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Learning how to grow tomatoes from seed is one of those things that sounds more complicated than it is. I’ve done it multiple times and the process is genuinely straightforward once you understand what tomato seedlings actually need — and more importantly, what kills them before they ever make it outside. The main thing that trips people up isn’t the growing part, it’s the timing. Start too early and you end up with leggy, root-bound seedlings with nowhere to go. Start too late and you’ve lost weeks of the growing season for no reason.
This guide walks through the full process from seed to transplant — what supplies you need, when to start, how to keep seedlings alive indoors, and how to get them into the ground without losing them to transplant shock.
Step 1 — Time Your Start Date Correctly
Tomato seeds should be started indoors 6 to 8 weeks before your last expected frost date. Not 10 weeks, not 12 — 6 to 8. I made the mistake of starting too early my first year and had tomato plants nearly two feet tall with nowhere to go, sitting in increasingly cramped containers while I waited for the weather to cooperate. They were stressed before they ever hit the ground.
Find your last frost date first. The Old Farmer’s Almanac has a free frost date lookup by zip code. In central Indiana, that’s typically around April 15 to April 30 — which means starting seeds indoors between late February and mid-March. Count back 6 to 8 weeks from your last frost date and that’s your seed-starting window.
Write the date on your calendar and don’t jump the gun. Six to eight weeks of healthy indoor growth produces a better transplant than ten weeks of cramped, overgrown seedlings every time.
Step 2 — Gather Your Supplies
You don’t need much to start tomatoes from seed, but what you do need matters. Here’s the short list:
- Seed starting trays with cells (not garden soil — seed starting mix)
- Seed starting mix (lightweight, sterile, drains well)
- Tomato seeds (more on variety selection below)
- A grow light or a very bright south-facing window
- A spray bottle for watering without dislodging seeds
- Plastic dome or humidity cover for the trays
- A heat mat (optional but helpful — tomato seeds germinate faster in warm soil)
The single biggest mistake beginners make at this stage is using regular potting soil or garden soil in seed trays. Both are too heavy, drain poorly, and often carry pathogens that kill seedlings before they establish. Seed starting mix is lightweight, sterile, and designed specifically to support germination and early root growth.
A seed starting kit that includes trays, a humidity dome, and seed starting mix bundled together is the most efficient way to get set up if you’re doing this for the first time.
Step 3 — Choose Your Tomato Variety
The variety you start from seed matters more than most beginners realize, because it determines everything about the plant’s growth habit, disease resistance, and harvest window. Two categories to understand before you choose:
Determinate tomatoes (also called “bush” types) grow to a fixed height, set all their fruit at roughly the same time, and stop. They’re good for canning or preserving because you get a concentrated harvest. Roma is the classic determinate variety.
Indeterminate tomatoes keep growing and producing until frost kills them. They need caging or staking, produce continuously all season, and are the type most home gardeners prefer for fresh eating. Sweet 100, Sun Gold, Brandywine, and Early Girl are all indeterminate and all good starting points.
For beginners growing from seed, I’d recommend a cherry or grape variety for your first year — they’re more forgiving, produce faster, and give you more feedback as you learn. Sun Gold in particular is hard to fail with and produces sweet orange cherry tomatoes that disappear at every cookout.
Step 4 — Plant the Seeds
Fill your seed starting cells with moistened seed starting mix — damp but not soggy, like a wrung-out sponge. Press the mix lightly to remove air pockets without compacting it.
Plant two seeds per cell, about 1/4 inch deep. Don’t overthink the depth — tomato seeds are forgiving. Cover lightly with mix, mist the surface with a spray bottle, and place the humidity dome over the tray.
Set the tray somewhere warm — tomato seeds germinate best at soil temperatures between 70°F and 80°F. A heat mat under the tray speeds germination significantly, often cutting the time from 7 to 10 days down to 5 to 7 days. Once you see sprouts, remove the dome and move the tray under your grow light immediately.
