Boulder, CO · zone 6a · ~135 frost-free days
When to plant in Boulder: frost dates and zone 6a.
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 May 22 | May 23 to Jul 3 | — |
| 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 | — |
| Cucumber, slicing | — | May 23 to Jul 12 | — |
| Pepper, bell | Apr 20 to May 25 | Jun 1 to Jul 6 | — |
| 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.
Boulder planting questions
What planting zone is Boulder, CO?+
Boulder is in USDA hardiness zone 6a. Growing zone, gardening zone, and planting zone all mean the same thing: the USDA number, 6a here. It tells you what survives winter, not when to plant. Frost dates set your planting time, and the free planner uses your exact ZIP code's frost dates, which can differ by weeks across one metro.
When is the last frost date in Boulder?+
The typical last spring frost in Boulder, CO is around May 16. That is an average from local history, not a guarantee, so tender crops like tomatoes and peppers are safer with a week of buffer after it.
When is the first frost date in Boulder?+
The typical first fall frost in Boulder, CO arrives around Sep 28, closing a growing season of roughly 135 frost-free days. Count backward from it to find the last date a fall crop can still finish.
What can I plant now in Boulder?+
It depends on the date. Cool-season crops like lettuce, kale, and spinach go out weeks before the last frost; warm-season crops like tomatoes and peppers wait until after May 16. The table above shows each crop's window; the free planner turns your ZIP into an exact, dated calendar.