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How much water is required for tomato seeds to germinate?

Jaime BeamerMarch 18, 2023

0 30 11 minutes read

When it comes to growing tomatoes from seed, the most crucial thing you can do is make sure you’re providing your seeds and seedlings the right quantity of water at the right time.

The objective is to water them just enough to aid germination and healthy development while avoiding saturating the soil and producing a soggy mess for your seedlings to sprout and thrive in.

But how much water is enough, and how frequently should seeds and seedlings be watered?

In other words, how can you know whether you’re watering them too much, too little, or just right?

As a general rule, tomato seeds and seedlings need a light watering once every 3-4 days since they do best in soil that’s consistently moist and around 75°F (23°C). Watering should be sufficient to keep the soil wet while avoiding overwatering, which may damage growing plants.

I’ve emphasized that you should water your plants “just enough” to ensure that they receive what they need without being overwatered, but it’s crucial to recognize that “just enough” isn’t a one-size-fits-all solution. In reality, how often you water your tomato seeds and seedlings will be determined by a variety of environmental conditions, including:

Where are you growing your plants?
What’s the temperature like in your growing area?
What kind of soil are you using?
What seed-starting supplies are you using?
How much natural or artificial light is reaching your seed trays?

Even while there is no ideal, one-size-fits-all solution, understanding basic rules as well as the ins and outs of watering tomato seeds and seedlings can aid you.

I’ve been producing tomatoes from seed for over ten years and have made several blunders along the way.

My objective with this post is to teach you all you need to know about providing your seeds and seedlings enough water to survive without accidentally stunting their development.

One quick note before we get to the detailed instructions below: If you want to do what I do, you’ll need to get your hands on a few basic supplies. The good news is that these initial investments will be repaid over time since you will reuse these materials to produce your favorite garden vegetables year after year.

There are several benefits to growing your tomato plants from seed rather than purchasing them from your local garden shop, so let’s take a look at how to properly hydrate your seeds and seedlings.

How Do You Know When Tomato Seeds and Seedlings Need Water?

When it comes to watering seeds and seedlings, there is a spectrum, as I said before.

On the one hand, too little watering causes dry soil, which prevents seeds from germinating and seedlings from developing strong root systems. On the other hand, if you water too much, your soil will be waterlogged, your seeds won’t get the oxygen they need to germinate properly, and your seedlings will likely yellow and begin losing leaves.

The goal is to water just enough to keep the soil from being too dry or overly moist. You want a nice moist soil like this Pro Mix variety, the kind that would retain its shape if you picked it up and squeezed it together.

Seeds will germinate in this soil while still having access to lots of oxygen, and seedlings will build healthy, powerful root systems that will provide them with the energy they need for appropriate development.

With this in mind, I’d want to take a deeper look at the materials I use to germinate and water my seeds, as well as the hallmarks of successful seed-starting, before discussing many things you can do to guarantee you’re properly watering your seedlings.

Seed and Seedling Supplies

As I previously said, you should water often enough to keep the soil wet so that the seeds may sprout effectively.

There are several methods for watering tomato seedlings, but my favorite includes a two-step process: I start my seeds in seed trays, then utilize a wicking technique to maintain continuous watering.

What I’ll do below is describe my procedure in as much detail as possible, then offer some information regarding previous top-watering methods that worked alright but not as well as I had planned.

Here’s how my current process works:

For starters, the water tray is the most critical aspect of my method. I rarely water my seeds or seedlings from above, like I did when I was a beginning gardener. Instead, I bottom-water by filling the water tray with water and allowing the soil in my seed trays to siphon up the water from below.

When it comes to seeds that haven’t yet germinated, I try not to fill the water tray more than a half inch because I don’t want to oversaturate the soil. While I’m cultivating seeds inside, I usually replenish the water tray once a week. If I’m growing them in my garage or shed, I may need to replenish the tray twice a week because of greater evaporation rates.

The Burpee self-watering device, which provides water to the seed tray through a wicking cloth, is my new favorite seed starting gadget.

