Do Your Tomato Seedlings Have Yellow Leaves? These 5 Common Mistakes Could Be the Cause, But Here’s How to Fix Them

Finding yellow leaves on plants is very common. The scientific term for the phenomenon is chlorosis. For mature plants, it might just mean you’re at the end of the growing season. For tomato seedlings, yellow leaves mean something is wrong – but don’t panic yet.

From nutrient deficiencies to incorrect watering, many of the factors that lead to tomato seedlings turning yellow are easy to reverse. This is one of the most common tomato seedling problems and once you determine the cause, you can resolve the issue.

I’ll share the top reasons why tomato seedlings yellow and how to fix them. Just avoid the care mistakes below that cause this problem. Then start working towards a solution so you can grow strong, healthy, green plants that produce tons of tasty tomatoes.

Why Are My Tomato Seedlings Turning Yellow?

Yellowing is very common among the possible problems when growing tomatoes and it's often fixable, so there's no need to stress. The list of care mistakes below will help you identify the cause of tomato seedlings turning yellow and provide solutions help plants regain their green color.

1. You're Overwatering

Overwatering seedlings is probably the most common underlying cause of yellowing leaves. It can be tricky to strike the right balance with water. Seedlings need consistent moisture, but too much water can cause roots to suffocate and ultimately rot. This leads to tomato seedlings turning yellow because they can’t absorb adequate nutrients.

When watering seedlings, aim for soil that never dries out entirely but is also never soggy or waterlogged. Your seedlings should be in trays or small pots with a light, well-draining potting mix like this one from Miracle-Gro and drainage holes, so water can’t get trapped.

If your seed trays have become waterlogged and the seedlings are yellowing, let them dry out a bit before watering again. If the container isn’t draining, repot your seedlings in a container that will drain excess water.

2. You're Underwatering

Tomato seedlings can also yellow if the watering balance tips in the other direction towards too dry. As with overwatering, underwatering prevents the roots from taking up enough nutrients to promote healthy, green growth.

If you catch this issue early, it’s very easy to fix. Aim to water your seed trays regularly, keeping the soil consistently moist but not soggy or drenched. When dryness is a problem, it’s important to check on seedlings daily. If you see any dry soil on top, give them some water.

A watering can that can direct water towards the roots and produces a fine spray, like this seedling watering can from Amazon, is good for delicate young tomato plants.

3. You're Not Fertilizing

Watering issues are common and easy to fix, but if you’ve mastered it and still have yellowing, it’s time to consider nutrient deficiencies. The deficiency most likely to cause yellow leaves is nitrogen.

Look at how the leaves have become yellow. If they started yellowing from the leaves at the bottom, followed by the upper leaves, then nitrogen is the likely culprit. Less commonly, a lack of iron or potassium can cause yellowing on the newest leaves.

To prevent nutrient deficiencies, you can use compost in your potting mix when starting seeds. This will provide them with a rich substrate to promote healthy growth.

Alternatively, you can fertilize seedlings with a balanced but diluted liquid fertilizer after the first true leaves emerge. This organic liquid fertilizer from Amazon is a good choice. Apply the diluted solution every two to three weeks as seedlings grow.

4. You're Not Providing Enough Light

Bright light is essential for healthy seedling growth. Are your tomato plants getting enough light? If not, they can definitely turn yellow from the reduced photosynthesis.

Another sign of inadequate light is leggy growth. This means the stems are growing tall yet thin as they stretch to reach any available light.

Be sure your seedlings are getting 12 to 16 hours of direct, bright light per day. If sunny windows are scarce, use artificial lights, like this affordable grow light from Amazon, to give tomatoes a boost.

Situate lights a few inches from the tops of the seedlings. If you are using light from a window, rotate the seed trays regularly so they get even sunlight.

5. Your Plants Have a Disease

Hopefully, you’ve found the cause of your yellowing seedlings in one of the above issues. Tomato diseases are much more serious and more difficult to manage. They're also less likely to be the problem compared to the typical issues of lighting, water, and nutrients.

Fungal infections are most common, but you might also have a bacterial issue. Less common are viral diseases in tomato seedlings. Diseases in tomato seedlings often begin with yellowing from the top of the plant down. They are also associated with other signs, like spots on leaves or wilting.

If you see signs of a fungal disease, like fusarium wilt or early blight, remove the affected leaves or entire seedlings. You can try a fungicide, like this one from Bonide, if the infection hasn’t spread too far.

Bacterial diseases, like bacterial leaf spot, often come from the seed. That's why it’s so important to buy from reputable sources. Burpee has tons of high-quality tomato seeds, including many disease-resistant varieties. If your seeds were infected, all your seedlings are likely impacted and you may need to start over.

To prevent diseases in tomato seedlings, start with good seeds. Use clean, sterile seed starting mix and seed trays. Avoid overcrowding your seedlings, which can lead to disease spread. Good air flow between plants is important, too. Water only at the soil level and remove any seedlings that look diseased as soon as you spot the signs of illness.

For tomato seedlings, yellow leaves can mean many things. It’s a common phenomenon and shouldn't be too worrying as long as you determine the cause. The most important thing you can do when growing tomatoes from seeds is to keep a close eye on the seedlings as they develop.

Check them at least once a day for signs of watering issues, nutrient deficiencies, diseases, and other problems. The earlier you catch these issues, the easier they are to fix and the more likely it is you’ll be able to save your little seedlings.