One of the best things about starting your tomato plants from seed is that you get to choose from hundreds of types of tomato plants. But even more importantly, when you start tomato seeds indoors, you have control over their care at every stage in their life.
You don’t know if plants bought at the nursery have been mislabeled, treated with chemicals, or stressed, but tomato plants you grow from seed at home receive your TLC from day one.
Before you get ready to start your tomato seeds, you first must choose some tomato varieties for your garden. I recommend that you read through ‘How To Choose The Best Tomato For Your Vegetable Garden.’ It will help you sort through the thousands of options and land on a variety that will suit your needs and growing conditions.
What you’ll learn in this post
- The ideal conditions for germinating tomato seeds
- How to take care of seedlings for their first weeks of life
- What you can do to prepare seedlings for transplanting out into the garden
First step: Gather supplies
You will need:
- Germinating mix
- Containers + dome lid or plastic bag
- Heat mat w/ thermometer (recommended, but not necessary)
- Shop light or grow light
Get help finding seed starting supplies.
Now it’s time to get those seeds in the ideal environment for sprouting.
- Germinating mix is a soilless mix that makes it easier for your seeds to send out roots and pop through the soil to open their first leaves. Although you don’t absolutely have to use a germinating mix, it is better for building a healthier root system.
- Most germinating mixes consist of peat moss/coconut coir + perlite + vermiculite
It should not be difficult to find germinating mix (also called seed starting mix) wherever you find gardening supplies.
Make your own germinating mix by mixing perlite and peat moss (or coconut coir) +/- vermiculite. Seeds will also germinate in vermiculite or peat moss alone. And if you’re using potting soil for starting seeds, sift out any sticks and clumps.
- For containers, you can use seed starting trays or any shallow container. Seed starting trays are great if you’re starting a dozen or more plants.
- You’ll also want a clear dome lid to cover your trays. Covering them helps trap heat and moisture and speeds up germination.
If you’re just starting 3 or 4 tomatoes for your garden, you can use some empty yogurt cups, toilet paper rolls, or cardboard egg carton.
Remember that we will be transplanting them into larger containers a week or two, so don’t worry about the container being too small.
Transplanting might seem like an extra step, but it takes literally seconds to do and it makes for healthier transplants in the spring.
- For labeling, you just need a way to mark which container has which plant in it.
So whether that is writing on the container, on popsicle sticks, or a piece of tape, just make sure your marking doesn’t get rinsed off when you water.
- A heat mat with thermometer offers you the ability to control the soil temperature. Tomato seeds prefer warm soil to germinate: 75-85 degrees F/23-29 C. We set our thermometer to 82 degrees Fahrenheit.
Just setting your seeds inside your house is probably warm enough even if it’s not quite 75 degrees in there. To increase the temp just a little, you can place your seed on top of your refrigerator and/or slip the whole container into a plastic bag to trap heat and moisture.
Don’t try to use a heating pad, they get way too hot and will cook your seeds and seedlings.
At cooler temperatures, you can expect germination to be slower and less consistent-meaning some of your seeds may not sprout. Give your seeds up to 10 days to germinate.
- You’ll need to set up a light for your plants within a week or so of planting them, so make sure you’ve got a plan for setting it up.
We like to use metal racks that make hanging lights over seedlings really easy, but you can use homemade or bought table top hangers and they even make clip on and desk lamp style grow lights that will work great for starting just a few seeds.
Step Two: Germinating seeds (4-8 weeks before planting in the garden)
By starting tomatoes from seed indoors, you are able to provide ideal conditions for germination and healthy growth during the early stages.
The ideal conditions for germinating tomato seeds include:
- A lightweight, fine grain seed starting mix
- A soil temperature of 80-82 degrees Fahrenheit
- Even moisture (do not let them dry out)
Before filling your containers with seed starting mix, I recommend pre-moistening your seed starting medium.
This allows you to stir the water in and make sure it is evenly distributed within your containers. Wet it down to the point that it mostly holds its shape when you squeeze it in your hand, but it’s not soggy or dripping wet.
- Using the premoistened seed starting mix, fill your seed starting containers to the top. To help the seed starting mix settle into your containers and remove any air pockets, lift your container about an inch and drop it. Top off if needed.
You don’t want to pack the mix into the container, but you want it filled completely. Air pockets within the mix are not good for root growth.
- Before you put your seeds into the mix, label the container with variety name and date. You might also include how many seeds are in the container.
For placing your seeds, you can either poke a hole first and set the seed in or you can set the seed on top and poke it in after. Either way, set your seed in about 1/2-1cm deep. I use my pointer finger to do this: up to the nail bed but not past the first knuckle.
How many tomato seeds per container?
This is up to you, but here’s my recommendation. If they are brand new seeds and you have a heat mat set up, you’ll probably get close to 100% germination. So plant exactly what you need plus maybe one or two more.
If the seeds are old, were saved at home not using fermentation, or if you don’t have a heat mat set up, plant a few extra just in case they don’t all come up.
If you plant extra seeds and they all pop, cut out the weakest sprouts and leave only what you need.
- Once they’re in the container, cover your seeds lightly with seed starting mix or vermiculite. Don’t press down, just sprinkle.
Then water your seedlings gently, cover them with a dome lid and put them on their heat mat (or put them in a plastic bag and store them in a warm place).
