Frost dates for 80306 — Boulder, CO
Last spring frostJun 21median date
First fall frostSep 2median date
Growing season73 daysfrost-free
Hardiness zone6aZone 6a
How Boulder compares
- In 8 years out of 10, the last spring frost in Boulder falls between June 7 and July 4, a 27-day window. The first fall frost falls between August 17 and September 15.
- The median last frost is 41 days later than the Colorado median of May 11.
- Its 73-day growing season is among the shortest of the 374 Colorado places with a regular frost season.
- The last frost is 53 days later than the zone 6a median of Apr 29. Hardiness zones measure winter cold, not season length, so use these dates rather than a zone 6a chart.
- The weather station records an average of 244 nights a year at or below 32°F.
- Compared with Arvada, CO, 39 km away, the last frost here is 47 days later than there and the growing season is 80 days shorter.
Frost probability
| Chance of frost after | Date |
|---|---|
| 90% (early) | June 7 |
| 50% (median) | June 21 |
| 10% (safe for tender plants) | July 4 |
| Chance of frost before | Date |
|---|---|
| 10% (early) | August 17 |
| 50% (median) | September 2 |
| 90% (late) | September 15 |
Frost here means 32°F (0°C). A median date means half of years have their last frost earlier and half later.
Planting calendar for Boulder, CO
Frost dates for 80306 — Boulder, CO — last frost Jun 21, first frost Sep 2, zone 6a. whenisthelastfrost.com
Crop
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Start indoors: Mar 29 to Apr 12Transplant outside: May 24 to Jun 7
Start indoors: Apr 12 to Apr 26Transplant outside: Jul 5 to Jul 19
Start indoors: Apr 12 to Apr 26Transplant outside: Jul 5 to Jul 19
Start indoors: Apr 26 to May 10Transplant outside: Jun 28 to Jul 12
Start indoors: Apr 26 to May 10Transplant outside: May 31 to Jun 14Sow outdoors: May 24 to Jun 28
Start indoors: Apr 26 to May 10Transplant outside: May 24 to Jun 7Sow outdoors: May 24 to Jun 7
Start indoors: Apr 26 to May 10Transplant outside: Jun 7 to Jun 21
Start indoors: Apr 26 to May 10Transplant outside: May 31 to Jun 14
Start indoors: Apr 26 to May 10Transplant outside: Jun 7 to Jun 21
Start indoors: May 10 to May 24Transplant outside: Jul 12 to Jul 26Sow outdoors: Jul 12 to Jul 26
Start indoors: May 10 to May 24Transplant outside: Jun 28 to Jul 12
Sow outdoors: May 10 to May 31
Sow outdoors: May 10 to May 31Plant for fall: Jul 8 to Jul 29
Sow outdoors: May 17 to Jun 28Plant for fall: Jul 22 to Aug 12
Start indoors: May 24 to May 31Transplant outside: Jul 5 to Jul 12Sow outdoors: Jun 28 to Jul 12
Start indoors: May 24 to May 31Sow outdoors: Jun 28 to Jul 12
Start indoors: May 24 to May 31Sow outdoors: Jun 28 to Jul 12
Start indoors: May 24 to May 31Transplant outside: Jul 5 to Jul 19
Sow outdoors: May 31 to Jun 28
Sow outdoors: May 31 to Jun 28
Sow outdoors: May 31 to Jun 21
Sow outdoors: Jun 7 to Jul 5
Sow outdoors: Jun 7 to Jun 28
Sow outdoors: Jun 28 to Jul 19
Sow outdoors: Jun 28 to Jul 12
Transplant outside: Jul 5 to Jul 26
Plant for fall: Aug 19 to Sep 23
Start indoorsTransplant outsideSow outdoorsPlant for fallMedian frost dates
When to plant vegetables in Boulder, CO
| Crop | Start indoors | Transplant | Sow outdoors | Fall crop |
|---|---|---|---|---|
| Onions | Mar 29 – Apr 12 | May 24 – Jun 7 | — | — |
| Peppers Warning: Your frost-free season is about 73 days, which is tight for peppers (75+ days). Choose early varieties and use transplants or row covers. | Apr 12 – Apr 26 | Jul 5 – Jul 19 | — | — |
| Eggplant Warning: Your frost-free season is about 73 days, which is tight for eggplant (75+ days). Choose early varieties and use transplants or row covers. | Apr 12 – Apr 26 | Jul 5 – Jul 19 | — | — |
