10 Tips for Strong and Healthy Tomato Seedlings

Tomatoes are often one of the first crops beginner gardeners grow in their vegetable patches. That’s because they are generally considered easy to grow, beginner-friendly, and versatile in the kitchen. Plus, there is a long list of fascinating varieties to choose from and experiment with. What’s not to love?

Unfortunately, as easy as tomatoes are to grow, I have always struggled with them. Whether it was battling powdery mildew or damping off, I could never seem to get tomato sowing right. Luckily, after years of trial and error, I have identified the many mistakes that can be made and how to avoid them.

If you want a productive tomato plant at the end of the season, you need to start with a healthy tomato seedling at the beginning of the season. Here are a few tips to get tomato seedlings right.

Sow at the Right Time

This tomato seedling tip applies not only to tomatoes but to any crop you plan to plant: follow the sowing instructions.

If you plant too early, you risk your seedlings being killed off by sudden cold snaps (or worse, hard frost). If you start too late and have a short season, your tomato plant won’t have time to mature before the first frost kills it off. When sowing indoors, incorrect timing can also cause your seedlings to get leggy as they stay in their trays for too long.

With that in mind, it’s important to sow at the right time for the variety that you’re growing to ensure the best possible start.

For most tomatoes, indoor sowing in trays is recommended. This gives you greater control over the growing environment (which we’ll discuss later). When it comes to timing, start your seeds around 4 to 6 weeks before you plan to transplant. Keep an eye on the weather for any sudden cold snaps to protect your young seedlings from damage.

Use Seed Starting Mix

One mistake new gardeners make with tomatoes is starting them in soil straight from the garden (or anything they have on hand). Garden soil is usually not the right consistency for seed starting. This can cause problems with moisture retention, weeds, and disease. Even a regular potting mix is often too chunky for seed starting, with large particles that hinder early root growth and impact germination.

If you want to give your tomatoes the strongest start, you need a seed starting mix. These mixes are designed to retain enough moisture to encourage germination. They are typically fine in texture to limit resistance to root growth.

You can purchase a seed starting mix from your local nursery. Alternatively, make your own by combining one part coconut coir or peat moss with one part perlite and one part sifted compost. The mixture should be fine without clumping together, and water should drain through it relatively quickly.

Check Spacing

When you purchase a packet of tomato seeds, you will see spacing guidelines in the planting instructions. This is easy to miss, but if you want healthy tomato seedlings, make sure you check it and follow those guidelines closely.

Spacing your tomatoes too far apart isn’t really a concern beyond aesthetics. However, spacing them too close together can cause a long list of problems. Disease is the most common issue, as plant diseases like powdery mildew and blight can spread far quicker between closely spaced plants due to the lack of airflow.

Tomatoes that are planted too close together also compete for resources, leading to growth problems and leggy stems. If this continues past the seedling stage, you’ll see far fewer tomatoes on your plants, reducing your harvest.

If you’re sowing in cell trays, you’ll simply sow one or two seeds per cell. However, if you’re direct sowing outdoors, make sure you follow the instructions on the seed packet to avoid issues with competition.

Provide Overhead Light

When it comes to environmental conditions, lighting is one of the most important for young tomato seedlings. Lack of light is one of the main causes of leggy stems (stems that stretch out and start to collapse).

If you’re sowing indoors on a sunny windowsill, you may notice your tomato seedlings stretching toward the nearest light source rather than growing upright. That’s where grow lights come in.

This tomato seedling tip is considered optional. Although you don’t necessarily need grow lights to grow strong, healthy tomatoes, it does make a big difference when you’re sowing indoors or in a covered greenhouse. Overhead light prevents issues with leggy growth caused by lack of sunlight, helping your tomato seedlings grow as quickly as possible. The stems will also be much stronger and ready to handle the outdoors.

Place your grow lights directly above your seed trays and adjust them as your tomato seedlings grow to ensure the light source is not too close to the leaves.

Use a Heat Mat

While grow lights help above your seedling trays, heat mats help below the seedling trays. Tomatoes are heat lovers and only germinate well in warm soils. The soil should be between 70°F and 90°F if you want the strongest and healthiest tomato seedlings possible.

