Chlorosis is the condition where the bright green color of herbs fades to yellow. This yellowing is a symptom, not a disease, indicating the plant cannot produce enough chlorophyll for photosynthesis. When chlorophyll production is impaired, the plant’s health declines. Diagnosing the specific pattern of chlorosis—which leaves are affected first and where the yellowing appears—helps correct the underlying issue.
The Most Common Culprit: Soil and Hydration Issues
Improper watering is the most frequent cause of chlorosis in container herbs, often leading to root dysfunction. Overwatering saturates the soil, displacing air pockets necessary for root respiration. Roots suffocate without oxygen and may rot, limiting the plant’s ability to absorb water and nutrients. This causes widespread yellowing, often starting with older, lower leaves that may feel limp or mushy.
Conversely, underwatering also causes yellowing, but symptoms are distinct. Drought stress forces the plant to sacrifice older leaves to conserve moisture, resulting in dry, crispy edges or overall wilting. The soil is visibly dry, sometimes pulling away from the pot sides. Use the “finger test” to check moisture an inch or two below the surface; if it is bone-dry, the plant needs a deep soak.
Soil structure and drainage significantly influence hydration levels. Dense, compacted soil or blocked drainage holes mimic overwatering by keeping the root zone waterlogged. Poor drainage inhibits gas exchange and promotes root disease. Ensure the pot has functional drainage and the soil mix is light and porous to prevent root suffocation and chlorosis.
Nutrient Deficiencies and Imbalances
If watering is consistent, the yellowing may be a sign of a mineral deficiency, which can be identified by the pattern of the chlorosis.
Mobile Nutrients (Nitrogen and Magnesium)
Nitrogen (N) deficiency causes uniform yellowing across the entire leaf blade, starting with older, lower leaves. Nitrogen is a mobile nutrient, meaning the plant moves it from older tissues to support new growth. Magnesium (Mg) deficiency also causes interveinal chlorosis (yellowing between veins), and since it is mobile, this yellowing also begins on older, lower leaves.
Immobile Nutrients (Iron)
Other deficiencies cause interveinal chlorosis, where tissue between the veins turns yellow while the veins remain green. Iron (Fe) deficiency is an immobile nutrient, meaning the plant cannot relocate it from old leaves. Consequently, this yellowing appears first and most dramatically on the newest growth at the top of the plant.
Nutrients may be present in the soil but unavailable due to incorrect soil pH. If the soil is too alkaline (above pH 7.0), essential micronutrients like iron become chemically “locked up” and cannot be absorbed. Herbs prefer a slightly acidic to neutral pH (6.0 to 6.5) for optimal nutrient uptake. Testing the soil pH may reveal an imbalance, requiring correction before fertilizer application is effective.
External Stressors and Infestations
Environmental factors and pests can also trigger chlorosis. Insufficient light exposure, common for indoor herbs, leads to a pale, yellowish-green color across the entire plant because it limits chlorophyll production. Conversely, too much direct, intense sun can cause sun scald, appearing as bleached, white, or yellow patches on the leaf surface.
Sudden or extreme temperature fluctuations induce stress, causing leaves to turn yellow and drop. Herbs are sensitive to both cold drafts and excessive heat, which disrupt normal cellular function. Moving a plant abruptly into bright sunlight or placing it near a heating vent can cause this shock-induced chlorosis.
Yellowing can signal an infestation from sap-sucking pests that physically damage leaf cells. Pests like spider mites or aphids draw out plant fluids, leaving small, speckled yellow or white dots, known as stippling. This damage reduces the leaf’s photosynthetic area, leading to overall yellowing in severe cases. Regular inspection of the undersides of the leaves helps catch these pests early.