How to Grow Tomato Seeds Indoors
By Steven Biggs
Learning How to Plant Tomato Seeds
When I was 10 years old my grandfather helped me sow tomato seeds. My first yellow-fruited tomatoes.
I had the perfect teacher. Dido was a life-long gardener and retired market gardener.
He was visiting us from Calgary that spring. We didn’t have much in the way of seed-starting supplies at our place. So he just grabbed an unused wash basin. We put a couple of inches of potting soil in it, sprinkled seeds on top. And then a thin layer of soil.
I gave lots of yellow tomatoes to the neighbours that summer!
His no-fuss approach to gardening coloured mine. There are lots of great supplies, gadget, and tricks…if you want. But you can also keep gardening—and growing tomato seeds—really simple.
In this article I share ideas about how to grow tomato seeds in a way that suits your situation.
Choose a Tomato Variety
Find out how to grow tomato seeds in a way that suits your situation.
Before planting, select a variety that gives you what you want.
Here are things I think about as I choose tomato varieties:
Colour
Size
Taste
How long it takes to mature
Plant stature
Disease Resistance
How I use the tomatoes (sauce, sandwiches, packed lunches)
Storage properties (there are storage—a.k.a. “keeper” tomatoes!)
Find out about “keeper” tomatoes.
Get tomato-choosing tips in this article by my tomato-crazy daughter, Emma.
When to Start Tomato Seeds Indoors
Grow tomato seeds indoors to get a head start on the growing season. These seedllings are growing in a wooden mandarin crate.
We grow tomato seeds indoors to get a head start on the growing season. That head start gives us an earlier harvest.
When it comes to timing, the date of the last spring frost is our guidepost. You’ll often see this date called the “last frost date” or “average last frost date.”
Find out the average last frost date for your area, and then count backwards 6 to 8 weeks.
For example:
The average last frost date around here is mid May. So working backwards 8 weeks, I know that I should be starting tomato seeds indoors around mid March.
This is not an exact science.
So don’t sweat the exact date. Aim for approximately 6-8 weeks. But don’t start too early, because you could end up with leggy seedlings.
Create Your Own Unique Edible Landscape
That fits for your yard, and your style!
Supplies for Growing Tomato Seeds Indoors
The supplies you need to grow tomato seeds depends on your approach to gardening. As I mentioned above, you can keep it pretty simple.
Cell packs are a good option if you’re growing a lot of seedlings.
Here are basic supplies:
Potting soil
Pots or containers (ideas below)
Labels
Seeds
Optional goodies:
Heat mat
Fan
Dome or cover
Containers for Starting Tomato Seeds
You can buy purpose-made containers for starting seeds. Or you might already have things you can reuse for seed-starting. (Horticulture creates a lot of plastic waste…and a bit of creativity with seed-starting containers is a great way to generate less waste.)
Here are ideas for seed-starting containers for tomatoes:
Plug trays are an option if you want to start lots of seeds.
Cell packs (the plastic containers with multiple holes, often used for bedding plants – a good option if you’re growing a lot of seedlings)
Plug trays (trays with a number of smaller holes, more commonly used in commercial greenhouses)
Pots
Crates (I’ve used wooden mandarin orange crates)
Newspaper pots (paper pots wick moisture and dry quickly, so adjust your watering accordingly)
Egg cartons (like egg shells below, too small for growing tomato plants to the final transplanting size, but if it’s all you have, they’re OK for getting seeds started before transplanting into a bigger pot)
Egg shells (There are lots of cutesy pictures online of seeds growing in egg shells. My suggestion is don’t bother, they’re impractical)
Newspaper pots are easy to make. They can be planted directly into the garden as the roots grow right through the newspaper.
If you’re using biodegradable, natural-fibre pots (peat pots are common, and I’ve even seen pots made from cow manure) a word of caution: Bury the entire pot when planting in the garden, or the whole thing is a wick, wicking water away from plant roots.
Soil for Starting Tomato Seeds
Top Tip: Don’t use garden soil.
That’s for two reasons: First, many garden soils have a structure that packs down, preventing little roots from growing. The other thing is that garden soil can harbour diseases that kill young tomato seedlings.
Use potting soil.
You might see potting soils specifically for seed-starting. These are made with ingredients that are more finely ground, so that there are no chunks of material blocking the way of little germinating seeds.
You don’t need the finely ground potting soils.
A general-purpose potting soil is fine. (A finely ground soil mix is important for commercial growers who need uniform, optimal seed germination; but in a home garden we usually have more seeds than we need, so having the odd coarse chunk in the potting soil is no big deal).
If the soil is dry, moisten it before using it.
Light for Your Tomato Seedlings
Lights for growing tomato seedlings. I use fluorescent lights. Note that the trays are propped up to be as close as possible to the lights.
Good light is important for growing compact plants. When there’s not enough light you end up with leggy plants that topple over.
There are lots of lighting options. The simplest and least expensive is to grow seedlings in a bright window (south facing is best.)