If you’re sowing early indoors, it isn’t always possible to create those conditions without the help of a heat mat. Heat mats are also useful for any warm-season crops you want to sow early indoors, like eggplants or peppers. They gently warm the soil, providing the perfect conditions for root growth.

If you’re direct sowing outdoors, you don’t have to worry about heat mats. But for indoor sowing, this is an essential tomato seedling tip I wouldn’t skip.

Water Carefully

One of the biggest risks to young tomato seedlings is incorrect moisture. Most often, watering too much is the biggest error. Soggy soil impacts germination and root growth and encourages diseases like damping off that will kill your seedlings before they get started.

Watering too little is also damaging, as lack of moisture halts early growth. If those young roots dry out and the leaves start to wilt, your seedlings might die off before they can bounce back.

Both extremes are dangerous and should be avoided if you want healthy tomato seedlings. To maintain the right moisture levels, I prefer watering from the bottom. This involves placing the seedling tray inside a bottom tray filled with fresh water. The soil absorbs the moisture through the drainage holes, and the excess can then be poured out to stop the roots from sitting in soggy soil.

This method also avoids dislodging the seeds or young seedlings as the roots are trying to anchor into the soil. Use your finger to gently determine the moisture levels or simply lift up the seedling tray to test how much water is still in the soil. Don’t water on a strict schedule, as environmental conditions have a huge impact on how much moisture remains.

Thin Seedlings

This tomato seedling tip goes hand in hand with spacing. After planting, whether indoors or out, you’ll likely plant more than one seed per hole or cell. This increases germination rates, so you end up with as many seedlings as you planned. However, that does mean you’ll likely end up with more than one plant per cell in some cases.

That’s where thinning comes in. You can’t leave two seedlings in the same cell to grow, as they will compete for space and resources. One of them needs to be sacrificed to ensure the other can grow to its full potential.

This might feel tough at first, but you’ll end up with far more healthy tomato plants than you would if you let them all grow as is.

When your tomato seedlings are a few inches tall, identify the strongest performer and pick out any that are too close. If you sowed outdoors, thin your seedlings to the recommended spacing on the seed packet so they can continue to grow without competition.

Fertilize, But Not Too Early

When you’re first planting your tomato seeds, you don’t need to worry about fertilizing. The seed contains everything it needs to germinate, and excess nutrients in the soil at this time can negatively impact the fragile roots. However, that doesn’t mean you shouldn’t fertilize seedlings at all.

Many gardeners only fertilize when they transplant or just after. But as long as you use the right fertilizer, you shouldn’t have to worry about burning the roots. Feeding seedlings once they are a few inches tall can boost root and leafy growth at the right time, delivering the strongest tomato seedlings possible.

The key is to use a diluted fertilizer so you don’t overdo it. Choose a balanced liquid fertilizer, applying it at the same time as you water your seedlings. Use one or two applications before you transplant.

Manage Airflow

Stagnant air is the enemy of healthy tomato seedlings. It’s a bigger concern in greenhouses or indoors when there isn’t a breeze.

Stagnant air encourages the proliferation of diseases. A lack of airflow also makes stems weaker and less able to handle the wind when they are transplanted outside.

To prevent diseases like damping off and ensure the stems can withstand wind once they move outdoors, provide airflow around your seedling trays if you’re sowing indoors or in a greenhouse. One way to do this is to use a small fan on a low setting to blow through the stems, which will strengthen them over time and limit your risk of disease.

If you can’t do that, just make sure there is an open window or some airflow around the trays (as long as it isn’t too cold).

Harden Off

Before you consider transplanting your healthy tomato seedlings (unless you are direct sowing outdoors), there is an essential tomato seedling tip you don’t want to skip: hardening off.

Hardening off is a process that allows your seedlings to acclimatize to outdoor environments before they are transplanted. Going from a highly controlled environment to the great outdoors can be a shock to plants, and you want to limit the shock as much as possible to give your tomatoes a strong start.

The transplanting process is stressful enough as it is. Hardening off ensures they will acclimatize to their outdoor spots quicker, avoiding any delays or adjustment periods in growth.

Start by exposing your seedlings to outdoor conditions a week or two before you plan on transplanting for a few hours a day. Gradually increase the time, bringing them indoors overnight to prevent any cold damage. After a period of acclimatization, your tomato seedlings will be ready for transplanting to continue to grow into healthy, productive plants.