Boulder, Colorado: planting zone & frost dates
Boulder is in USDA plant hardiness zone 6a, where the average coldest winter night reaches -10 to -5°F (-23.3 to -20.6°C). The typical last spring frost is around May 11 and the first fall frost around Sep 30.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 18 | Sep 21 |
| Typical planning date (30% risk) | May 11 | Sep 30 |
| Median (50%) | May 6 | Oct 5 |
How to read this: after the spring "10% risk" date, a 32°F frost happens in only about 1 year in 10. In fall, the "10% risk" date is the early end — a frost before it happens about 1 year in 10.
Frost data comes from NOAA station BOULDER, CO (USC00050848), 2.4 mi (3.8 km) from the city center and 233 ft higher than the city center (+71 m). Record used for the 1991–2020 normals: 26 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Boulder
All 7 ZIP codes covering Boulder are in zone 6a.
Vegetable planting calendar for Boulder
Dates are counted from the 30%-risk frost dates above (last frost May 11, first frost Sep 30). Shift a week later in cold spots or low-lying ground.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Mar 16 – Mar 30 | May 18 – Jun 1 | — | — |
| Pepper | Mar 2 – Mar 16 | May 25 – Jun 8 | — | — |
| Lettuce | Mar 30 – Apr 13 | Apr 27 – May 18 | Apr 13 – May 25 | Jul 22 – Aug 19 |
| Green Beans | — | — | May 18 – Jun 22 | Jul 8 – Jul 22 |
| Sweet Corn | — | — | May 18 – Jun 15 | — |
| Zucchini & Summer Squash | Apr 13 – Apr 20 | May 18 – Jun 1 | May 18 – Jun 8 | Jul 8 – Jul 22 |
| Cucumber | Apr 13 – Apr 20 | May 25 – Jun 1 | May 18 – Jun 8 | Jul 8 – Jul 22 |
| Carrot | — | — | Apr 20 – May 25 | Jul 8 – Jul 22 |
| Onion | Feb 16 – Mar 2 | Apr 13 – Apr 27 | — | — |
| Potato | — | Apr 13 – May 4 | — | — |
| Basil | Mar 30 – Apr 13 | May 25 – Jun 8 | May 25 – Jun 15 | — |
| Spinach | — | — | Mar 30 – Apr 27 | Aug 5 – Sep 2 |
| Kale | Mar 16 – Mar 30 | Apr 13 – May 4 | Apr 13 – May 4 | Jul 22 – Aug 19 |
| Broccoli | Mar 16 – Mar 30 | Apr 27 – May 18 | — | Jul 8 – Jul 22 |
| Peas | — | — | Mar 30 – Apr 20 | Jul 22 – Aug 5 |
| Eggplant | Mar 2 – Mar 16 | May 25 – Jun 8 | — | — |
| Cabbage | Mar 16 – Mar 30 | Apr 13 – Apr 27 | — | Jul 8 – Jul 22 |
| Radish | — | — | Apr 6 – May 25 | Aug 19 – Sep 16 |
| Strawberry | — | Apr 13 – May 4 | — | — |
| Beets | — | — | Apr 20 – Jun 1 | Jul 22 – Aug 5 |
Nearby cities in Colorado
- Longmont — zone 5b, last frost May 13, first frost Oct 1
- Broomfield — zone 6a, last frost May 7, first frost Oct 4
- Arvada — zone 6a, last frost May 11, first frost Sep 29
- Westminster — zone 6a, last frost May 7, first frost Oct 4
- Thornton — zone 6a, last frost May 7, first frost Oct 4
- Lakewood — zone 6a, last frost May 8, first frost Oct 2
More: all Colorado cities · other zone 6a cities: Chicago, IL, Denver, CO, Milwaukee, WI, Colorado Springs, CO, Omaha, NE, Aurora, CO, Lincoln, NE, Fort Wayne, IN
Boulder gardening questions
When is the last frost in Boulder, CO?
Plan around May 11. At NOAA's Boulder, CO station, 2.4 mi from the city center, there is a 30% chance of a 32°F frost after that date. Half of springs have their last frost by May 6, and after May 18 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around Apr 27. The last frost is fairly predictable here: only 24 days separate the 90% and 10% dates (median for cities on this site: 36 days).
When should I plant tomatoes in Boulder, CO?
Start seeds indoors Mar 16 – Mar 30 and set transplants outside May 18 – Jun 1, once nights stay above 50°F and the soil is near 60°F. That leaves 142 days between the 30% frost dates, enough for most varieties including 80–85-day slicers. More: tomato planting dates by zone.
How long is the growing season in Boulder?
About 151 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 131 days, and in 1 year in 10 it runs 170 days or more. In fall there is a 10% chance of frost by Sep 21 and a 30% chance by Sep 30. That ranks 13th longest of 16 Colorado cities on this site.
When should I plant a fall garden in Boulder?
Count back from the first fall frost (Sep 30): transplant broccoli Jul 8 – Jul 22; sow kale Jul 22 – Aug 19; sow lettuce Jul 22 – Aug 19; sow radishes Aug 19 – Sep 16. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 15. How the math works: succession and fall planting.
Did Boulder's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Boulder is zone 6a on both the 2012 and the 2023 USDA maps (PRISM/OSU ZIP listings). About 42% of U.S. ZIP codes kept their half-zone in 2023. Why the 2023 map is warmer.
How does Boulder compare with Broomfield, CO?
Both are in zone 6a. The typical last spring frost in Boulder (May 11) comes about 4 days later than in Broomfield, CO (May 7), 12 miles away, and the first fall frost is Sep 30 versus Oct 4. The two cities use different NOAA stations.