| Tomatoes Warning: Your frost-free season is about 73 days, which is tight for tomatoes (70+ days). Choose early varieties and use transplants or row covers. | Apr 26 – May 10 | Jun 28 – Jul 12 | — | — |
| Lettuce | Apr 26 – May 10 | May 31 – Jun 14 | May 24 – Jun 28 | — |
| Kale | Apr 26 – May 10 | May 24 – Jun 7 | May 24 – Jun 7 | — |
| Broccoli | Apr 26 – May 10 | Jun 7 – Jun 21 | — | — |
| Cabbage | Apr 26 – May 10 | May 31 – Jun 14 | — | — |
| Cauliflower | Apr 26 – May 10 | Jun 7 – Jun 21 | — | — |
| Okra Warning: Your frost-free season is about 73 days, which is tight for okra (60+ days). Choose early varieties and use transplants or row covers. | May 10 – May 24 | Jul 12 – Jul 26 | Jul 12 – Jul 26 | — |
| Basil Warning: Your frost-free season is about 73 days, which is tight for basil (60+ days). Choose early varieties and use transplants or row covers. | May 10 – May 24 | Jun 28 – Jul 12 | — | — |
| Peas | — | — | May 10 – May 31 | — |
| Spinach | — | — | May 10 – May 31 | Jul 8 – Jul 29 |
| Radishes | — | — | May 17 – Jun 28 | Jul 22 – Aug 12 |
| Cucumbers | May 24 – May 31 | Jul 5 – Jul 12 | Jun 28 – Jul 12 | — |
| Zucchini & Summer Squash | May 24 – May 31 | — | Jun 28 – Jul 12 | — |
| Winter Squash & Pumpkins Warning: Your frost-free season is about 73 days, which is tight for winter squash & pumpkins (100+ days). Choose early varieties and use transplants or row covers. | May 24 – May 31 | — | Jun 28 – Jul 12 | — |
| Melons & Watermelon Warning: Your frost-free season is about 73 days, which is tight for melons & watermelon (85+ days). Choose early varieties and use transplants or row covers. | May 24 – May 31 | Jul 5 – Jul 19 | — | — |
| Carrots | — | — | May 31 – Jun 28 | — |
| Beets | — | — | May 31 – Jun 28 | — |
| Potatoes | — | — | May 31 – Jun 21 | — |
| Swiss Chard | — | — | Jun 7 – Jul 5 | — |
| Cilantro | — | — | Jun 7 – Jun 28 | — |
| Green Beans | — | — | Jun 28 – Jul 19 | — |
| Sweet Corn Warning: Your frost-free season is about 73 days, which is tight for sweet corn (80+ days). Choose early varieties and use transplants or row covers. | — | — | Jun 28 – Jul 12 | — |
| Sweet Potatoes Warning: Your frost-free season is about 73 days, which is tight for sweet potatoes (100+ days). Choose early varieties and use transplants or row covers. | — | Jul 5 – Jul 26 | — | — |
| Garlic | — | — | — | Aug 19 – Sep 23 |
Dates come from the median frost dates and common extension-service timing. Adjust for your soil temperature, microclimate and the varieties you grow.
About these dates
Based on 1991–2020 NOAA climate normals for the Boulder 14 W, CO weather station, 15 km away (elevation 9,829 ft (2,996 m)).
ZIP codes in Boulder: 80301 80302 80303 80304 80305 80306 80307 80308 80309 80310 80314
Frost dates near Boulder
- Jamestown Jun 21 · Sep 2
- Ward Jun 21 · Sep 2
- Hygiene May 7 · Oct 5
- Lyons Jun 14 · Sep 12
- Nederland Jun 21 · Sep 2
- Allenspark Jun 14 · Sep 12
- Niwot May 7 · Oct 5
- Pinecliffe Jun 5 · Sep 16
Frequently asked questions
When is the last frost in Boulder, CO?
The median last spring frost in Boulder, CO is June 21. There is still about a 10% chance of frost after July 4, so wait until then to plant frost-tender crops like tomatoes and peppers.
When is the first frost in Boulder, CO?
The median first fall frost is September 2, with about a 10% chance of frost before August 17.
How long is the growing season in Boulder, CO?
About 73 days between the median last spring frost and first fall frost.
What planting zone is Boulder, CO?
Boulder, CO is in USDA hardiness zone 6a.