Why Seedling Leaves Turn Yellow and How to Fix It
When you first sprout vegetable seeds indoors, those delicate seedlings depend on the right conditions to thrive. One of the most common problems growers face is yellowing leaves—a sign that something in the seedling's environment needs adjustment. Understanding what causes this and how to correct it can mean the difference between healthy transplants and stunted growth.
The Science Behind Leaf Colour in Seedlings
Healthy seedling leaves are green because of chlorophyll, the pigment that captures light energy for photosynthesis. When leaves lose their green colour and turn yellow, it's usually because the plant isn't producing enough chlorophyll or can't access the nutrients it needs to maintain it. This process is called chlorosis, and it's your seedling's way of signalling distress.
Nitrogen Deficiency: The Most Common Culprit
Nitrogen is essential for chlorophyll production, and seedlings in nutrient-poor soil or water often show yellowing first on their oldest (lowest) leaves. If your seedlings are in basic seed-starting mix without added fertiliser, they'll quickly deplete what little nitrogen is available. Start feeding seedlings with a diluted, balanced liquid fertiliser once they develop their first true leaves.
Light Intensity Makes All the Difference
Seedlings stretching toward a window are telling you they're not getting enough light. Weak, insufficient light slows photosynthesis and weakens overall plant structure. Invest in a grow light positioned 5–10 cm above seedlings, and keep it on for 14–16 hours daily. This encourages compact, sturdy growth and vibrant green leaves instead of pale, yellowed foliage.
Watering and Root Health
Both overwatering and underwatering damage roots and prevent nutrient uptake. Seedling soil should be consistently moist but never waterlogged. Allow the surface to dry slightly between waterings, and use room-temperature water. Poor drainage or compacted soil suffocates roots, making it impossible for the plant to absorb the nitrogen and magnesium needed for healthy leaf colour.
Temperature and Humidity Control
Cold seedlings struggle to photosynthesize efficiently, and their leaves often look pale or yellowed as a result. Keep seedlings in a warm environment—around 18–24°C—to support active growth. High humidity also helps reduce stress on young plants, though air circulation is important to prevent fungal issues.
Recovery and Prevention Strategies
Once you've identified the problem, correction is straightforward. Improve light with a grow lamp, fertilise every 7–10 days with half-strength liquid feed, and check your watering routine. Most seedlings recover quickly once their environment improves, showing new green growth within days.
By staying ahead of these common issues, you'll grow strong, vibrant seedlings ready to thrive in the garden.