The Mandevilla is a popular tropical vine, known for its glossy green foliage and showy, trumpet-shaped blooms. When leaves turn yellow, a condition known as chlorosis, it signals that the plant is under stress and unable to produce sufficient chlorophyll. This discoloration is not a disease but a visual symptom pointing to an underlying cultural, environmental, or nutritional issue.
The Impact of Improper Watering and Drainage
Watering errors are the most frequent cause of yellowing leaves in Mandevilla plants. Overwatering is particularly harmful because it saturates the soil, displacing oxygen and causing the roots to effectively drown and begin to rot. When root rot occurs, the damaged roots cannot absorb water and nutrients, mimicking the symptoms of a dry plant. Older, lower leaves turn soft, mushy, and yellow before dropping.
The plant may also exhibit soft, discolored stems at the base if the problem is severe and chronic. Conversely, if the Mandevilla is consistently underwatered, the plant conserves moisture by sacrificing foliage. The leaves will dry out, become brittle, and develop crispy, brown edges as the plant dehydrates.
To determine the true cause, use the “finger test” by pushing your finger a couple of inches into the soil. If the soil is soggy or wet at that depth, the plant is likely overwatered; if it is bone-dry, it needs immediate water. Mandevillas require consistently moist, but not waterlogged, soil, so ensuring the container has unobstructed drainage holes is paramount to prevent standing water from suffocating the roots.
Essential Nutrient Deficiencies and Soil pH
Mandevillas are considered heavy feeders, and the pattern of chlorosis often reveals which specific nutrient is lacking. A general yellowing that starts on the older, lower leaves and progresses upward is a classic sign of nitrogen deficiency. Nitrogen is a mobile nutrient, meaning the plant pulls it from older tissues to support new growth, causing those older leaves to yellow first.
A more distinct symptom is interveinal chlorosis, where the leaf tissue turns yellow while the veins remain a sharp green color. If this pattern appears on the plant’s newest growth, it generally indicates an iron deficiency. If the same interveinal yellowing appears on the older leaves, it is more likely a magnesium deficiency, as magnesium is also a mobile nutrient that is relocated to new tissues.
The availability of micronutrients like iron is highly dependent on the soil’s acidity. Mandevillas prefer a slightly acidic to neutral soil pH, ideally ranging from 6.0 to 6.5. If the soil becomes too alkaline (high pH), micronutrients can become chemically “locked up,” making them unavailable for the plant to absorb. A soil test kit can confirm pH levels, and applying a balanced, slow-release fertilizer or one formulated for acid-loving plants can help correct the imbalance.
External Stressors: Temperature, Light, and Pests
As tropical natives, Mandevillas are sensitive to sudden shifts in their external environment. Temperatures below 50°F (10°C) can cause cold shock, which results in leaf yellowing and dropping as the plant struggles to cope with the stress. Cold drafts or placing the plant too close to a cold window during winter can trigger this same reaction.
While the plant needs bright light, a sudden move into harsh, direct afternoon sunlight can lead to sun scald. This manifests as bleached, scorched, or spotty yellow patches on the leaves, which are damaged by the intensity of the light. Mandevillas thrive best when protected from the most intense midday sun, especially in hot climates.
Pests also cause yellowing by feeding on the plant’s sap. Spider mites cause a localized, stippled yellowing and may leave behind telltale fine webbing on the undersides of the leaves. Scale insects appear as small, immobile bumps on the stems and leaves, and their feeding can cause spotty chlorosis and the excretion of a sticky substance called honeydew. Initial treatment for these pests involves physically dislodging them with a strong jet of water or applying horticultural oil or insecticidal soap.