Free Growli tool
Seed starting date calculator
Enter your ZIP and what you're growing — get the exact date to start seeds indoors, when to move them outside, and roughly when you'll harvest. All anchored to your local frost averages.
Enter your ZIP code above to get exact seed-starting dates for tomatoes.
Seed starting date calculator — frequently asked questions
How do I know when to start my seeds indoors?
Count backwards from your last spring frost date. Most warm-season crops start indoors 6-10 weeks before last frost — tomatoes at 6 weeks, peppers and eggplant at 9-10 (they germinate and grow slowly), cucurbits at 4 weeks (they resent root disturbance). This calculator does the maths for you: enter your ZIP and crop and it returns the exact calendar date, derived from NOAA 1991-2020 frost averages for your area.
What is my last frost date and why does it matter?
Your last spring frost is the date after which a damaging frost is unlikely in a typical year (a 50% probability average). It is the anchor for the entire spring planting schedule: start tender seeds indoors a set number of weeks before it, and transplant or direct-sow a set number of days after it. Get the frost date wrong and you either start too early (leggy rootbound seedlings) or too late (a shortened season).
Which seeds should I NOT start indoors?
Root crops (carrots, radishes, beets) and legumes (beans, peas) dislike transplanting and do best direct-sown. Cucurbits (cucumbers, squash, melons) can be started indoors but only 3-4 weeks ahead in deep cells, because their roots are sensitive. Garlic is fall-planted from cloves, not seed-started at all. This calculator flags the right method for each crop automatically.
How accurate are these dates?
The frost dates are 50%-probability NOAA climate normals (1991-2020), so real local frost varies by roughly ±10 days year to year, and microclimates (urban heat, slopes, proximity to water) shift them further. Treat the calculator output as a reliable planning baseline, then watch your actual forecast in the final two weeks before transplanting. The Growli app does this watching for you.
Can I use this for a fall or second planting?
This version anchors on the spring season (last frost) for most crops and on the first fall frost for fall-planted crops like garlic. For succession sowing and full fall planting windows, the month-by-month planting calendar and the per-zone planting calendars cover the second half of the season.
Why does the calculator sometimes show next year's dates?
If this season's start date for your chosen crop has already passed, the calculator rolls forward to the next season so the dates are always actionable — you never get a date in the past. That is why a crop you have missed for this spring will show next spring's schedule.
Crops covered
10 crops with calibrated timing. More rolling out — each links to its full growing guide.