5 Reasons to Thin Your Tomato Seedlings this Season

Every winter, I dream of biting into juicy tomato sandwiches and snacking on cherry tomatoes as I complete garden tasks. But before you or I can enjoy homegrown tomatoes, we must start with seeds and seedlings. If you’re planting tomato seeds at home, one aspect to think about is thinning your seedlings.

There are many ways to grow seedlings, and thinning isn’t always necessary. For example, if you start single seeds in plug trays, every resulting seedling will have the room it needs to grow. But if you crowd multiple seedlings in a single area, removing weaker seedlings or bumping up the plants into larger containers keeps them healthy and gives them more room to grow.

In this article, I’ll cover when, why, and how to thin your tomato seedlings this growing season.

Should You Thin Your Tomato Seedlings?

Before I dive into why you should thin your teeny, tiny tomato plants, let’s determine whether or not you should complete this task. After all, there’s no need to do unnecessary work!

Whether or not you should thin your seedlings largely depends on the type of seedling container you use and the space between the seedlings. You have lots of options when it comes to choosing a container for seed starting, and one isn’t necessarily better than another. As long as the container is large enough to hold soil and contains drainage holes for excess water to escape, it’s suitable for seed starting.

Many gardeners choose to use designated seed-starting trays since these trays create nice soil plugs that you can pot out of the trays and plant into the ground. However, since these trays take up a fair bit of space, you can also start seeds in open soil flats and then pot the seedlings into individual containers. For example, while you can only fit 15 or so mature tomatoes in a 10×20 tray, you can fit hundreds of seedlings in the same area.

If I’m starting more than a few seeds, I often start them up in open flats and then pot them into larger containers. This method saves space and ensures all the heat-loving tomato seeds can fit on a heat mat. People don’t always consider potting up a form of thinning, but it does give small plants extra space.

Even if you start your tomatoes in the pots you plan to keep them in, you may still need to thin them. Starting a single seed in each pot is sometimes risky since 100% germination is rare. Planting two or three seeds increases the odds you’ll end up with a healthy seedling in each pot. However, if you end up with more than one seedling in each pot, you should remove the extra tomato seedlings so the remaining plant can thrive.

When and How to Thin

The best time to thin your tomato seedlings is when they have two sets of true leaves. When they first germinate, they produce two long leaves called cotyledons. These leaves come from the seed and fall off as the plant grows. They aren’t true leaves!

The true leaves appear after the pair of cotyledons and have a different shape. They have serrated edges and a classic tomato leaf shape. When you notice four of these true leaves, it’s time to thin your seedlings.

The best way to thin seedlings depends on what you want to do with the discarded plants. If you just want to get rid of them, use a sharp knife or pair of scissors to cut the seedling at the soil surface. This will remove the unwanted seedlings without disturbing the roots of the remaining plants.

If you plan to repot the seedlings into larger containers, gently remove the seedlings and the surrounding soil from the container. Once the plants are out of the container, use your fingers to gently tease apart the seedlings’ roots. You can then repot each plant into an individual container.

Reasons to Thin Tomato Seedlings

Removing unwanted seedlings or potting the plants up into larger containers provides them with the proper benefits.

Improves Airflow

When tomato seedlings are packed close together, air has difficulty reaching all sides of the seedlings. This can lead moisture to stick around, which is not what you want. Increased moisture on and around tomato leaves increases the likelihood of fungal problems.

Thinning increases the space around each plant, allowing air to reach all sides of the plant. This extra airflow increases evaporation, decreases moisture, and keeps your plants healthy.

Imagine you’ve been caught in a rainstorm, and your clothes are soaking wet. Fortunately, it’s a windy day, so you can count on the wind to help dry your clothing. But you’re surrounded by hundreds of other people facing the same soggy predicament. If you’re in the middle of this tightly packed crowd, you’ll feel only a small amount of wind. But if there are five feet of space between you and the people around you, you’ll feel the stronger wind and dry out more quickly. The same logic applies to seedlings.

Even if you thin your seedlings, I recommend using a fan to increase airflow. Not only will this moving air help dry out excessive moisture, but it will also help the seedlings acclimate to the wind they’ll experience outdoors. Run the fan on a low setting for at least a few hours each day.

Decreases Chances of Disease

Many common fungal diseases are most likely to occur in stagnant, moist environments. Since thinning increases airflow and decreases moisture, it also limits the chances that fungal diseases will develop.

Some diseases that are more likely to occur when seedlings remain moist include Pythium and Rhizoctonia, both of which can lead to damping off. Damping off causes seedlings to flop over at the base and eventually die. Thinning allows the soil surface to dry out and is one part of preventing damping off. You should also use disease-free potting mix and seedling trays, keep the soil warm, and avoid overwatering.

Thinning your seedlings also decreases the chances of foliar diseases like powdery mildew and septoria leaf spot. While these tomato diseases are more likely to impact mature plants growing in the ground, they can also infect seedlings. Maintaining good airflow is a key part of preventing these diseases, so proper thinning is a key part of a prevention strategy.

Decreases Competition for Nutrients

While some people believe seedlings don’t require nutrients, this is a myth! Seedlings, like mature plants, require a range of nutrients to grow and thrive. If you want to make sure your baby tomato plants can access the nutrients they need, take a moment to properly thin them.

Just like plants compete for light and water, they also fight for nutrients. Even if you’re using a nutrient-rich potting mix packed with nitrogen, potassium, and all the other necessary nutrients, tightly packed seedlings will have difficulty absorbing the nutrients they need.

If your seedlings show signs of nutrient deficiency, you may want to add some extra fertilizer. While this is sometimes an acceptable solution, don’t assume light green or yellow plants mean you need to add fertilizer! If your seedlings are packed tightly together, there’s a good chance the soil has enough nutrients, but the crowded plants can’t reach the nutrients they need.

Giving each plant extra space will decrease competition for nutrients and allow each plant to take up the nutrients it needs.

Increases Light Exposure

If tomato seedlings are packed closely together, there’s a good chance not all of the plants will have equal access to light. Seeds that germinate a day later than others will become shaded and compensate by extending their stems to reach the light. While this may sound good, it results in tall, spindly plants with weak stems.

Even if all your seeds germinate and emerge simultaneously, tightly packed seedlings will still compete for light. Each plant will quickly grow upward in an attempt to outcompete the surrounding plants and reach bright light. This growth leads to “leggy” plants with weak stems.

Thinning your seedlings increases the amount of space around each seedling and helps it access more light. Therefore, plants remain short and stocky rather than tall and leggy. While thinning your seedlings helps increase light exposure, make sure your plants have access to bright light.

Provides Room for Growth

While this one may be a little obvious, thinning your tomato seedlings ensures the plants have room to grow and expand. If your plants are packed tightly together, they’ll remain stunted or grow up rather than out. This will lead to weak plants less likely to survive outdoors.

Thinning also gives the plants room to develop strong root systems. These systems help the plants anchor into the soil and increase water and nutrient uptake. When you transplant these seedlings into their final home, they’ll be more likely to thrive.