Frost Date Math

When to Plant Tomatoes: Start Seeds and Move Transplants Out

By Tam Vu · Updated

On this page
  1. What the source says about tomato seeds
  2. The math
  3. Three stations, same rules
  4. If you want to be safer
  5. Before moving them out
  6. Sources

Tomatoes are a warm-season crop. The University of Idaho Extension guide says they can be moved into the garden about 1 to 3 weeks after the average date of last frost, and that they need 5 to 9 weeks growing indoors first. Find your own last frost date with the frost date tool, or use the seed starting calculator.

What the source says about tomato seeds

Tomatoes
Ideal soil temperature for germination 75 to 80 °F
Days to germination 7 to 14
Weeks to grow indoors 5 to 9
Move outdoors about 1 to 3 weeks after the average last frost

The math

  • Move-out window = last frost + 1 week to last frost + 3 weeks.
  • Earliest start = earliest move-out date − 9 weeks.
  • Latest start = latest move-out date − 5 weeks.

Three stations, same rules

All dates use NOAA's 50% last freeze at 32 °F and a 2027 calendar.

Weather station Average last freeze (50%) Move tomatoes outdoors Start seeds indoors
Dallas-Fort Worth AP, Texas Mar 8 Mar 15 to Mar 29 Jan 11 to Feb 22
Port Columbus Intl AP, Ohio Apr 18 Apr 25 to May 9 Feb 21 to Apr 4
Denver Intl AP, Colorado May 5 May 12 to May 26 Mar 10 to Apr 21

Check one row: Columbus, April 18 + 1 week = April 25, + 3 weeks = May 9. April 25 − 9 weeks = February 21. May 9 − 5 weeks = April 4.

If you want to be safer

The 50% date is an average, so a freeze after it is about as likely as not. NOAA also gives a 10% date, after which a freeze is unlikely. Counting from the 10% date instead pushes everything later:

Weather station Last freeze (10%) Move tomatoes outdoors Start seeds indoors
Dallas-Fort Worth AP, Texas Mar 26 Apr 2 to Apr 16 Jan 29 to Mar 12
Port Columbus Intl AP, Ohio May 5 May 12 to May 26 Mar 10 to Apr 21
Denver Intl AP, Colorado May 19 May 26 to Jun 9 Mar 24 to May 5

The extension guide counts from the average date, so this is a more cautious reading, not the guide's own rule. What frost dates mean explains the trade-off.

Before moving them out

Indoor seedlings are soft. The guide says they need to be hardened off by moving the pots outside for progressively longer periods each day, which usually takes at least a week. Protect new plants from sun, wind and weather extremes.

Sources

  • University of Idaho Extension, Start Seeds Indoors with Success (CIS 1209): Table 1 and transplant timing.
  • NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, stations named in the tables.