If you don’t have a bright window, you can start seeds under lights. There are many types of lights available.
Here’s the thing to know: Your tomato seedlings don’t need the same light as an indoor hydroponic vegetable crop.
You’re not trying to create perfect conditions to grow a plant right through to harvest. The lights just need to be good enough to give you healthy, fairly compact tomato transplants.
So save yourself some money and don’t go overboard on lighting.
My lighting for growing tomato seedlings is fluorescent shop lights. You don’t need full spectrum lights, nor do you need the strongest lights. Remember: You’re growing a young plant to transplant outdoors – where it will spend the rest of its life in sunlight.
Some light are adjustable, allowing you to move the lights close to the seedlings. Mine aren’t, so instead, I prop up trays of plants closer to the lights by putting something underneath them.
Use a timer so that you don’t have to remember to turn the lights on and off every day. I leave mine on for 16 hours a day.
Landscape with Fruit
That’s easy to grow in a home garden!
Planting Tomato Seeds
Fill your container loosely with potting soil, leaving a bit of space at the top.
If you’re planting an individual seed, place it on top of the soil
If you’re planting a number of seeds in one container, space them out on top of the soil
Then place a thin layer of potting soil over the top. You don’t need to put much potting soil over the seed: Cover with a depth of soil similar to the seed width…up to about one-quarter inch. (Too deep and they might not grow.)
Then tamp very gently and water.
What we’re trying to do by covering with soil and tamping is to make sure the seed is in contact with soil, which helps with uniform germination.
(NOTE: A lot of people “bottom water” seedlings. This just means sitting a container in water so that water wicks upwards. The reason is that a strong jet of water from a watering can can move around seeds and soil. So feel free to do the bottom watering if you like, although I find that watering gently, by putting my finger over the tip of the watering can to slow the flow of water, is enough to prevent making a potting soil sinkhole.)
Tomato Seeding Density
Depending on how many plants you’re growing and how much space you have, you can seed more or less densely:
Grow 2-3 seeds per pot, and then thin out extras as the plants begin to grow.
Low Density
If you have lots of space and aren’t growing a lot, give each seed its own pot or section within a cell pack. One seed per pot takes up far more space initially. But if you want to keep things very simple, this is a good way to do it because it can cut out the step of transplanting later on.
Remember that a bigger pot dries out more slowly; water them accordingly so they aren’t waterlogged.
Tip: It’s rare to have every single seed germinate. To avoid having empty pots where seeds haven’t germinated, place 2-3 seeds per pot, and then thin out extras as the plants begin to grow.
High Density
If you want to start more seeds in less space, plant a number of seeds together in a container, and then separate and transplant them as they get bigger.
Labelling Tomato Seeds
A tray of labelled tomato seedlings.
If you’re growing more than one variety, label them as you plant them. There are years I was sure I’d remember what was what…and I forgot. You can buy purpose-made plastic labels; or, if you want to use less plastic, wooden popsickle sticks or wooden tongue depressors work well (but don’t last all summer.)
Hygiene
Damping off disease can wipe out a whole crop of seedlings quickly. It’s a fungal disease that moves around your growing area along with runoff from watering plants. It gets there in contaminated soil and on previously used equipment and pots.
The easiest way to avoid damping off is to use disease-free potting soil and clean supplies. If you make your own potting soil using compost, consider sterilizing it. Many commercial potting soils are made with peat and inert ingredients—and these tend to be disease free.
New pots and cell packs are likely disease free, but if you’re reusing pots and cell packs, sterilize them. First, scrub off any soil that’s caked on. Then dunk them in a solution of 10 parts water with 1 part bleach, and then let them air dry. (Note: This is a strong smelling solution, so I do this outdoors or in my garage.)
How Many Tomato Seeds to Grow
Planting extra tomato seeds is cheap insurance against plants lost to accidents.
I always grow extra plants.
Then I share leftovers with friends.
Conditions for Germinating Tomato Seed
Temperature
Find a warm spot to germinate tomato seeds. Pictured here: Germinating a flat of tomato seeds beside a heat duct near the basement ceiling — a warm location.
Room temperature is fine for germinating tomato seeds, but you can speed things up and get a more uniform germination if the temperature is warmer.
Here are ways to give your seeds warmer conditions:
Place them on a heated floor
Set them on top of a hot-water radiator
The fluorescent fixtures in my light tray give off heat, so if I put seed containers on the rack above
Some appliances give off heat – check to see if the top of your fridge is warm
Or get a heat mat (a water-resistant heating pad for plants)
Once a half to three quarters of the seeds are up, I remove the container from the heat.
Humidity
The seedlings on the left are uncovered, to get air circulation. The seeds on the right are covered with a plastic cover to keep the humidity high during germination.
As a seed germinates, it needs moist surroundings until it grows roots and can take up water on its own. If it dries out at this vulnerable stage, it’s game over.
