Nasturtiums are known for their vibrant, edible flowers and unique foliage. However, yellowing leaves are a common concern, indicating an underlying issue. Understanding the reasons behind this discoloration is the first step toward restoring your nasturtium to its healthy state.
Watering Imbalances
Improper watering is a frequent cause of yellowing nasturtium leaves. Both overwatering and underwatering can lead to similar symptoms. Nasturtiums prefer soil that is consistently moist but not waterlogged.
Excess water can lead to root rot due to a lack of oxygen, preventing roots from absorbing nutrients. This causes leaves to yellow, often appearing soft or mushy. Ensure pots have drainage holes or amend garden soil with organic matter to improve drainage. Insufficient water also causes leaves to yellow, often with a crispy texture and wilting. Nasturtiums need about an inch of water per week, though this varies with temperature. Allow the top inch of soil to dry before watering again.
Light Conditions
Nasturtiums need ample sunlight. Both inadequate light and excessive, harsh sun can cause leaves to yellow. These plants require at least six to eight hours of direct sunlight daily for optimal growth.
Insufficient light causes leaves to become pale and yellow as the plant struggles to produce chlorophyll, especially in shaded areas. Move container-grown nasturtiums to a brighter spot or ensure proper spacing for in-ground plants. Conversely, intense, scorching sunlight, particularly in hotter climates, can also yellow leaves or cause a withered appearance from sun scorch. Provide afternoon shade during the hottest parts of the day to protect plants.
Nutrient Deficiencies
While nasturtiums can thrive in less fertile soil, a lack of essential nutrients can still cause yellowing leaves. Both overly rich soil and a complete absence of nutrients can be problematic.
Nitrogen deficiency is a common cause, making older, lower leaves turn pale green and then yellow as the plant reallocates nitrogen to new growth. The plant may appear thin and weak. Other deficiencies, like magnesium or iron, also cause yellowing. Magnesium deficiency often shows as interveinal chlorosis on older leaves, where areas between veins yellow while veins remain green. Iron deficiency can cause similar interveinal yellowing but typically affects newer leaves. A balanced fertilizer (e.g., 15-15-15) can address general nutrient needs, though soil testing offers precise guidance.
Pests and Diseases
Pests and diseases can also cause yellowing nasturtium leaves. These stressors weaken the plant, leading to discoloration. Regular inspection helps with early detection and treatment.
Aphids are common pests that suck sap from leaves, causing them to yellow and distort. A strong water spray or insecticidal soap can manage infestations. Whiteflies, found on leaf undersides, also feed on sap, leading to yellowing and potentially black sooty mold. Certain bacterial and fungal diseases can cause yellowing. For example, bacterial leaf spot results in water-soaked lesions and yellowing, often due to high humidity and poor air circulation. Ensure good air circulation and avoid overhead watering to prevent fungal issues like powdery mildew.
Temperature Stress
Nasturtiums prefer cooler temperatures and react poorly to extreme heat or cold, often by yellowing. Prolonged exposure to high temperatures causes stress.
In hot weather, leaves may yellow and wither from heat stress. Move container plants to a shadier spot during peak heat. Cold temperatures and frost also damage nasturtiums, as they are frost-sensitive. In colder climates, they are grown as annuals, dying with the first hard frost. Protecting plants from unexpected cold snaps with covers can prevent yellowing. Nasturtiums grow best between 55°F and 75°F (13°C to 24°C).
Other Potential Causes and Prevention
Beyond environmental factors, pests, and diseases, other issues can cause nasturtium leaves to yellow. Sometimes, yellowing is a natural part of the plant’s life cycle. Older, lower leaves may yellow and drop as the plant prioritizes energy for new growth. This is a normal process unless widespread.
Transplant shock can also cause temporary yellowing, especially when nasturtiums are moved. They are sensitive to transplanting, so using peat pots to minimize root disturbance is recommended. Improper soil pH can hinder nutrient uptake, leading to yellowing. Nasturtiums prefer a slightly acidic to neutral soil pH, typically between 6.0 and 8.0. Choosing the right planting location with appropriate light and well-draining soil from the start can prevent many common problems.