When to plant in Boulder, CO
USDA Zone 6aEverything below — frost dates, hardiness zone, and what to plant when in Boulder, Colorado — is derived from the closest NOAA station with complete climate normals.
In zone 6a, Boulder gardeners can grow many hardy perennials, while the last and first frost still dictate the annual vegetable calendar.
Frost probability
BOULDER · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Boulder’s own odds, recorded at BOULDER.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | Jun 2 | May 17 | May 5 | Sep 13 | Sep 28 | Oct 10 |
| 32°F | May 18 | May 6 | Apr 24 | Sep 21 | Oct 5 | Oct 18 |
| 28°F | May 10 | Apr 27 | Apr 10 | Sep 30 | Oct 15 | Oct 30 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Full-year planting calendar
Each bar is the exact window to take a planting action in Boulder, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 18 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Boulder planting FAQ
When is the last spring frost in Boulder, CO?
On average, the last spring frost in Boulder is around May 6 (50% probability at 32°F, from 1991–2020 NOAA normals). Wait until after this date to set out tender plants like tomatoes and peppers.
When is the first fall frost in Boulder, CO?
The first fall frost in Boulder typically arrives around October 5 (50% probability at 32°F). Harvest or protect frost-sensitive crops before then.
What hardiness zone is Boulder in?
Boulder is in USDA hardiness zone 6a. In zone 6a, winters are moderate — most common vegetables grow well in season.
How long is the growing season in Boulder?
Boulder has about 152 frost-free days — a moderate growing season — between the average last spring frost (May 6) and first fall frost (October 5).
When should I plant tomatoes in Boulder?
For Boulder, sow tomatoes indoors about March 11–March 25 and move the seedlings out around May 13, after the last spring frost.
Never miss a window in Boulder
An email when it’s time to start seeds, transplant, and sow — timed to Boulder’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reach- Gunbarrel · 8 km
- Louisville · 11 km
- Superior · 13 km
- Lafayette · 13 km
- Erie · 18 km
- Broomfield · 19 km
- Longmont · 20 km
- Westminster · 23 km
Frost dates recorded at BOULDER, 4 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Boulder, CO — Frost Dates & Zone 6a." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USC00050848. Retrieved from https://blissgarden.com/colorado/boulder.