Boulder, CO planting zone & frost dates
Boulder is in USDA hardiness zone 6a. The last spring frost usually comes around May 6 and the first fall frost around October 5, for a growing season of about 151 days.
Planting calendar for Boulder
Dates are counted from the average last spring frost (May 6) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.
| Crop | When |
|---|---|
| Tomatoes | Start seeds indoors: Mar 11 – Mar 25 Transplant outdoors: May 13 – May 20 |
| Peppers | Start seeds indoors: Feb 25 – Mar 11 Transplant outdoors: May 20 – May 27 |
| Eggplant | Start seeds indoors: Feb 25 – Mar 11 Transplant outdoors: May 20 – May 27 |
| Broccoli | Start seeds indoors: Feb 25 – Mar 11 Transplant outdoors: Apr 8 – Apr 22 |
| Cabbage | Start seeds indoors: Feb 25 – Mar 11 Transplant outdoors: Apr 8 – Apr 22 |
| Onions (sets) | Plant outdoors: Apr 8 – Apr 22 |
| Peas | Sow outdoors: Mar 25 – Apr 8 |
| Spinach | Sow outdoors: Mar 25 – Apr 8 |
| Lettuce | Sow outdoors: Apr 8 – Apr 22 |
| Carrots | Sow outdoors: Apr 8 – Apr 22 |
| Potatoes | Plant outdoors: Apr 8 – Apr 22 |
| Beans | Sow outdoors: May 13 – May 20 |
| Sweet corn | Sow outdoors: May 13 – May 20 |
| Cucumbers | Sow outdoors: May 13 – May 20 |
| Squash & zucchini | Sow outdoors: May 13 – May 20 |
| Basil | Start seeds indoors: Mar 25 – Apr 8 Transplant outdoors: May 13 – May 20 |
Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Aug 10 (8 weeks before the average first fall frost on Oct 5).
Frost risk in detail
| 90% chance | 50% chance | 10% chance | |
|---|---|---|---|
| Last 32°F frost in spring is after | Apr 24 | May 6 | May 18 |
| First 32°F frost in fall is before | Oct 18 | Oct 5 | Sep 21 |
How to read it: in spring there is a 10% chance of a frost after May 18, so that is a safe date for tender plants. In fall there is a 10% chance of frost before September 21. A hard freeze (28°F) usually ends around April 27.
Hardiness zone by ZIP code in Boulder
- 80301 zone 6a
- 80302 zone 6a
- 80303 zone 6a
- 80304 zone 6a
- 80305 zone 6a
- 80306 zone 6a
- 80307 zone 6a
- 80308 zone 6a
- 80310 zone 6a
- 80314 zone 6a
Nearby cities
- Superior zone 6a, last frost May 6
- Louisville zone 6a, last frost May 6
- Lafayette zone 6a, last frost May 2
- Broomfield zone 6a, last frost May 2
- Erie zone 5b, last frost May 7
- Longmont zone 5b, last frost May 7
- Golden zone 6a, last frost May 3
- Arvada zone 6a, last frost May 5
Questions
What growing zone is Boulder, CO?
Boulder is in USDA Plant Hardiness Zone 6a (average extreme winter low -10°F to -5°F), according to the USDA 2023 map.
When is the last frost in Boulder?
The average last spring frost (32°F) in Boulder is May 6. There is still a 10% chance of frost until May 18.
When can I plant tomatoes in Boulder?
Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 13 – May 20. Start seeds indoors 6–8 weeks before the last frost.
Frost dates: NOAA U.S. Climate Normals 1991–2020, station Boulder, CO (USC00050848), 2.6 km (1.6 mi) from the city center, elevation 1672 m, based on 26 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.