
A phosphorus deficiency can occur if the soil lacks the element, but it is more frequently caused by a soil condition that prevents the plant from accessing available P. Cold soil temperatures in the spring can temporarily inhibit the roots’ ability to absorb the nutrient, which is why purple leaves are sometimes seen on young plants early in the season. A greater issue is a pH imbalance, as phosphorus becomes less available for uptake in highly alkaline or highly acidic soil.
A different deficiency that can cause reddish discoloration is a lack of magnesium (Mg), which presents with a more distinct pattern. Magnesium deficiency typically causes interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins remain green. In hydrangeas, this yellowing can be accompanied by red or purple margins, most often affecting the older leaves first.
Diagnosing a nutrient issue should begin with a professional soil test to determine the levels of P and Mg, and the soil pH. If a phosphorus deficiency is confirmed, applying a high-phosphorus or balanced fertilizer can help correct the imbalance. For a severe deficiency, a foliar application of a water-soluble fertilizer can offer temporary, immediate relief while waiting for soil amendments to take effect.
Environmental Stressors and Management
Environmental factors can trigger a red or bronze discoloration as a protective response. Sunscald, caused by excessive exposure to intense afternoon sun, is a common issue for hydrangeas, which generally prefer morning sun and afternoon shade. The red or bronze patches that appear are the plant’s way of producing anthocyanins to filter the excess light and shield the chlorophyll from damage.
Water stress is another frequent cause, stemming from either too much or too little moisture. Under-watering can cause the leaves to develop brown and crispy edges, which may be preceded by a reddish tinge as the plant attempts to conserve moisture. Conversely, overwatering can lead to root rot, which damages the roots and prevents them from absorbing both water and nutrients, resulting in symptoms similar to a nutrient deficiency.
Managing environmental stress involves assessing the plant’s location and irrigation schedule. Plants suffering from sunscald may need to be relocated to a shadier spot or provided with afternoon shade. Proper watering involves ensuring the soil is consistently moist but never waterlogged; checking the soil a few inches down before watering is the most reliable method. Applying a layer of organic mulch helps regulate soil temperature and moisture, reducing the likelihood of water-related stress.
Pest and Disease Triggers
Red or purple spots on hydrangea leaves, often with distinct borders, are frequently the result of a fungus or bacterium. Fungal leaf spots, like those caused by Cercospora, initially appear as small, circular, purple or reddish-brown spots on the lower leaves. These spots often develop a tan or gray center with a distinct reddish border as the disease progresses.
Another common fungal culprit is Anthracnose, which causes irregular-shaped brown or black leaf spots that may also have a red-brown margin. For both fungal diseases, the red color is the plant’s reaction to the infection, and these conditions are often promoted by warm, humid weather and overhead watering. Management involves removing and destroying infected leaves and avoiding overhead irrigation to minimize leaf wetness.
Pests can also contribute to a reddish discoloration, particularly spider mites, which thrive in hot, dry conditions. These tiny pests feed on the underside of the leaves, causing a fine stippling that may appear as bronzing or reddish color, and severe infestations can be identified by fine webbing. Viruses, though less common, cause mottled red or purple patterns that are untreatable, requiring removal of the entire plant to prevent spread.