The sight of bright green onion seedlings turning a pale, sickly yellow is a common worry for gardeners who start their plants indoors. This change in color, known as chlorosis, indicates the plant is struggling to produce enough chlorophyll, the pigment responsible for capturing light energy. While the symptom is always yellowing leaves, the underlying problem stems from various environmental factors or deficiencies. Understanding the location and pattern of the yellowing is the first step toward diagnosing and correcting the issue.
Nutrient Shortages in the Soil
Chlorosis in onion seedlings frequently signals that one or more macronutrients are unavailable in the soil. The most common cause of yellowing is a deficiency in nitrogen (N), a mobile nutrient within the plant. A lack of nitrogen first appears as a uniform yellowish-green color on the older, lower leaves as the plant moves the limited supply to new growth. If the deficiency is severe, these older leaves may also become thin, straight, and wilted.
Another nutrient involved in chlorosis is sulfur (S), a component of amino acids and proteins. Unlike nitrogen, sulfur is immobile, meaning a deficiency appears first on the newest growth at the top of the seedling. These upper leaves show a generalized, uniform yellowing. Corrective action for suspected nutrient deficiency involves a light application of a water-soluble, balanced liquid fertilizer or a diluted fish emulsion.
Improper Watering and Drainage
Water management problems are a frequent cause of chlorosis in indoor-started onion seedlings. Overwatering quickly leads to waterlogging, where excess moisture displaces air pockets in the soil. Deprived of oxygen, the roots suffocate and are damaged, preventing nutrient uptake and mimicking deficiency symptoms. Seedlings affected by overwatering typically sit in heavy, soggy soil and may display yellowing tips or soft bulbs.
Conversely, underwatering also causes yellowing and wilting, often presenting as dry, crumbly soil and crispy, brown tips on the leaves. The solution requires providing adequate drainage, ensuring the container has holes to allow excess water to escape. Maintaining an even moisture level—moist like a wrung-out sponge—is necessary for healthy root growth and nutrient absorption.
Temperature and Light Stress
Onion seedlings thrive within a cool temperature range for vegetative growth, ideally between 12°C and 24°C (55°F to 75°F). Exposure to temperatures below 0°C (32°F) can damage tender young stalks. Even without freezing, a cold snap slows the plant’s metabolism and temporarily impedes its ability to absorb nutrients, causing cold-induced yellowing.
Light is also important, especially for indoor seedlings. Inadequate light results in etiolation, causing the plant to grow tall and spindly with pale, weak growth as it stretches. Moving seedlings abruptly from a protected indoor environment into harsh, direct sunlight can cause sun scald or bleaching. When using artificial lights, placing them too far away (more than 10 to 15 centimeters) will not provide the intensity needed for robust, green growth.
Identifying Pests and Disease
While less common than environmental issues, biological factors can also cause onion seedlings to turn yellow. One fungal issue is Damping Off, a soil-borne disease that causes seedlings to collapse at the soil line. This problem is associated with overly wet soil and poor air circulation, which stresses the plants and allows the fungus to proliferate.
Pests like thrips are minute insects that feed on plant sap, causing stippling or silvery streaks that appear yellowed or discolored. Another disease, Fusarium basal rot, causes yellowing and wilting of the lower leaves, along with brown discoloration and rotting near the root plate. If yellowing is accompanied by twisting, stunting, or distinct lesions, it may indicate a viral or fungal infection requiring the removal of affected plants to prevent spread.