The air in centrally heated homes over the winter is often quite dry. You can keep the humidity higher right around the seeds by covering them. I use the clear plastic domes designed to go over top of plant trays. Or, cover a tray with a sheet of glass or plastic; or cover an individual pot with a plastic bag.
Light
Don’t worry about light while germinating tomato seeds.
So in summary, as you’re getting your tomato seeds to germinate, think warm and humid. (In a previous house, I’d germinate my seeds atop a shelf in the basement, near the heat duct at ceiling level, where it was nice and warm.)
Conditions for Growing Tomato Seedlings
Once your tomato seedlings are up and growing, the best conditions are different from what we want when germinating seeds.
Light and Temperature
As seedlings begin to grow, we want bright light and cooler temperatures. That’s because with warmer temperatures, growth is lanky. With cooler temperatures (combined with bright light) the plants will be more compact and sturdy.
See if there’s a cooler spot near a window. In my case, my grow lights are in the basement, in a room that is cooler than the rest of the house. It’s perfect for growing tomato seedlings.
Humidity and Air Circulation
High humidity while seed germinate is good. But as the plants grow, we want lower humidity and some air circulation, which reduces the chance of fugal diseases. The moving air also gives stronger stems. (Some people use a fan to improve air circulation.)
So in summary, once the plants are growing, you want cooler, brighter conditions than for germination.
Caring for Tomato Seedlings
Water
It’s easy to kill seedlings by overwatering them.
Keep the soil moist, but not sopping wet.
If in doubt about the amount of moisture in the soil, use your finger. (Whatever you do, don’t waste your money on a gadget that tells you soil moisture!)
(If you forget to water and your seedlings flop over, there might still be hope. I remember the seedlings I grew with Dido: we came home one day and they were flopped right over…looked like they were doing yoga. He shrugged and watered them—and they sprang back up within an hour.)
Feeding Tomato Seedlings
Check your potting soil mix to see if it contains fertilizer. Some do. And if it contains fertilizer, you might not need to feed.
Otherwise, once your plants have a few leaves, you can begin to feed them. I use a water-soluble fertilizer.
Don’t overfeed. It can damage delicate seedling roots. (I feed at half of the rate recommended on the label.)
Note: Don’t worry about feeding before the seedlings have a couple of sets of leaves. They’re still drawing from stored energy in the seed.
Thinning and Repotting
Transplant tomato seedlings after they get “true leaves.” These plants are beginning to grow true leaves.
If you planted a few seeds together in a container, once they get big enough to handle, you can separate them and give them more space.
Transplant tomato seedlings after they get “true leaves.” (The first leaves that appear are “seed leaves” so it’s the third and fourth leaves that are the true leaves. You’ll quickly see the difference.
I use a popsicle stick or pencil to tease apart soil and gently lift up a seedling and its roots. (There are purpose-made gadgets for this…not necessary.)
When picking up seedlings, hold them by the leaves. That’s because it’s very easy to crush the stem. (If you crush a leaf, it can grow a new leaf…if you crush the stem, it’s game over.)
Transplanting Tomato Plants in the Garden
Moving tomato seedlings outdoors to a cold frame in the spring so that they are hardened off for planting into the garden. If the light is bright, we cover the top of the cold frame with something to shade the plants below.
When it’s time to move your tomato seedlings to the garden, there’s one last thing to remember. Your seedlings have grown in the house, in moderate conditions. Once they’re outdoors, the light is brighter, the temperature swings, and there’s wind.
So we “harden off” seedlings, which simply means we get them used to outdoor conditions. We do this by putting the plants outdoors, in the shade, for a few hours each day. Give them a longer stint in the sun each day, and keep doing this for at least a week.
FAQ - How to Grow Tomato Seeds
Can I plant tomato seeds directly in the garden?
Yes, but your harvest will be weeks behind plants started indoors. (Tomato plants sometimes come up on their own in the garden where tomatoes fell to the ground the previous year…but the harvest is late.)
Should I soak my tomato seeds?
No, it’s not necessary.
Can I save my own seed?
Absolutely. It can be as simple as smearing some seeds on a paper towel, or you can clean them more through a fermentation process. In our household, I do the former, while my daughter does the latter.
Note: If you save seeds from hybrid tomato varieties, the seeds you end up with will be different from the parent plant. If you plan to save seeds, look for “open pollinated” varieties.
Is it too late if I start my tomato seeds only 4 weeks before the last frost?
No, but your harvest won’t be as early.
Can I start my tomato plants 10 weeks before the last frost?
Yes, but when they’re started indoors early, there’s more chance of ending up with lanky plants, unless you have very good growing conditions. If I had a greenhouse, I’d consider starting a few plants earlier—for an earlier harvest.
Where can I find the average last frost date for my area?
The easiest way is to do an online search. Some seed companies list dates, as do some master gardener groups and extension agencies.
More About Growing Tomatoes
Hear Tips from Tomato Experts
One More Idea…
We always grow microgreens indoors over the winter. It’s a perfect crop to grow under lights with your tomato seedlings.
Find out how to grow microgreens indoors.