Step 5 — Give Them Enough Light
This is where most indoor seedlings fail. A window — even a bright south-facing one — almost never provides enough light intensity for tomato seedlings. The result is etiolation: seedlings stretch upward toward the light source, growing tall and spindly with weak stems that can’t support the plant once it’s outside.
A dedicated grow light set 2 to 3 inches above the seedlings for 14 to 16 hours per day solves this entirely. As the seedlings grow, raise the light to keep it 2 to 3 inches above the top leaves. A simple outlet timer takes the guesswork out of the on/off schedule and ensures consistent light without you having to remember to flip a switch.
If your seedlings are already stretching toward the light and looking leggy, lower the light source immediately and add a small fan nearby — gentle air movement strengthens stems and helps prevent damping off, a fungal condition that kills seedlings at the soil line.
Step 6 — Water and Feed Carefully
Overwatering kills more tomato seedlings than anything else. The goal is consistently moist soil, not wet soil. Let the top of the mix dry slightly between waterings and water from the bottom when possible — pour water into the tray and let the cells absorb from below. This keeps the soil surface drier, which dramatically reduces the risk of damping off.
Tomato seedlings don’t need fertilizer until they’ve developed their first true leaves — the second set of leaves after the initial seed leaves. Once true leaves appear, feed weekly with a diluted liquid fertilizer at half the recommended strength. Full-strength fertilizer on young seedlings burns roots.
A soil moisture meter takes the guesswork out of when to water and prevents the overwatering that kills most indoor seedlings.
Step 7 — Thin to One Seedling Per Cell
Once seedlings are an inch or two tall and have their first true leaves, thin to one plant per cell. If two seeds germinated in the same cell, snip the weaker one at the soil line with scissors — don’t pull it out, which can disturb the roots of the one you’re keeping.
Choose the stockier, darker green seedling over the taller, leggier one. Height at this stage isn’t a sign of strength — it’s usually a sign of reaching for light.
Step 8 — Harden Off Before Transplanting
Hardening off is the process of gradually introducing indoor seedlings to outdoor conditions before transplanting. Skip this step and you’ll lose plants — I’ve done it and it’s a frustrating way to lose six weeks of work.
Start about 10 to 14 days before your planned transplant date. Set the seedlings outside in a sheltered spot with indirect light for one to two hours on the first day. Add an hour each day, gradually introducing more direct sun and wind over the two-week period. Bring them in if temperatures drop below 50°F or if strong wind or rain is forecast.
By the end of the hardening period, the seedlings should be spending the full day outside and coming in only at night. They’ll look noticeably sturdier and darker green than when you started — that’s the plant adjusting to real outdoor light intensity.
Step 9 — Transplant Into the Garden
Tomatoes are one of the few vegetables that can be planted deeply — bury the stem up to the lowest set of leaves and the buried portion will develop into additional roots, giving the plant a stronger foundation than it had in the cell tray.
Dig the hole deep enough to bury the stem, set the plant in, backfill, and water in well. Space tomatoes 18 to 36 inches apart depending on variety — indeterminate types need more room than determinates. Install your cage or stake at planting time rather than waiting — driving stakes into the ground around an established plant damages roots.
A tomato cage installed at planting prevents the stem damage that comes from trying to support a plant that’s already falling over.
What to Expect After Transplanting
Transplanted tomatoes often look rough for the first week — some wilting, some yellowing of lower leaves, and slower growth than you’d expect. This is normal transplant shock and doesn’t mean the plant is dying. Keep the soil consistently moist and resist the urge to fertilize heavily during the adjustment period.
Once the plant settles in and starts producing new growth — usually 7 to 14 days after transplanting — it will take off quickly. From there, consistent watering is the most important thing you can do. Irregular watering causes blossom end rot and fruit splitting, two of the most common tomato problems that have nothing to do with disease and everything to do with moisture consistency.
If you grew your tomatoes in a raised bed, our guide on how to build a raised garden bed covers irrigation setup, soil mix, and bed depth — all of which directly affect how well your tomatoes perform once they’re in the ground.
For a deep dive into pairing tomatoes with plants that help them thrive and naturally deter pests, our companion planting chart covers the best tomato companions and what to keep away from them.