Garden centers sell maybe a dozen tomato varieties in spring. Start your own from seed and you’ve got thousands to choose from, for a fraction of the price of buying plants.
That’s the real reason to grow tomatoes from seed. You get the rare heirlooms, the odd colors, the flavors you’ll never find as a transplant. And it isn’t hard. You need a few basics and a little patience, and this guide covers all of it, from the day you sow to the day the seedlings go in the ground.
Should you start from seed or just buy plants?
Fair question, and honestly, buying transplants is a perfectly good choice if you only want two or three common plants and would rather skip the fuss. No shame in it.
Starting from seed pays off when you’re after variety, quantity, or savings. One packet costs about the same as a single nursery plant and gives you dozens of seeds. And it’s the only way to grow the interesting stuff, the heirlooms and oddball colors that never show up on a garden center bench. Grow an heirloom once and you can even save its seeds for next year, free. If that’s you, read on.
When to start tomato seeds
Timing is the first thing to get right, and it hangs on one date: your average last frost.
Start your seeds indoors about six to eight weeks before that last frost. Garden.org puts the sweet spot at six to seven weeks. That gives the seedlings enough time to grow into sturdy young plants, ready to head outside once the danger of frost has passed.
Start too early and you end up with overgrown, root bound plants stuck inside waiting on the weather. Start too late and they’re still tiny when planting time arrives. If you’re not sure of your frost date, our planting date calculator works it out from your location and tells you exactly when to sow.
What you’ll need
Nothing fancy. Here’s the short list:
- Tomato seeds. This is where you get to go wild, with far more types of tomatoes than any store will ever carry.
- A seed starting mix, not garden soil. More on that in a second.
- Containers or cell trays with drainage holes. Clean yogurt cups work fine if you poke holes in the bottom.
- A grow light. This is the one thing people skimp on and regret. A sunny window rarely cuts it.
- Warmth for germination. A seedling heat mat helps, but a reliably warm spot in the house works too.
A small fan and a clear plastic cover are handy extras, and I’ll explain why as we go.
Use a seed starting mix, not soil
Don’t fill your trays with dirt from the garden, and don’t reach for regular potting soil either. Use a proper seed starting mix.
These mixes are light, fluffy, and free draining, so delicate new roots push through easily. They’re also sterile, which matters a lot. Garden soil carries the fungi that cause damping off, the disease that can flatten a whole tray of seedlings overnight. A clean, sterile mix is your first line of defense.
How to sow the seeds
This part is quick.
Moisten your seed starting mix first, so it’s damp like a wrung out sponge. Fill your containers and firm it lightly. Sow the seeds about a quarter inch deep, roughly the diameter of the seed itself, and cover them over.
Drop two or three seeds per cell to hedge against a dud, then thin to the strongest one later. And label everything now, because I promise you will not remember which cup is which in three weeks.
Cover the tray with a clear dome or a loose sheet of plastic. That traps humidity and keeps the surface from drying out while you wait for germination.
Warmth gets them germinating
Tomato seeds sprout on warmth, not light. They need no light at all until they break the surface.
Aim for a soil temperature around 70 to 75 degrees. At that warmth, most tomato seeds pop up within about five to ten days. Let it drop below 60 and germination crawls, taking weeks instead of days, if it happens at all.
A seedling heat mat under the tray is the easy way to hit that temperature. No mat? The top of the fridge, or any steadily warm spot, does the job. The moment you see green breaking through, the warmth stage is done and the light stage begins.
Light is everything (this is where people fail)
Here’s the single biggest mistake in starting tomatoes: not enough light.
The second your seedlings emerge, pull the cover off and get them under strong light right away. Not tomorrow. Now. A seedling that sits in dim light for even a few days stretches tall and thin reaching for more. You’re left with weak, floppy, leggy plants, and our leggy seedling guide covers the rescue if it happens.
A sunny windowsill sounds good but almost never delivers enough, especially in late winter when the days are short. A grow light earns its keep here. Keep a fluorescent or LED grow light just two to three inches above the tops of the seedlings. Run it 14 to 16 hours a day. As the plants grow taller, raise the light to hold that gap.
Get the light right and your seedlings stay short, stocky, and strong. That one thing separates good tomato starts from sad ones.
Watering without killing them
Seedlings want steady moisture, never a swamp. Keep the mix damp like that wrung out sponge, and don’t let it dry out completely or sit soggy.
