Tomato Seedling Leaves Turning Yellow: Causes and Solutions

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When the leaves of your tomato seedlings start turning yellow, it can be concerning. The seedlings are young and fragile, so it may seem like they won’t recover. But the truth is that most of the time, tomato seedling leaves turning yellow can be fixed. So let’s find out what causes tomato plant seedling leaves to turn yellow and provide solutions to fix the problem.

Tomato Seedling Leaves Yellow Causes and Solutions
Tomato Seedling with Yellow Leaves

What Causes Tomato Seedling Leaves To Turn Yellow?

The most common reasons tomato seeding leaves turn yellow are due to their cotyledon leaves falling off, watering problems, inadequate sunlight, too much fertilizer and disease.

Let’s take a look at each of these issues in detail and how you can treat them to get your tomato seedlings back to excellent health.

Tomato Seedling Cotyledons Leaves Falling Off

When tomatoes grow from seed, they start with a small set of rounded leaves called cotyledons (this is true for other plants too). These starter leaves are essential for the growth of the tomato plant and getting it the nutrients it needs.

Once the tomato seedling gets a bit older, it starts to grow its first set of true leaves. These are the oval, multi-lobed leaves that grow on tomato plants. And it is these true leaves that provide even more support for the plant. This is when tomato seedlings really start taking off and growing.

At this point, the cotyledons are no longer necessary for the plant, and they can yellow, wilt, and fall off. So if the yellowing leaves on your plant are the cotyledons, remember, the ones that aren’t shaped like most tomato leaves, then don’t worry! That’s actually a sign that your tomatoes are growing well.

If the other leaves on your seedlings are turning yellow, though, then it could be one of the other issues that tomato seedlings face.

Too Much Water Causes Tomato Seedling Leaves to Yellow 

How to Fix Tomato Seedling Leaves Yellow
Overwatered Tomato Seedling Turns Leaves Yellow

Tomato seedlings are young and therefore often more fragile than mature plants. They’re more sensitive to stress and changes in care.

Because of this, overwatering is a very common cause for tomato seedling leaves turning yellow. Too much water can stress out the roots, which causes stress for the whole tomato plant. If too stressed, the leaves may start to die.

Too much water can also lead to root rot which causes the roots of tomato plants to become less solid, more droopy, and often the roots turn black.

I’ve experienced this with young tomato seedlings planted out in the vegetable garden. We had weeks of rain causing the soil to become waterlogged and this turned my tomato seedling leaves yellow.

If overwatering has occurred, you can still try to save the tomato seedlings.

Carefully remove the tomato seedlings from their pots, or the garden (although waiting for drier weather may also be a good option), and brush excess soil away from the roots. Gently wipe off the roots, getting as much waterlogged soil off as you can. Then plant the tomato seedlings in new soil that is drier.

Make sure the soil drains well because tomato seedlings will suffer in water-logged soil.

Not Enough Water Causes Tomato Seedling Leaves To Yellow

Of course, inadequate watering of tomato seedlings can also be the cause of yellow leaves. 

If the soil completely dries out, then the tomato seedling might not get enough nutrients, and that can also cause leaves to start dying. And when they dry out they’ll turn yellow before they turn brown. So don’t let the soil dry out too much between waterings.

Tomato seedlings should receive a regular supply of water, enough to make the soil a bit wet, but not overly moist, and let it start to dry a bit before watering again. The soil should remain slightly moist but not water-logged. I like a drip irrigation watering system I can set and forget.

Lack Of Sunlight Causes Tomato Seedling Leaves To Yellow

Why are my Tomato Seedling Leaves Yellow
Tomato Seedling with Yellow Leaves

Tomato seedlings have a lot of growing to do, and often grow very fast once they’re established. To support this growth, tomato seedlings need sufficient sunlight to keep them going.

Generally, around 12 hours of light a day is optimal. If you are starting your tomato seedlings indoors, this could be obtained through a bright window or grow lights like these ones. Outside, you have less control over sunlight hours, but try to choose a sunny spot to place your tomato seedlings for optimal growth.

It’s also important to note that at least 8 hours should be in direct sunlight. If all the light is indirect, it might not be enough for the tomato plants to properly grow.