Inside the water tray, there’s a plastic insert, and a thin cloth lays across the insert and hangs down into the tray. Just set your seed trays on top of the insert before filling the water tray.

When you do this, the cloth will wick water from the water tray and get saturated. That moisture is then pulled from the wicking cloth into the seed trays by the soil and eventually the plants’ roots too.

Avoid using the plastic dome. It’s crudely constructed, and you don’t need it. Also, be sure to cut your 72-cell tray into 8 9-cell pieces. It’ll be much easier to get your seedlings out that way.

As the tomato seeds germinate, I leave the seedlings in the seed tray until they’re about 3 inches tall and have four small branches. By this time, the first little leaves (known as cotyledons) have usually started to die off, and this tells me that it’s time to pot up so that the developing seedlings have enough space to grow into healthy young plants.

If you leave the seedlings in the Burpee seed trays for too long, two things will happen:

They will first begin to deplete the available nutrients in each little seed cell. When this happens, the leaves of the seedling will get very yellow, and they might even shed one or both of their lowest tiny branches.

Tomato Seedlings That Need to Be Potted Up

This indicates that you should pot up your seedlings right now. If you wait any longer, you’ll stunt your plants’ growth by depriving them of the nutrients they need.

Second, as the tomato seedlings grow taller and larger, their root systems will expand. As the roots extend, they’ll run out of room in the small seed cell and begin twisting around the perimeter, causing the plant to become root bound, as it’s called.

They can also climb through the bottom of the seed cell and get stuck to the wicking cloth, so when it comes time to remove and pot up the plant, they’ll get pulled off in the process. This isn’t a big problem, but it doesn’t help your plant develop to its maximum potential.

What’s good about the wicking process is that it makes watering a breeze when done correctly. All you have to do is ensure the wicking cloth is always moist, which will happen if there’s water in your water tray.

So just keep the water tray filled with a half inch or so of water, and the wicking cloth will have all the water it needs to keep your soil nice and moist. Bottom watering, even if you are not employing the wicking technique, will result in more constant watering for all of your seeds and seedlings.

One more thing: Seeds don’t need sunlight to germinate, but once seeds have germinated and produced cotyledon leaves, they’re going to need plenty of light. If you want to understand more about the LED grow lights I use and how to ensure your tomato seedlings receive enough light, click on the link above.

Seed-Starting Soil

When it comes to producing tomatoes from seed, you’ll get the greatest results if you utilize a light and airy seed-starting mix.

Last year, I did an experiment and attempted to grow tomato plants in soil that came directly from my garden, with no amendments whatsoever. I planted another batch of tomato plants on a seed tray using a good seed starting mix.

You may probably guess what followed: I had abysmal germination rates when attempting to grow plants in unamended garden soil. The soil was pretty thick, and it didn’t hold water effectively in seed trays, so the seeds didn’t have what they required to germinate and develop correctly.

Thankfully, I had a second tray of tomato seeds that I’d put in excellent seed-starting mix. My germination rates with those plants were fantastic (around 95%), and the experiment confirmed for me that it was worth purchasing quality seed-starting soil each year.

Of course, if you’re willing to amend your garden soil, you can definitely use that soil to grow seeds. But you must change it.

I recommend doing so with either coconut coir or peat moss, adding some perlite and vermiculite as well, plus a little touch of Osmocote slow-release fertilizer. This is what I do:

Soil	Coconut Coir	Perlite	Vermiculite	Fertilizer
2 parts	4 parts	1/2 part	1/4 part	Osmocote
4 parts	8 parts	1 part	1/2 part	Osmocote
6 parts	12 parts	1 1/2 parts	3/4 part	Osmocote
8 parts	16 parts	2 parts	1 part	Osmocote

Recipe for Homemade Seed Starting Soil in a Table

What’s lovely about outstanding soil is that it can withstand overwatering better than bad soil, retaining moisture to some level even if you forgot to water when you should have or watered a bit too much inadvertently.

But whatever method you end up using, you’ll get the best germination rates if you place heat mats underneath your water trays. These mats will keep the water warm, which will stimulate germination.

