Why Are My Tomato Seedlings Turning Yellow

If you’re starting seeds indoors, seeing yellow leaves can be worrying. Let’s figure out why are my tomato seedlings turning yellow and how to fix it. This guide will walk you through the most common causes, from simple fixes to more serious issues. With a little detective work, you can usually get your seedlings back to a healthy green.

Yellowing leaves, or chlorosis, is a plant’s distress signal. It means your seedling is struggling with something in its environment or diet. The good news is that many causes are easy to correct if you catch them early. The key is to look at the whole plant and its conditions.

1. Overwatering or Poor Drainage (The Most Common Culprit)

This is the number one reason for yellow seedlings. Tomato roots need oxygen as much as they need water. Soggy soil suffocates roots, leading to root rot. The first sign is often yellowing of the lower, older leaves.

Signs: Yellow lower leaves that may feel limp or soggy. Soil is constantly wet to the touch. Pot feels heavy. A musty smell or green algae on the soil surface.
The Fix: Let the soil dry out! Check moisture by sticking your finger an inch into the soil. Only water when it feels dry at that depth. Ensure your containers have drainage holes. Use a well-draining potting mix, not garden soil.

2. Not Enough Light (Leggy and Yellow)

Tomato seedlings are sun lovers. Without sufficient light, they become weak, stretch tall (leggy), and their leaves turn pale green or yellow because they can’t produce enough chlorophyll.

Signs: Overall pale or yellow color, spindly stems stretching toward the light source. New growth may be small.
The Fix: Provide 14-16 hours of bright, direct light daily. A sunny south-facing window is often not enough. Use a grow light placed just 2-4 inches above the seedlings, raising it as they grow.

3. Nutrient Deficiencies – Hunger Pangs

Seedlings use up the energy stored in the seed quickly. Once they have a few true leaves, they need food from the soil. Yellowing can point to a lack of key nutrients.

Nitrogen Deficiency: Classic symptom is yellowing of the older, lower leaves while veins may stay green. The plant redirects nitrogen to new growth.
Iron Deficiency: New leaves at the top turn yellow but veins remain sharply green.
The Fix: Feed with a balanced, water-soluble fertilizer diluted to half strength. Apply once every 1-2 weeks. Ensure your potting mix isn’t just plain peat, which has few nutrients.

Is It Nitrogen or Overwatering?

Since both cause lower leaf yellowing, check the soil. If it’s wet, it’s overwatering. If it’s dry and the plant is stunted, it’s likely hunger. Sometimes, overwatered roots can’t absorb nutrients, causing a double problem.

4. Temperature Stress – Too Hot or Too Cold

Tomatoes are sensitive to temperature extremes. Cold drafts from a window or heat from a vent can shock them.

Cold Stress: Leaves can turn purple-tinged or yellow, growth stalls.
Heat Stress: Wilting and yellowing, especially if under strong lights.
The Fix: Keep seedlings in a stable environment between 65-75°F (18-24°C). Keep them away from drafty windows and direct heat sources. A small fan on low can strengthen stems and improve air circulation.

5. Pot-Bound Roots – Cramped for Space

If seedlings stay in a small cell or pot too long, roots circle the bottom, become congested, and can’t function properly. This leads to yellowing and stalled growth.

Signs: Roots growing out the drainage holes, soil that dries out extremely fast, general yellowing.
The Fix: Gently slide the seedling out of its container to check root growth. If roots are densely packed, it’s time to transplant into a larger pot (at least 3-4 inches).

6. Soil pH Problems – Locked Out Nutrients

Tomatoes prefer slightly acidic soil (pH 6.0-6.8). If the pH is too high (alkaline) or too low (acidic), the plant cannot absorb nutrients, even if they are present in the soil. This leads to deficiency symptoms like yellowing.

The Fix: Test your potting mix’s pH with a simple test kit. To lower pH (make more acidic), you can use a fertilizer for acid-loving plants. To raise pH, a little garden lime can help. Most quality potting mixes are pH balanced, but it’s worth checking if other issues are ruled out.

7. Diseases and Pests

While less common in seedlings, these can occur.

Damping Off: A fungal disease that rots the stem at the soil line, causing the seedling to collapse and yellow. It’s fatal and caused by overly moist conditions.
Pests: Aphids or spider mites can suck sap, weakening the plant and causing yellow speckling.
The Fix: For damping off, prevention is key: use clean containers, well-draining soil, and avoid overwatering. For pests, a strong spray of water or insecticidal soap can help.

Your Step-by-Step Diagnostic Checklist

Check the Soil Moisture: Is it sopping wet or bone dry? Adjust your watering immediately.
Evaluate Light: Are stems leggy? Move lights closer or increase duration.
Inspect the Roots: Gently check if they are crowded or brown/smelly (rot).
Look at the Yellow Pattern: Lower leaves (overwatering, nitrogen) or new top leaves (iron, light)?
Consider Temperature: Is there a cold draft or hot vent nearby?
Remember Last Feeding: Have you fertilized at all in the last 2 weeks?

How to Save Your Yellowing Seedlings

Once you’ve identified the likely cause, take action. Most seedlings are resilient and will recover with proper care.

Correct Watering: Establish a “finger-test” routine. Water deeply until it runs out the bottom, then let dry appropriately.
Provide Steady Nutrition: Begin a half-strength fertilizer schedule if you haven’t already.
Transplant if Needed: If root-bound, pot up carefully into a larger container with fresh mix.
Prune if Severe: If the lowest yellow leaves are very bad, you can snip them off to focus energy on new growth. Don’t remove to many at once though.

Prevention is the Best Medicine

To avoid yellowing from the start, follow these best practices:

Start with a quality seed-starting mix, not heavy garden soil.
Use containers with excellent drainage holes.
Provide strong light from day one, using grow lights.
Water from the bottom to encourage strong roots and avoid wet leaves.
Begin fertilizing when the first set of true leaves appear.
Use a fan for air circulation to strengthen stems and prevent disease.

FAQ: Tomato Seedling Yellow Leaves

Q: Should I remove yellow leaves from my tomato seedlings?
A: Yes, if they are fully yellow and at the bottom. Carefully pinch them off. This helps the plant focus on healthy growth and improves air flow. Don’t remove to many at once though.

Q: Can tomato seedlings recover from yellow leaves?
A> Absolutely. If the cause is fixed, new growth will come in green. The already yellow leaves may not recover and can be removed. The plant’s overall health will improve quickly.

Q: What is the best fertilizer for yellowing tomato seedlings?
A: A balanced, water-soluble fertilizer (like a 10-10-10 or 5-5-5) is a good start. For suspected nitrogen deficiency, one with a higher first number (like 15-5-5) can help. Always dilute to half strength for seedlings.

Q: Why are only the bottom leaves of my seedling turning yellow?
A: This almost always points to either overwatering or a nitrogen deficiency. Check your soil moisture first—it’s the most common reason by far.

Q: Are yellow leaves a sign of overwatering or underwatering?
A: Both can cause yellowing! Overwatering causes yellow, limp leaves in wet soil. Underwatering causes yellow, dry, crispy leaves in dry soil. Always touch the soil to know for sure.

Seeing yellow leaves on your tomato seedlings is a common part of the learning process for every gardener. Don’t panic. By systematically checking light, water, food, and roots, you’ll find the answer. With a few ajustments, your plants will likely bounce back, ready for a strong transition to the garden.