When to Start Tomato Seeds Indoors: Dates by Source and Last Frost

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What each saved source says about starting tomatoes indoors (each row is one source; none is averaged)

Source	Weeks indoors	Start seeds	Set out	Counted from
University of New Hampshire	6 to 8 weeks	Count back from the day you set out	1 to 2 weeks after the last frost	The last frost
North Dakota State University	6 weeks	6 weeks before the last frost	2 weeks after the last frost	The last frost
Montana State University	6 to 10 weeks	Count back from your planting date	Planting window opens 1 week before the last frost	The last frost
University of Minnesota	5 to 6 weeks	Early April (Minnesota)	Not given	A Minnesota calendar
NC State	6 to 8 weeks (any transplant)	Count back from the day you transplant	Transplant window opens April 15 (central North Carolina)	A central North Carolina calendar
University of Georgia	6 weeks	February (middle Georgia)	April (middle Georgia)	A middle Georgia calendar

In short: Tomato seeds go in about five to ten weeks before the plants go out, depending on which Extension source you follow, and the plants go out around the last spring frost. Pick your weather station and the page counts the dates for each source that times tomatoes from the last frost.

The short answer

When to start tomato seeds indoors is a date counted back from your last spring frost, not a fixed month. Tomatoes are frost-tender, so the plants go outside around the time frost is over, and the seeds go in before that by however many weeks a tomato seedling needs to reach transplant size. The sources below put that wait between 5 weeks and 10 weeks, and the picker above does the counting for the three that time tomatoes from the last frost.

How many weeks before last frost to start tomatoes

Each source answers a slightly different question, so they are listed as printed and never averaged. The University of New Hampshire says tomatoes take 6 to 8 weeks from sowing until transplant and are safe to set out 1 to 2 weeks after the last frost. North Dakota State University sows tomatoes 6 weeks before the last frost and transplants them 2 weeks after the last frost. Montana State University gives 6 to 10 weeks to transplant size and opens its tomato planting window 1 week before the last frost.

The other three are calendars for one state's climate. The University of Minnesota starts tomatoes in early april for 5 to 6 weeks indoors. The University of Georgia says tomatoes take 6 weeks to reach transplant size, in a February sowing for middle Georgia. NC State advises 6 to 8 weeks before transplanting for any crop, and its central North Carolina calendar opens tomato transplanting on April 15. Those months belong to those states, so the page does not turn them into dates for your garden.

When to plant tomato seeds indoors: the temperature to aim for

Germination has its own temperature. North Dakota State's chart gives 80 to 85 °F as the best temperature for tomato germination and 7 to 10 days until the seeds come up. Alabama Extension's soil table is for germination too: it lists a minimum of 50°F, a best range of 70°F to 95°F and an optimum of 85°F.

Why the sources differ

They count from different days. New Hampshire counts back from the day the plants go out. North Dakota counts from the last frost to the sowing day. Montana counts back from the day its planting window opens, which is the earliest of the three because it starts before the last frost; New Hampshire's fact sheet says not to seed or transplant frost-sensitive plants before that date. The climates also differ, and none of the sources says which weeks a particular tomato variety prefers, so the seed packet is the next place to look.

Worked example: a last frost of May 10

Suppose your nearest station gives May 10 as the late last-frost date. Following New Hampshire, the plants go out May 17 to May 24, and the seeds go in March 22 to April 5: the earlier day for the full wait, the later day for the shortest. Following North Dakota, you sow on March 29 and transplant on May 24. Following Montana, the planting window opens May 3, and the seeds go in February 22 to March 22.

Pick one source and keep to it from sowing to set-out. Mixing one source's sowing date with another's planting date throws away the arithmetic each built in.

Setting out is about the soil as well

Georgia's tomato circular says tomatoes have a number of problems when planted in soil colder than 60°F. A soil thermometer settles that better than a calendar. The frost dates lookup shows the odds of a late frost for your station, and the spring planting planner counts the same frost date for the other vegetables.

Before the seedlings go out

Seedlings that have lived indoors need a few days outside before they stay there, and the hardening off checklist walks through it. If the tray turns tall, thin and pale before then, the leggy seedlings fix starts with light. The dates here are long-term odds, not a forecast, so look at the coming week's weather before the plants go out.