How long does it take for tomato seeds to sprout?
- It will take as few as 2 and up to 10 days for your seeds to sprout.
You must check on them every single day. Gently water them as needed. They need to stay moist but not soggy.
While you wait, go ahead and get set up to provide them with light. A sunny window might work, but for best results, use a dedicated light for your plants.
- An LED grow light is great, that’s what we use, but a simple shop light also does the trick (we have those, too).
Hang your light so that it sits 2-4 inches above your dome lid so your seedlings get light as soon as they pop. Leave your light on 24/7 for now.
Once they sprout, continue to check on your seedlings daily.
- Water lightly when the top of the soil appears dry.
- Keep them on the heat mat until all of the seeds have sprouted.
After all the seeds have sprouted (or 10 days) you can remove the lid and pull them off the heat mat/top of the refrigerator. At this point, light is the most important thing for producing strong seedlings for the garden.
- If your seedlings seem to have a really long stem, then they are either too hot or too far from light. We call that stretching and refer to the seedlings as leggy.
To remedy that, remove them from the heat mat (assuming they’re inside your home) and/or pull the light down closer to the seedling.
How to care for tomato seedlings after they sprout
Sprouted tomato seedlings simply need water and light for the next week or two.
You’ll soon see they’ll grow new leaves from the top. The new leaves will look very different from the baby leaves.
Those baby leaves are called cotyledons, and they develop within and pop out of the seed. They’ll grow a little larger, but soon the true leaves start to form.
True leaves are the type of leaves you see on adult plants that are regular or potato leaf.
- Once you see the true leaves, you’ll know it’s nearing time to transplant your seedlings.
Step 3: Transplanting Seedlings (2-6 weeks before planting in the garden)
The benefits of transplanting seedlings are:
- Provides the seedlings with more space for their roots to grow.
- Adding fertilizer in the new container encourages root growth and feeds the plants.
- Allows you to bury the stem of the seedling to encourage root growth from the stem.
It’s all about encouraging the seedling to make stronger roots so that it has a better start when it gets transplanted out in the garden. Stronger roots mean that you have sturdier plants that are better prepared to seek out nutrients and water from the soil once they’re out in the ‘real world.’
- For transplanting your seedlings, choose a container that is at least twice as big as your seed starting container (a 4-inch pot is perfect).
You’ll be planting your young plants from this container into your garden or their permanent home. So make sure your container is easy to remove without disturbing the roots of your young tomato plant.
- Use half potting soil and half seed starting mix in your new container.
Potting soil contains some clumps and sticks as well as fertilizer (usually). Young seedlings are still very tender, and they’ll grow stronger in the long run if you baby them a little right now. So we dilute the potting soil with the seed starting mix to make a more gentle transition.
Instead of potting soil, you could use high-quality compost (50:50 mix) or add some worm castings (3/4 seed mix + 1/4 worm castings).
- Premoisten the soil and tap it down into the container like we did when we started the seeds.
Make a hole in the soil in your new container with your finger. Go down as deep as you can without hitting the bottom or burying the entire plant. Tomatoes love to have their stem buried, so get as much of it down in the soil as you can.
- When you’re moving your seedling from one location to the other, DO NOT grab it by the stem.
Grabbing it by the stem can:
- Dislodge it from its roots and kill the plant
- Permanently damage the vascular system within the stem
Loosen around the edges of your seedling’s container by gently squeezing, turning it upside down, or poking in through a hole at the bottom. Then pinch the true leaves between 2 fingers and lift up. It should slide out easily.
- Set your seedling into the hole in the new container. Bury it deep, and leave only the leaves on the top sticking out.
If your seedling has grown up a bit and you need to remove some leaves, that is fine. Leave at least 2 sets above ground, but don’t bury any leaves. Just gently pinch them off as you set it in its new home.
- Fill around it with your soil mix and press gently to set it in. Then water and place it back under the light.
Step 4: Harden off your seedlings (1-3 weeks before planting in the garden)
The process of hardening off just means that we are going to prepare our seedlings to get used to the harsher environment that exists outside your home.
They need to develop a tolerance for direct sunlight and wind, but this must be done gradually so that the plants are not harmed. If you don’t harden them off, you can have plants that wilt, develop sunburn, or fall over.
To harden off your seedlings, simply move them outdoors for 1-2 hours for a few days. Start by placing them where they will receive indirect light such as in a greenhouse or under a porch or eave.
Overcast days in early spring are perfect for hardening off tomatoes.
If you don’t have any indirect sunlight, direct is okay. If it seems too harsh to you, it probably is. Just shorten their time out there to about 30 minutes for a day or two.
After a couple of days, gradually move every day until they are in direct sunlight. After 4 days, increase the time to 3-4 hours, then 4-6 hours, and so on until they can tolerate 8 hours of bright direct sunlight per day.
If your seedlings get some sunburn spots on them, don’t panic. Give them a day or two to recover indoors and then start over.
Once your seedlings have been hardened off, you’re all set to plant them out in your garden!
- Gather seed starting supplies
- Plant your seeds
- Move them to light when they germinate (if they aren’t already under lights)
- Transplant seedlings when they have true leaves
- Harden off over 1-3 weeks before planting out
Questions about starting tomato seeds?
Cool. I’m on it! Leave me a comment or join my Facebook group to get help.