If you can’t get the tomato seedlings enough naturally, consider buying grow lights. They can really make a difference, and make it a lot easier to give your tomato plants as much light as they need.

Too Much Fertilizer Causes Tomato Seedling Leaves To Yellow

As tomato seedlings are small, they often have no need for additional fertilizer. As such, applying fertilizer, especially applying a lot, can damage the tomato plant as it builds up excess nutrients. This causes more stress to the tomato seedling, leading to growth problems and possibly yellowing the leaves.

If you see white clumps around the drainage holes in your pots, it’s a big sign that there may be too much fertilizer in the soil. To fix this, you need to first stop apply more fertilizer, and then flush the plant.

This means giving it a big watering and letting the excess drip out the bottom. Try doing this a couple of times, each one spread out by a few hours, and see if that helps. But make sure the pot is well-draining, you don’t want too much water either.

If you haven’t applied any fertilizer, your seedlings are yellowing, and the other things listed here don’t seem to be the issue, then perhaps it does need a little boost. In this case, you can give the plant a bit of fertilizer, but not a lot. It can be a good idea to add homemade compost which will boost the soil nutrients while being gentle on the tomato seedlings.

Disease Causes Tomato Seedling Leaves to Yellow

Why Tomato Plant Leaves Turn Yellow
Yellow Leaves on Tomato Plant

There are a few diseases that can affect tomato plants but not too many of them show up as yellowing leaves. Most of them have other signs and symptoms. So if your tomato seedling leaves are yellowing, there are just a few possible diseases.

Damping Off and Fusarium Wilt are both fungal diseases that can manifest in the yellowing leaves of tomato seedlings. Luckily, as fungal diseases, they’re very similar to each other and can be treated the same way.

First, try to avoid them by controlling the moisture level of your tomato plants. If it’s overly humid or the soil is too moist, it can be easier for the fungus to take hold and spread. Also, try to avoid getting water on the leaves themselves, only watering the soil instead.

Once these diseases take hold, the steps above will still help prevent them from spreading, but the tomato plants may need more treatment. Mixing baking soda into water in a spray bottle can help. A ratio using 1 gallon of water mixed with 4 teaspoons of baking soda and a few drops of gentle dish-washing liquid. The spray will help kill the fungus on the plant without harming the seedlings themselves.

These diseases aren’t as common as the other problems, but they can still play a part. So take care not to create favorable conditions for fungal diseases, and fight them when you can.

As we said, most other diseases will show up in other ways. So if yellowing leaves are your only issue, you shouldn’t have to worry about those. But if your tomato plants have other symptoms, look into what other diseases might affect tomato seedlings.

Transplant Tomato Seedlings To Fix Yellow Leaves

Tomato Plant Yellow Leaves

Lastly, if you still can’t figure out what the issue is with your tomato seedlings, try transplanting them. Although they may be young, sometimes seedlings benefit from a new environment, which may provide improved soil, soil drainage and better lighting.

This is especially helpful if your tomato seedlings are growing indoors, as taking them outside may be just what they need. An increase in sunlight, fresh air, and new soil may really help your tomato plants.

New tomato seedlings can also outgrow a pot and require a larger pot or planting out in the garden. This has certainly been an experience of mine and I’ve found transplanting tomato seedings from starter pots to either larger containers or out in the garden has given yellowing tomato seedlings the boost they needed to really thrive.

Of course, you have to be gentle doing this, especially if the seedlings are young. You should harden them off to the outside elements first. You can read how to do that in our article: Hardening Off Seedlings (& Avoiding Transplant Shock). The tomato seedlings, especially their roots, are still fragile. You have to treat them with care, and try not to jostle the roots too much.

Growing tomato seedlings in biodegradable pots like these ones, are a great choice because it removes a lot of the potential stress of transplanting. If your pots are biodegradable, you can simply pop them in the ground without having to disturb the roots of the plant.

Caring For Yellowing Tomato Seedlings

As you can see, there are a lot of reasons tomato seedlings might turn yellow, and you may have to do some work to figure out exactly what is plaguing your plants. But once you do, you should be able to fix the issue and be back on track to have big, healthy tomato plants and a thriving harvest.

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Further reading:

Fix Tomato Seedling Yellow Leaves