Boulder, CO · zone 6a · ~135 frost-free days
When to plant a garden in Boulder.
In Boulder, the typical last spring frost lands around May 16 and the first fall frost around Sep 28, giving you about 135 frost-free days. Cool-season crops go out before the last frost; warm-season crops wait until after it.
The 2026 windows
Sow dates for Boulder, keyed to your frost.
| Crop | Start indoors | Spring sow / set out | Fall sow |
|---|---|---|---|
| Tomato, slicing | Apr 11 to Jun 11 | May 23 to Jul 23 | — |
| Lettuce, leaf | — | Apr 8 to May 23 | Jul 15 to Sep 13 |
| Carrot | — | Apr 8 to May 11 | Jul 3 to Jul 29 |
| Bush Bean | — | May 16 to Jul 6 | Jun 14 to Jul 17 |
| Cucumber, slicing | — | May 23 to Aug 22 | May 28 to Jun 28 |
| Pepper, bell | Apr 20 to Jun 11 | Jun 1 to Jul 23 | — |
| Kale | Feb 25 to Mar 25 | Apr 8 to May 6 | Jun 28 to Aug 13 |
| Spinach | — | Mar 25 to May 6 | Jul 15 to Sep 2 |
Boulder sits in Zone 6a, but two gardens in the same city can be a week or two apart. The planner works from your ZIP code's frost dates, not the zone, so the dates land right for your block, not the metro average.
Build your Boulder garden plan.
Tell the free planner what you want to eat and who you're feeding. It returns plant counts, spacing, and frost-safe sow dates for ZIP 80302 in about 60 seconds.