Watering Methods

I mentioned the wicking watering technique above since it is my preferred way for growing tomatoes from seed, but there are two more common methods I’d like to mention:

Top Watering
Bottom Watering

Most people think of top watering when they think of watering. When you top water, you take a watering container, and you wet the plants from the top. This works fine with larger plants, and even larger seedlings too, but I think it’s an inadequate way to water tomato seeds. It causes inconsistencies in moisture levels, which might reduce germination rates.

Bottom watering, on the other hand, is similar to the wicking approach in that you put water to the water tray rather than watering the plants directly. The downside to bottom watering your tomato seeds is that you can unintentionally fill your water tray too high with water and overwater the seed trays, making the soil overly soggy.

Bottom watering is a lot more constant and successful watering strategy than wicking, but if you have the resources on hand, wicking is the best way to obtain the most germination rates out of your seedlings.

If you’ve already potted up the seedlings from the original seed tray into 3-4 inch pots and your seeds have germinated and your seedlings have grown at least 4 to 6 inches tall, you may top water or bottom water them. Since there is so much more soil at that stage, it will not dry up as quickly.

Even if you’ve previously potted up your tomato seedlings, I still suggest bottom watering, but either method works since the plants will grow stronger and have better established root systems at this stage.

Can You Overwater Tomato Seeds?

The goal when it comes to watering seeds is to achieve a balance between dry and wet. This is why I describe the ideal soil as moist: it is neither too dry nor too wet.

Tomato seeds may be overwatered as a result of excessive top or bottom irrigation. At this point, the soil becomes so soggy that the seeds can’t get the proper amount of oxygen, which can negatively impact germination rates.

I’ve already blogged about how to prevent overwatering tomato seeds. Check out that article if you’d like to learn more about when and how to water to get the best results.

How Do I Make Tomato Seeds Germinate Faster?

Quality seeds, wet soil, and warm temps are three essential factors for achieving the best and quickest germination rates.

You may germinate seeds in a variety of methods, but this combination is likely to result in high germination rates.

The paper towel technique is the quickest but most time-consuming approach to germinate tomato seeds. Instead of planting seeds directly in soil, seeds are instead placed in voiced paper towels and sealed up in small plastic bags, then left in a temperature controlled environment.

Here’s a quick step-by-step guide to what you do:

Use one plastic bag for each tomato type. Try not to germinate many tomato kinds in the same bag. You’ll just make your life more difficult as a result.
Mark your bags ahead of time using a Sharpie pen. This is much easier than trying to label them once you’ve got paper towels and seeds inside.
Wet a paper towel. Don’t entirely wet it. Just wet it enough so that, when folded over and placed in the bag, the paper towel has enough water to get completely saturated.
Wait 3-4 days before checking your seedlings. After that, check them once per day.
After they germinate, gently take them from the paper towel using tweezers and put them in a seed tray filled with appropriate seed-starting soil.

What you’re looking for ideally is a seed that has sprouted and produced its first root (the radicle), which is a precursor to the growth of the stem and its cotyledon leaves. In case you’re curious in what tomato seeds look like at this time, I’ve written about it.

After you observe the radicle, you’ve had a successful germination and may place the seed in soil.

If you wait too long to examine your seeds, the seed will form its radicle, then its stem and cotyledon leaves right on the paper towel.

Once that’s happened, your seeds are still fine, but you’ll need to be very careful when planting them, or you’ll damage the main stem and kill the tiny seedling. In these cases, I extract the seed from the paper towel with tweezers and delicately plant it in soil, leaving a portion of the main stem and leaves above the soil line.

My seeds have done just fine in these situations, but as I said earlier, you’ll have to work slowly, and in some cases, the seed will stick to the paper towel, and the main stem will get damaged when you attempt to separate them. Yet, if you’re cautious, your small seedlings will start to appear normal after 4-6 days in the soil and a chance to establish themselves.

This is not the sole method for germinating tomato seeds. Seeds may germinate in water. Fresh tomato seeds may be germinated. You can even plant whole tomatoes, if you see one in your garden that’s not worth eating but don’t want to compost or trash it.