The vibrant foliage of Coleus, scientifically known as Plectranthus scutellarioides, makes it a popular ornamental plant prized for its intense color patterns. When the brilliant leaves begin to turn yellow, this discoloration is known as chlorosis, indicating an underlying physiological problem. Chlorosis occurs because the plant cannot produce sufficient chlorophyll, the green pigment necessary for photosynthesis. Identifying the specific pattern of this yellowing is the first step in diagnosing the stressor affecting the plant’s health.
Watering and Soil Moisture Imbalances
Improper watering is the most frequent cause of yellowing leaves, as the Coleus plant is particularly sensitive to fluctuations in soil moisture. Overwatering creates a saturated, anaerobic environment where the roots cannot access oxygen, leading to root suffocation and decay. This condition, often called root rot, causes the plant to fail at absorbing water and nutrients, resulting in soft, limp, yellowing leaves, typically starting with the older, lower growth. If the soil remains consistently soggy, check for drainage holes and allow the top inch of soil to dry out completely before watering again.
Conversely, underwatering causes the plant to conserve moisture by sacrificing older leaves, leading to yellowing that is often accompanied by dry, brittle leaf edges and tips. The entire plant may appear wilted, but the leaves will feel dry rather than mushy. To remedy this, the plant requires a deep soak until water runs out of the drainage holes, followed by establishing a consistent watering routine. Monitor the soil regularly, only reintroducing water when the top layer is dry to the touch.
Light Exposure and Temperature Fluctuations
Coleus colors are directly tied to light intensity, and extremes at either end of the spectrum can induce chlorosis. Too much direct, intense sunlight can overwhelm the chlorophyll, leading to a breakdown of the pigment and resulting in sun scorch. Symptoms include bleached, washed-out yellow or white patches, particularly on the upper leaves that are most exposed. Moving the plant to a location that receives bright, indirect light or morning sun only can prevent further damage.
In contrast, insufficient light causes the plant to uniformly reduce chlorophyll production across all leaves, resulting in a pale, generalized yellow-green color. The plant may also become “leggy,” stretching out with long stems between leaves as it attempts to reach a light source. To correct this, relocate the Coleus to a brighter spot, ensuring the light is filtered to avoid the bleaching effects of direct sun.
Coleus plants are tropical in origin and thrive best in stable temperatures above 50°F (10°C). Exposure to sudden cold drafts or prolonged cold temperatures can cause immediate stress and shock. This environmental strain manifests as yellowing and subsequent dropping of leaves. Keeping the plant away from drafty windows, air conditioning vents, or external doors helps maintain the stable, warm environment it requires.
Nutrient Deficiencies and Fertilization
A lack of specific mineral elements in the soil can disrupt the synthesis of chlorophyll, leading to distinct patterns of yellowing that point directly to the deficiency. Nitrogen (N) is a mobile nutrient, meaning the plant can relocate it from older tissues to support new growth. A nitrogen deficiency therefore causes uniform yellowing to begin in the older, lower leaves first while the new growth remains green.
If the yellowing appears between the leaf veins while the veins themselves remain green, this is known as interveinal chlorosis, which signals a micronutrient problem. When this pattern appears on the older, lower foliage, it often indicates a deficiency in magnesium (Mg). Magnesium is a central component of the chlorophyll molecule, and its lack impairs the photosynthetic process.
When interveinal chlorosis appears on the new or upper leaves, it frequently points to a deficiency in iron (Fe) or, less commonly, manganese. Iron is an immobile nutrient, meaning the plant cannot reallocate it from older leaves, so its effects are visible only in the newest growth. Ensuring the soil pH is maintained between 5.5 and 6.2 is important, as a pH that is too high can lock up iron and prevent its absorption by the roots.
Applying a balanced, water-soluble fertilizer, such as a 10-10-10 or 20-20-20 NPK ratio, is the most direct way to correct these nutritional imbalances. However, applying too much fertilizer can also cause yellowing, known as fertilizer burn. This happens when excess mineral salts draw moisture out of the roots, mimicking drought stress and causing leaf margins to turn brown or yellow.
Pest Damage and Natural Leaf Drop
Sap-sucking pests feed by piercing the leaf tissue and extracting the plant’s fluids, which causes localized damage that appears as yellowing. Spider mites, which thrive in dry conditions, leave behind tiny yellow or white stipples on the leaves, often accompanied by fine webbing. Aphids and whiteflies cluster on new growth and the undersides of leaves, causing the foliage to weaken, yellow, and sometimes distort.
Regularly inspecting the undersides of the leaves and the junctions where the leaf meets the stem allows for early detection of these small insects. Treating an infestation can involve washing the plant with a gentle stream of water or applying a horticultural product like insecticidal soap or neem oil to smother the pests.
Observing the oldest, lowest leaves turning yellow and occasionally dropping is a normal biological process called senescence. As the Coleus matures, it naturally sheds these leaves to divert energy to new stems and growth tips. If only a few old leaves exhibit this behavior, and the rest of the plant appears healthy and vibrant, no intervention is necessary.