Tomato plants are one of the popular plants to grow in a container. But the leaves can turn yellow if you don’t take care of them.

Your potted tomato plants are turning yellow because you are overwatering them. Too much water will cause the roots to drown. This means fewer nutrients and oxygen reaches the leaves causing them to turn yellow. The leaves can also turn yellow due to underwatering that will cause similar problems.

I’ve written details below that will help you figure out why your potted tomato plant leaves are turning yellow and how you can fix the problem.

Overwatering

The most common problem with potted tomato plants is overwatering. Plants in a pot can’t spread their roots into the soil and extract the moisture they need.

So you may get worried and keep watering the plant feeling satisfied that you’re giving them as much as possible. The problem with overwatering is that the roots will drown due to excess water in the potting soil.

Nutrients, oxygen, and moisture can’t move as well from the soil through the roots to the leaves. This will cause stunted growth and yellowing of the leaves.

The excess moisture in the potting soil will lead to humid conditions that attract fungal diseases. Root rot is a common problem due to overwatering.

The roots will start rotting and turn brown or black in color with a soggy texture. They will also have a foul smell due to the fungus. You can figure out root rot by checking your potting soil for such a smell.

You can also dig the potting soil and check the roots whether they are brown or black in color.

The best solution for overwatering is to prevent it in the first place. Make sure there are sufficient drainage holes at the bottom of the pot.

Check the moisture in the potting soil before you think about watering. Stick your finger 1-2 inches into the potting soil and water it only if the finger comes out dry.

If the root rot has already affected the roots, you need to take the plant out of the potting soil. Leave the healthy white roots and trim all the dead, soggy, black roots.

Then repot the plant in a fresh batch of potting soil in a sterilized pot. You can sterilize a pot by soaking it in a solution of 1 part bleach and 9 parts water for an hour.

Rinse it well with clean water and let it dry before you repot the tomato plant into it. You can trim the yellowing leaves and dead parts of the plant as well.

Water the repotted tomato plant well till the water drains out from the drainage holes at the bottom.

Underwatering

Underwatering the potted tomato plant will cause the yellowing of the leaves because the plant is not getting enough water.

The lack of water will also lead to a lack of nutrients and oxygen from the roots to the leaves.

This problem is easy to solve as you just need to water the potting soil as required. Check your potted tomato plant every day in the morning.

Stick your finger 1-2 inches into the potting soil to check for moisture. If the finger comes out dry, you need to water the plant.

If you have a busy schedule and just can’t water the plant every day as required, you can consider options like drip irrigation or a soaker hose.

You can set this watering system up once and attach a timer. This will slowly drip water into the potting soil on a schedule.

Make sure to add mulch on top of the potting soil as this will help it retain moisture longer. You can use any organic material like dried leaves, grass clippings, hay, straw, or wood chips as mulch.

Lack of sunlight

Tomato plants need full sun to grow well. This means at least 6-8 hours of sunlight every day.

If your potted plants are growing near tall plants or obstructions like a fence or wall, they won’t get the required sunlight.

This will cause the leaves to turn yellow as they struggle to produce their nutrients through photosynthesis.

You need to remove such obstructions by cutting the branches of tall plants. You can try to adjust the angle of the plants so they can receive better sunlight.

Or you need to move the potted tomato plants to another location that gets better sunlight.

Lack of nutrients

Tomatoes are a resource-hungry plant like most other vegetable plants. You need to provide sufficient nutrients or the leaves will start turning yellow.

This can be a problem with potted tomato plants because they can’t spread their roots into the ground and access more nutrients. You are responsible for providing them what they need.

I recommend adding compost to the potting soil when preparing it. You can also add a slow-release fertilizer that is rich in nitrogen. The fertilizer will release into the potting soil when you water it.

The nitrogen will help the foliage of the tomato plant grow well. You can keep adding nitrogen-rich fertilizer every month to the potting soil as the plant is growing.

Once the tomato plant has matured, you should stop adding nitrogen-rich fertilizer. Otherwise, the plant will keep growing foliage and have slow growth of tomatoes.

Now you can start adding a fertilizer that is low in nitrogen but higher in potassium. This will encourage the potted tomato plant to start developing flowers and tomatoes.

Overfertilization

If the soil has too much fertilizer, it can cause a nutrient overload for tomato plants in pots that lead to yellowing leaves.

The excess nitrogen can burn the roots and damage the plant’s ability to take up nutrients and moisture from its roots.

I recommend you follow the manufacturer’s instructions on the amount of fertilizer to use and how often.

It would be good if you can use organic fertilizer instead of a synthetic one as that releases slowly and have less chance of causing fertilizer burn.

As a general rule, don’t use a slow-release fertilizer more than once a month. And don’t use a liquid fertilizer more than once every two weeks.

Poor potting soil

Potting soil is probably the most important component of growing good potted tomato plants. If the potting soil is poor, it can cause issues like overwatering, underwatering, or compaction.

This may happen if the potting soil contains a lot of clay or sand. I suggest using potting soil you buy from a garden center or nursery.

Don’t use garden soil to grow potted tomato plants because it will contain more clay or sand. And it will be heavy causing soil compaction.

The garden soil may contain pests, diseases, and chemicals that may harm your potted tomato plants.

If you are having problems with the potting soil, you need to repot the tomato plant. Take it out of the pot and throw away the potting soil.

Sterilize the pot using a solution of 1 part bleach and 9 parts water. Then add a fresh batch of good potting soil in it.

Now you can plant the roots of the tomato plant in this new potting soil and cover them up.

Pests and diseases

If your potted tomato plants are attacked by pests or diseases, the leaves may start turning yellow.

Pests like aphids and spider mites will suck the sap from the leaves. And if there is a large infestation, the leaves will lose their nutrients and turn yellow.

You can get rid of such pests by spraying water on the leaves and dropping them onto the ground. They won’t be able to get back to the leaves again. Make sure to check under the leaves as well where they may hide.

Your potted tomato plants can get infected with fungal and bacterial diseases such as Fusarium rot, blight, and septoria leaf spot causing leaves to turn yellow.

The best solution to these diseases is to choose tomato seeds that are resistant to them.

You can prevent fungal diseases by avoiding water on the foliage. Be careful when watering the plant and don’t splash water or soil on the leaves.

Water at the base of the plant with the watering can. It’s better if you can use a drip-irrigation or soaker hose for the watering.

It’s best if you can water in the morning so that the excess water will evaporate once the sun comes out.

If you find that some leaves have been attacked by pests and diseases, it’s best to cut them off the plant.

In some types of diseases, this will help the plant to recover and grow back the lost leaves. But some of the diseases like fusarium or verticillium wilt cannot be reversed.

The plant will get fully infected and eventually die. It’s best to take the plant out and dispose of it. These diseases can remain in the soil for several years so you should not use the same location to grow tomatoes again for a few years.

Lack of growing space

If your potted tomato plant does not get the required growing space in the pot, the leaves will turn yellow due to a lack of nutrients.

You need to grow the tomato plant in a pot that is suitable for the variety. Typically, a 15-20 gallon pot works well for a tomato plant.

If the pot is small for the plant, the roots won’t be able to get the required nutrients and moisture from the potting soil for all the foliage.

I would also recommend growing only one tomato plant per pot. Growing more than one will cause overcrowding and the plants competing with each other for nutrients and resources.

The overcrowding will also lead to humid conditions that will attract pests and fungal diseases to the potted tomato plants.