Frost dates for 80310Boulder, CO
The average last spring frost in Boulder is May 6 and the average first fall frost is October 5, giving a typical growing season of about 151 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 6
- Average first fall frost
- Oct 5
- Growing season
- 151 days
Safer after May 18 (1-in-10 risk)
Possible by Sep 21 (1-in-10 risk)
Last frost typically in 187 days
Frost risk by date
NOAA reports frost dates as probabilities. The middle date is the average (a 50% chance). For spring, the 10% date is a safer planting date: frost after it happens in about 1 year in 10. Light frost (36°F) can damage tender plants; a hard freeze (28°F) kills most annuals.
| Temperature | Last spring: 90% / 50% / 10% chance | First fall: 10% / 50% / 90% chance | Growing season (50%) | Years with at least one |
|---|---|---|---|---|
| Light frost (36°F) | May 5 · May 17 · Jun 2 | Sep 13 · Sep 28 · Oct 10 | 130 days | 100% |
| Frost / freeze (32°F) | Apr 24 · May 6 · May 18 | Sep 21 · Oct 5 · Oct 18 | 151 days | 100% |
| Hard freeze (28°F) | Apr 10 · Apr 27 · May 10 | Sep 30 · Oct 15 · Oct 30 | 171 days | 100% |
Planting calendar for Boulder
Planting windows from the University of Maryland Extension calendar, shifted to your average frost dates. Use them as a starting point and adjust for seed packet instructions and local conditions.
| Crop | Task | Window |
|---|---|---|
| Tomatoes | Start seeds indoors | Apr 5 – Jun 11 |
| Tomatoes | Transplant outdoors | May 22 – Jun 15 |
| Peppers | Start seeds indoors | Apr 5 – May 6 |
| Peppers | Transplant outdoors | Jun 5 – Jul 6 |
| Eggplant | Start seeds indoors | Apr 22 – May 21 |
| Eggplant | Transplant outdoors | Jun 5 – Jul 6 |
| Cucumbers | Sow outdoors | Jun 5 – Jun 25 |
| Summer squash | Sow outdoors | May 22 – Jul 6 |
| Winter squash & pumpkins | Sow outdoors | Jun 5 – Jul 6 |
| Snap beans | Sow outdoors | May 22 – Jul 18 |
| Sweet corn | Sow outdoors | May 22 – Jun 15 |
| Watermelon & cantaloupe | Sow outdoors | Jun 5 – Jul 6 |
| Okra | Sow outdoors | May 29 – Jun 21 |
| Peas | Sow outdoors (spring) | Apr 5 – May 21 |
| Potatoes | Plant outdoors (spring) | Apr 5 – Jun 5 |
| Onions | Start seeds indoors (spring) | Feb 22 – Mar 21 |
| Onions | Transplant outdoors (spring) | Apr 22 – May 21 |
| Broccoli | Start seeds indoors (spring) | Mar 18 – Apr 21 |
| Broccoli | Transplant outdoors (spring) | Apr 22 – May 21 |
| Broccoli | Start seeds indoors (fall) | Jun 5 – Jun 15 |
| Broccoli | Transplant outdoors (fall) | Jul 5 – Jul 16 |
| Cabbage | Start seeds indoors (spring) | Mar 8 – Apr 5 |
| Cabbage | Transplant outdoors (spring) | Apr 5 – May 6 |
| Cabbage | Start seeds indoors (fall) | May 26 – Jun 25 |
| Cabbage | Transplant outdoors (fall) | Jun 25 – Jul 26 |
| Leaf lettuce | Sow outdoors (spring) | Apr 5 – Jun 21 |
| Leaf lettuce | Sow outdoors (fall) | Jun 25 – Aug 11 |
| Spinach | Sow outdoors (spring) | Mar 29 – May 12 |
| Spinach | Sow outdoors (fall) | Jul 12 – Aug 16 |
| Radishes | Sow outdoors (spring) | Apr 5 – May 26 |
| Radishes | Sow outdoors (fall) | Jun 25 – Aug 26 |
| Kale | Sow outdoors (spring) | Apr 22 – May 12 |
| Kale | Sow outdoors (fall) | Jun 18 – Jul 26 |
| Carrots | Sow outdoors (spring) | May 6 – Jun 21 |
| Carrots | Sow outdoors (fall) | May 26 – Jul 11 |
| Garlic | Plant outdoors (fall) | Sep 25 – Oct 26 |
Where these dates come from
ZIP code 80310 is matched to the NOAA station Boulder, CO, 2.6 km away, at 5,484 ft elevation (166 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Boulder, CO (used) | 5,484 ft | May 6 | Oct 5 |
| Gross Rsvr, CO | 7,971 ft | May 24 | Sep 23 |
| Coal Creek Canyon, CO | 8,951 ft | Jun 5 | Sep 16 |
| Ralston Rsvr, CO | 5,900 ft | May 3 | Oct 14 |
Frost dates near Boulder
- Boulder, CO 80309 · last May 6, first Oct 5
- Eldorado Springs, CO 80025 · last May 3, first Oct 14
- Louisville, CO 80027 · last May 6, first Oct 5
- Niwot, CO 80544 · last May 7, first Oct 5
- Lafayette, CO 80026 · last May 2, first Oct 11
- Jamestown, CO 80455 · last May 24, first Sep 23
- Pinecliffe, CO 80471 · last May 24, first Sep 23
- Erie, CO 80516 · last May 7, first Oct 5
Frequently asked questions
- When is the last frost in Boulder, CO?
- For ZIP code 80310, the average last spring frost (32°F) is May 6, based on NOAA 1991-2020 climate normals for the Boulder, CO weather station. There is a 1-in-10 chance of a frost after May 18.
- When is the first frost in Boulder, CO?
- The average first fall frost (32°F) is October 5. There is a 1-in-10 chance of frost by September 21, so tender plants are at risk from that point on.
- How long is the growing season in 80310?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 151 days long.
- Are these frost dates a forecast?
- No. They are 30-year averages and probabilities from NOAA weather station records. Use them to plan, then watch your local forecast before planting tender crops or leaving them uncovered.