The safest way to water is from the bottom. Set the tray in a shallow pool of water for a few minutes and let the mix drink it up, then pour off what’s left. Bottom watering keeps the surface drier, which discourages disease, and it draws the roots downward.
Watch out for damping off
Damping off is the seedling killer worth knowing about. It’s a fungal disease that strikes at the soil line, and its signature is brutal. Healthy looking seedlings suddenly keel over, their stems pinched thin and dark right where they meet the soil. They don’t come back.
It thrives in wet, still, crowded conditions, so you beat it by doing the opposite:
- Start with sterile seed starting mix, not garden soil.
- Don’t overwater. Damp, not soaked.
- Give them airflow. A small fan on low, or even just an open door, keeps air moving and surfaces drier.
- Don’t sow too thickly, since crowded seedlings trap moisture.
- Take the humidity cover off the moment the seeds sprout.
Do those and damping off mostly stays away.
Thinning and potting up
The first leaves a seedling opens are the seed leaves, and they don’t look like tomato leaves at all. The next set, the ones that look like real tomato foliage, are the true leaves. That’s your cue.
Once a seedling has its first set or two of true leaves, it’s ready to move into a bigger pot. Something like a 3 or 4 inch cup gives the roots room to grow. If you sowed a few seeds per cell, snip the weaker ones off at the soil line and keep the strongest. Yanking them out tears the roots of the one you’re keeping.
Feeding seedlings
Seed starting mix has little or no food in it, so once your seedlings have true leaves, they’ll want a bit of feeding. Give them a diluted, half strength liquid fertilizer every week or two. Keep it light. Seedlings want a snack, not a feast, and too much fertilizer does more harm than good at this stage.
Hardening off before they go outside
Here’s the step people skip, and it costs them plants. You can’t take a pampered indoor seedling and drop it straight into the garden. The sun, wind, and cool nights will shock it, and it may never recover.
So you harden them off, which just means introducing them to the outdoors gradually over about seven to ten days. Start with an hour or two in a sheltered, shady spot, then add more time and more sun and wind each day. By the end of the week or so, they’re spending full days outside and are tough enough to plant.
Planting them out
Once frost is safely past and the soil has warmed, your hardened off seedlings are ready for the garden or their final pots.
Plant them deep, right up to the top few leaves, since tomatoes grow roots along any buried stem and you want that big root system. From there, here’s exactly how to plant them out, and our full guide to growing tomatoes from seed to harvest takes over. Headed the container route instead? Here’s how to grow tomatoes in pots.
Quick troubleshooting
A few common hiccups and what they’re telling you:
- Seeds never came up. Usually old seed or soil that was too cold. Tomato seed stays good for years, but warmth is non negotiable, so check your temperature and try fresh seed if needed.
- Fuzzy white mold on the soil surface. A sign of too much moisture and too little air. Ease off the water, run a fan, and pull any cover off.
- Seedlings turning pale or yellow. Often they’re hungry once past the true leaf stage, or short on light. A weak feed and stronger light usually sort it out.
- Seedlings flopping at the soil line. That’s damping off, covered above. Prevention is the only real cure, so focus on airflow and easing off the water.
Frequently asked questions
When should I start tomato seeds indoors?
About six to eight weeks before your average last frost date. That gives seedlings time to grow strong before they go outside. Use your local frost date to count back and set your sowing day.
How long do tomato seeds take to germinate?
Usually five to ten days when the soil is kept around 70 to 75 degrees. Cooler than that and they take much longer, sometimes a few weeks.
Do tomato seeds need light to germinate?
No. They need warmth to germinate, not light. But the instant they sprout, they need strong light right away, or they turn leggy and weak.
Why are my tomato seedlings tall and skinny?
Not enough light. Leggy seedlings are stretching toward a weak light source. Move them under a grow light kept two to three inches above them, running 14 to 16 hours a day, and the new growth will come in stockier.
Can I start tomato seeds in egg cartons or cups?
Yes, any small container with drainage holes works, cups and cell trays included. Just be ready to pot up into something bigger once the seedlings have true leaves, because they outgrow tiny containers fast.
Bottom line: start about six to eight weeks before your last frost, with a sterile seed starting mix. Keep them warm to germinate, then get them under strong light the second they sprout. Water gently, harden them off, and you’ll grow better tomato plants than you can buy, in varieties you’d never find on a shelf.