While growing my tomato seedlings, I found some of the leaves were curling. I panicked a bit and wanted to find out what is causing it and how to fix this. This post contains my research on figuring out a solution to this problem.

Your tomato seedling leaves are curling because of too much sunlight, heat, or wind. The leaves try to protect themselves from the dryness and heat by curling up and reducing the surface area that loses moisture due to evaporation.

If you find your tomato seedlings are curling, don’t panic. Find out what is the cause of this problem and you can figure out a solution that helps your tomato seedlings.

1. Check your seedlings for environmental stress

The most common reason your tomato seedlings will have leaf curl is because of environmental stress. The seedlings that suffer from such stress have their leaves curl upward.

The best way to understand if this is the problem is to check if only the mature leaves at the bottom have curled up. The fresh leaves at the top will remain unaffected.

One of the reasons could be too much heat on the tomato leaves. This causes the leaves to curl up so they can protect the insides from burning.

Another cause of environmental stress is a windy situation. This causes the water to evaporate from the leaves. And the leaves curl up to protect themselves.

Some other causes of environmental stress include severe pruning and transplant shock. If you prune a lot of the leaves and branches of the seedling, the plant comes under stress and the leaves curl up.

The same happens when you transplant the seedlings from a pot to your garden outside without hardening them off. The stress from the transplant shock is too much for the tomato seedlings and the leaves can curl.

The good thing about leaf curl due to environmental factors is you don’t need to do anything. Such leaf curl does not cause harm to the tomato seedling. It’s just a response from the plant to protect its leaves from the situation.

When the heat or wind reduces and you prevent severe pruning or transplant shock, the leaves will come back to their normal looking state.

2. Make sure you’re giving the right amount of water

Your tomato seedlings need plenty of water to grow their best. But you need to be careful and prevent overwatering as well.

Both underwatering and overwatering can cause the leaves of your tomato plants to curl.

The underwatering will cause a lack of moisture to the tomato seedlings and the leaves will curl up. The leaves will get back to normal once you start watering well. So this is not a serious situation and your seedlings can recover.

The curling of the tomato leaves due to overwatering is a more serious problem. In this case, either you are giving the soil too much water or your soil is full of clay and retaining too much water.

In both situations of overwatering, the roots of your tomato seedlings will suffer from root rot. This means the roots won’t be able to absorb moisture and get constricted.

The tomato seedlings won’t be able to get the required moisture and nutrients because the roots are rotting. And the leaves will start to curl down, wilt, turn yellow, and even fall off.

To solve this problem, you need to make sure your tomato seedlings are getting the right amount of water.

Stick your finger 1-2 inches inside the soil and check the moisture. If the tip of your finger does not feel moisture, that’s the time to water your tomato seedlings well.

To know if your tomato seedlings are facing root rot, you need to dig up the soil to check the roots. If you see that the roots are soft, soggy, and smell, it’s probably root rot.

You need to take the tomato seedling out of the soil and trim off the spoiled roots. You then need to transplant the seedling in fresh, clean potting soil.

3. Ensure the plant gets enough fertilizer

The tomato seedlings can suffer leaf curl due to a lack of nutrients or excess of them. The leaves will curl down when the plant faces this problem.

The lack of nutrients like boron is a situation that can cause the tomato leaves to curl. But this is not a common issue and it’s not a serious one either. Adding a little bit of compost or fertilizer to your plants will solve this problem.

The addition of excess fertilizer will cause the plant’s roots to burn and make the leaves curl down. This is a major issue that you need to be careful about.

Always use the fertilizer as per the instructions provided by the manufacturer. You need to dilute the fertilizer to the right levels before using on your tomato seedlings.

Once the plant’s roots have burned off, the damage would be too hard for the plant to recover.

You should use slow-release fertilizer just 1-2 times during the growing season of the plant. And if it’s liquid fertilizer you should not use it more than once every 2-4 weeks.

4. Protect your tomato seedlings from insects

If your tomato seedlings are infected by insects like aphids or whiteflies, the leaves may start to curl down. The problem is that these insects suck the sap from the tomato leaves causing them to curl.

Ants farming aphids on my pepper plants
Ants farming aphids on my pepper plants

Insects like caterpillars might make a nest in the leaves and they tend to curl the leaves for their protection.

The other problem that an insect infestation causes is they may bring a viral, bacterial, or fungal disease to the tomato plant. And these will also cause the leaves of the tomato seedlings to curl down.

The solution to the problem of insects is to monitor your tomato seedlings every day. As soon as you see an insect infestation starting, you need to take care of them.

It’s easy to take care of some insects like aphids or whiteflies with water. You spray the leaves with water and drop the insects into the soil. This ensures they cannot come back to the leaves.

Make sure to spray water on all sides of the leaves because the insects tend to hide on the underside.

white flies on pepper plant leaves
Whiteflies on my pepper plant leaves

5. Check if the seedlings have a fungal disease

If your tomato seedlings suffer from a fungal disease, the leaves will curl down due to the infection.

Some of the common fungal problems your tomato plants may suffer include Septorial wilt, Fusarium wilt, and leaf blight.

The most common reason tomato seedlings get infected by fungal diseases is splashing water and soil on the foliage.

This happens when you’re watering the tomato plant with a garden hose, or a sprinkler. You need to make sure only to water the base of the tomato plants.

The best way to do this is with a watering can, soaker hose, or drip-irrigation system that targets the water to the base of the plant.

watering can
Watering can at the HortiPro Exhibition

If the water stays on the foliage, it creates a humid environment that attracts fungal diseases to the plant.

I would also suggest to water the tomato seedlings in the morning. So even if there is a little excess water on the plant, the sun will evaporate it.

If you water the plants in the evening, you need to be extra careful not to splash it on the plant. If the water remains on the plant throughout the night, there’s a good chance of fungal infection.

Another reason your tomato seedlings may get fungal diseases is planting too early in the season. It’s best to wait till the night temperatures are at least 50 degrees Fahrenheit before you plant your tomato seedlings.

Make sure there’s enough air circulation between the leaves of the tomato plant. A lack of air also creates an environment that is conducive to fungal growth.

If your tomato seedlings do get infected by fungal diseases, the best thing to do is get rid of the leaves. If you monitor your plants every day, you can take care of this problem fast so the infection is limited to a few leaves and branches.

If a major portion of the plant is infected, there’s not much you can do. You’ll need to dispose of the plant and the soil.

6. Check if the tomato seedlings have a virus

The most serious problem of leaf curl happens when your tomato seedlings are infected with a virus. This will cause the tomato leaves to curl down and the plant to eventually die.

One of the main reasons your tomato plants may get infected with viral disease is because of insects like aphids and whiteflies. These are carries of a virus such as the leaf curl virus or the tomato mosaic virus.

Once your plant is infected by these viruses, there is no way to reverse the damage. The plant will try to grow more suckers to overcome the problem but it’s just a matter of time.

The good thing is that your tomato plants will continue to grow and produce fruits. And you can harvest the tomatoes as the virus only affects the foliage of the plant.

But once the growing season ends, you need to dispose of the plant and the soil. Make sure not to grow tomato plants in the same spot for a few years.

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