Starting Tomatoes Indoors: A Week-by-Week Schedule
Table of Contents
Every year, the conversation about when to start tomatoes ends up in the same place: people who started too early are dealing with root-bound plants in four-inch pots with nowhere to go, waiting for a transplant date still three weeks out. And people who started too late are watching neighbors put plants in the ground while their seedlings are still thin and pale on a windowsill.
Six weeks before your last frost date is the right starting point for most tomatoes. Compact determinates and fast-maturing varieties like Siletz or Early Girl are ready right at that mark. Large-fruited indeterminates (Brandywine, Cherokee Purple, most beefsteak types) grow more slowly in their early weeks and can benefit from seven to eight weeks. If you go past eight, you’re racing against the rootbound problem: a tomato that’s been in a small container too long develops a stunted root system that struggles to establish after transplant, regardless of how good the plant looks above soil. If you start a large heirloom at eight weeks and it’s filling its container by week six, up-pot it rather than letting it stall.
That window exists for a reason. Tomatoes started at the right time will be stocky, well-rooted, and hardened off by transplant day. Tomatoes started too early get leggy, develop thick stems that struggle to establish, and often sit in shock for two weeks after transplanting while a six-week plant walks past them.
Here is what the six weeks look like, what happens in each one, and what to watch for so you don’t lose the whole tray to a preventable mistake.
The six-week window is not a starting point for early tomatoes. It is the right amount of time to produce a transplant-ready plant. Large-fruited heirlooms may need seven to eight weeks, but never more, or the rootbound clock starts running.
How to find your date
Your last frost date is the average date of the last 32°F reading in spring for your area. It is a probability, not a guarantee — in most zones, there is roughly a 50% chance of frost after that date, which is why experienced growers add a buffer and watch the forecast in the final two weeks.
The USDA hardiness zone map gives general guidance, but your county extension service will have more precise last frost data for your specific area. Most extension services publish 10%, 50%, and 90% probability frost dates — the 10% date (only a 10% chance of frost after this point) is the one worth targeting for tomatoes.
Once you have your last frost date, count back six weeks for most varieties, or seven to eight for large indeterminates. Write it down. The most common seed-starting mistake isn’t failing at germination — it’s starting at the wrong time because you didn’t count backward from a fixed date.
The week-by-week schedule
The table below assumes a last frost date (LFD) of April 12 — adjust your actual dates by counting back from yours.
| Week | Timing (example) | What to do |
| Week -6 | ~March 1 | Sow seeds in trays. Soil temp 70–75°F (80–85°F for large indeterminates). No light needed yet. |
| Week −5 | ~March 8 | Germination. Move to light immediately at first sprout. 14–16 hrs/day. |
| Week −4 | ~March 15 | First true leaves emerging. Begin watering with dilute fertilizer (quarter strength). |
| Week −3 | ~March 22 | Up-pot if roots are circling or growth slows. 3–4” containers. |
| Week −2 | ~March 29 | Begin hardening off. One hour of outdoor exposure daily, protected from wind. |
| Week −1 | ~April 5 | Increase outdoor time daily. Overnight temps must stay above 50°F. |
| Week 0 | ~April 12 (LFD) | Transplant after last frost, but confirm soil temperature first. See below. |
What the table can’t show is what to watch for inside each week. That’s where most losses happen.
Why your last frost date isn’t the same as your planting date
The last frost date tells you when it’s probably safe to put tomatoes in the ground without losing them to cold. It doesn’t tell you whether the ground is ready to grow them.
Soil temperature at transplant depth (six to eight inches down) needs to be at least 60°F for tomatoes to establish well. Below that, root development stalls. The plant sits. It doesn’t die, but it doesn’t grow either, and a tomato that spends two weeks in cold soil in May often gets caught and passed by a plant put in three weeks later into warm ground. You waited out the frost date and still lost the race.
In Zone 5 and colder, the last frost date and the soil-ready date can be two to three weeks apart. A May 10 last frost date doesn’t mean the soil six inches down is 60°F on May 11. It might not get there until late May or even early June, depending on the spring, how much rain has fallen, and whether your beds are raised or in-ground.
The fix is simple: take the soil temperature before you plant, not just the air temperature. Push a soil thermometer to transplant depth in the morning, soil is coldest then, and if you’re reading below 60°F, wait. Cover the bed with black plastic or dark landscape fabric for a week to absorb heat and you can often gain five to seven degrees in that time.
The gardeners who consistently get early tomatoes aren’t necessarily planting earlier. They’re planting into warm soil, which is a different thing. As the seasons progress, you’ll learn when to plant based on the weather and your own knowledge of the land. We didn’t have soil thermometers on the farm years ago. We judged when to plant by how the weather changed that year. It didn’t always work, and so making use of a thermometer is a good way to start out with a little more confidence.
The frost date is the earliest possible date. Warm soil is the actual condition. Don’t confuse the two.
What each week actually looks like
Week −6: Sowing
Fill your cells or trays with a seed-starting mix, not potting soil, which is too coarse. Moisten it before you sow, not after. Dry mix repels water and takes a long time to wet evenly once seeds are in place.
Sow two seeds per cell, a quarter inch deep. Tomatoes are reliable germinators at the right temperature; you will likely need to thin. Cover loosely and maintain soil temperature at 70 to 75°F for most varieties, or 80 to 85°F if you’re starting large indeterminate heirlooms. See our guide to soil temperature if you don’t have a heat mat, there are several ways to get there without one.
No light needed at this stage. A covered tray in a warm spot is all you need.
Week −5: Germination
Most tomato seeds germinate in five to ten days at proper temperature. Check daily from day four on.
The moment you see the first seedling arch above the soil, remove any cover and move the tray to your light source. Not the next morning. Not after dinner. Immediately. A seedling that stretches toward distant light in its first hours will be leggy from the start, and that doesn’t recover.
Position grow lights two to three inches above the seedling tops and run them fourteen to sixteen hours a day. If you’re using a south-facing window, the seedlings will still lean toward the glass. Rotate the tray ninety degrees every day to keep them upright.
Week −4: True leaves
The first pair of leaves that emerge are cotyledons, seed leaves, not true leaves. The first true leaves appear in the second week, and when you see them, the seedling has shifted from relying on the seed’s stored energy to photosynthesizing on its own.
This is when you begin fertilizing, lightly. Quarter-strength liquid fertilizer once a week is enough. Over-fertilizing at this stage produces soft, lush growth that’s more susceptible to disease and handles transplanting poorly.
If two seeds germinated in the same cell, thin to the strongest one now. Use scissors. Cut at the soil line rather than pulling, which can disturb the roots of the seedling you’re keeping.
Week −3: Up-potting (if needed)
Check whether your seedlings need more room. Signs they do: roots visible through the bottom of the cell, growth that seems to have stalled, or yellowing despite adequate water and fertilizer.
Move them to three- or four-inch containers with standard potting mix. When you transplant, bury the stem up to the lowest leaves. Tomatoes produce roots along their buried stems, and this makes for a much stronger plant.
Weeks −2 to −1: Hardening off
Hardening off is the process of gradually introducing seedlings to outdoor conditions. Indoor plants have thin, soft tissue that hasn’t developed the waxy cuticle needed to handle direct sun, wind, and temperature fluctuation. Moving them outside without hardening first causes leaf scorch or death within a day.
Start with one hour of outdoor exposure in a sheltered, partially shaded spot, out of direct afternoon sun, protected from wind. Add an hour each day. By the end of the second week, the plants should be spending most of the day outside and coming in only if overnight temperatures drop below fifty degrees. If you have a cold frame, use it for the final nights.
Week 0: Transplant day
Check both the ten-day forecast and the soil temperature before you plant. A hard frost after transplant can kill established plants. Cold soil (below 60°F at transplant depth) will stall even a healthy plant. If either condition isn’t right, wait.
Transplant in the evening or on a cloudy day. Water deeply at planting and again the next day. Bury the stem up to the lowest true leaves. If the plant has grown tall and leggy, you can plant it in a trench at an angle. The buried stem will root, and the plant will straighten toward the sun within a few days.
The mistakes that cost entire trays
| The mistake | What it produces and how to fix it |
| Starting too early | Overgrown, rootbound plants that stall after transplant. Six to eight weeks is enough. Ten or twelve is too many. |
| Ignoring soil temperature | Plant sits in cold ground for two to three weeks while later-planted neighbors catch up. Check at transplant depth before planting. |
| Skipping hardening off | Transplant shock, leaf scorch, or death. One week outside, gradually, is not optional. |
| Not enough light | Leggy, pale seedlings that won’t recover. Grow lights 2–3” above the canopy, 14–16 hours a day. |
| Wet growing mix | Damping off – a fungal collapse at the soil line that kills entire trays overnight. Water from below. Let the surface dry between waterings. |
A note on variety timing
This schedule works for any tomato variety, but some take longer to mature from transplant than others. Days-to-maturity figures on seed packets count from transplant, not from seeding. A 75-day tomato started six weeks before your last frost date will be ready about eleven weeks after transplant day.
In a short-season zone (Zone 5 or colder), that math matters. A 90-day tomato started on the right schedule will push ripening into September. Short-season varieties bred for northern climates (Siletz, Stupice, Early Girl) are worth knowing if your season is tight.
Staggering your plantings, putting a first wave in at the frost-free date and a second wave two weeks later, hedges against late frost losses and spreads your harvest window across the season rather than compressing it into two weeks in August. See our guide to succession planting tomatoes for how to plan the calendar.
What you’re actually building in those six weeks
A transplant-ready tomato is a specific thing: eight to ten inches tall, with a thick stem that doesn’t bend, true leaves that are dark green and compact, and roots that fill the container without circling. It has spent time outside. It has been stressed mildly, by air movement, by temperature variation, by the reduction in fertilizer in the final week, and it is tougher for it.
That plant will establish in the garden faster, branch earlier, and set fruit sooner than a seedling that’s been coddled indoors for ten weeks. The six-week discipline exists because it produces the best plant, not because it’s the most convenient schedule.
Our family grew tomatoes on the same ground for three generations before I ever bought a seed packet. They didn’t have grow lights or heat mats or apps to calculate last frost dates. What they had was a fixed start date written on the kitchen calendar, a hand in the soil before planting to check the ground was warm enough, and the discipline to wait when it wasn’t. The method is older than the tools, and it still works.
If you’re still working out your germination setup, see What Temperature Do Seeds Actually Need to Germinate? for a rundown of four ways to hit the right soil temperature without a heat mat.
Front Garden Back Forty | Old knowledge. New season. Same good ground.