Broomfield, Colorado: planting zone & frost dates
Broomfield 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 7 and the first fall frost around Oct 4.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 14 | Sep 26 |
| Typical planning date (30% risk) | May 7 | Oct 4 |
| Median (50%) | May 2 | Oct 11 |
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 NORTHGLENN, CO (USC00055984), 4.3 mi (6.9 km) from the city center and 72 ft higher than the city center (+22 m). Record used for the 1991–2020 normals: 26 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Broomfield
Broomfield spans more than one half-zone. Share of the city's land area by zone (via its 4 ZIP codes): 6a (89%), 5b (11%). The headline zone is the one covering the most land. Look up your ZIP for your exact zone.
Vegetable planting calendar for Broomfield
Dates are counted from the 30%-risk frost dates above (last frost May 7, first frost Oct 4). 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 12 – Mar 26 | May 14 – May 28 | — | — |
| Pepper | Feb 26 – Mar 12 | May 21 – Jun 4 | — | — |
| Lettuce | Mar 26 – Apr 9 | Apr 23 – May 14 | Apr 9 – May 21 | Jul 26 – Aug 23 |
| Green Beans | — | — | May 14 – Jun 18 | Jul 12 – Jul 26 |
| Sweet Corn | — | — | May 14 – Jun 11 | — |
| Zucchini & Summer Squash | Apr 9 – Apr 16 | May 14 – May 28 | May 14 – Jun 4 | Jul 12 – Jul 26 |
| Cucumber | Apr 9 – Apr 16 | May 21 – May 28 | May 14 – Jun 4 | Jul 12 – Jul 26 |
| Carrot | — | — | Apr 16 – May 21 | Jul 12 – Jul 26 |
| Onion | Feb 12 – Feb 26 | Apr 9 – Apr 23 | — | — |
| Potato | — | Apr 9 – Apr 30 | — | — |
| Basil | Mar 26 – Apr 9 | May 21 – Jun 4 | May 21 – Jun 11 | — |
| Spinach | — | — | Mar 26 – Apr 23 | Aug 9 – Sep 6 |
| Kale | Mar 12 – Mar 26 | Apr 9 – Apr 30 | Apr 9 – Apr 30 | Jul 26 – Aug 23 |
| Broccoli | Mar 12 – Mar 26 | Apr 23 – May 14 | — | Jul 12 – Jul 26 |
| Peas | — | — | Mar 26 – Apr 16 | Jul 26 – Aug 9 |
| Eggplant | Feb 26 – Mar 12 | May 21 – Jun 4 | — | — |
| Cabbage | Mar 12 – Mar 26 | Apr 9 – Apr 23 | — | Jul 12 – Jul 26 |
| Radish | — | — | Apr 2 – May 21 | Aug 23 – Sep 20 |
| Strawberry | — | Apr 9 – Apr 30 | — | — |
| Beets | — | — | Apr 16 – May 28 | Jul 26 – Aug 9 |
Nearby cities in Colorado
- Westminster — zone 6a, last frost May 7, first frost Oct 4
- Thornton — zone 6a, last frost May 7, first frost Oct 4
- Arvada — zone 6a, last frost May 11, first frost Sep 29
- Boulder — zone 6a, last frost May 11, first frost Sep 30
- Longmont — zone 5b, last frost May 13, first frost Oct 1
- Denver — zone 6a, last frost May 8, first frost Oct 3
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
Broomfield gardening questions
When is the last frost in Broomfield, CO?
Plan around May 7. At NOAA's Northglenn, CO station, 4.3 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 2, and after May 14 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around Apr 20. The last frost is fairly predictable here: only 26 days separate the 90% and 10% dates (median for cities on this site: 36 days).
When should I plant tomatoes in Broomfield, CO?
Start seeds indoors Mar 12 – Mar 26 and set transplants outside May 14 – May 28, once nights stay above 50°F and the soil is near 60°F. That leaves 150 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 Broomfield?
About 160 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 142 days, and in 1 year in 10 it runs 180 days or more. In fall there is a 10% chance of frost by Sep 26 and a 30% chance by Oct 4. That ranks 4th longest of 16 Colorado cities on this site.
When should I plant a fall garden in Broomfield?
Count back from the first fall frost (Oct 4): transplant broccoli Jul 12 – Jul 26; sow kale Jul 26 – Aug 23; sow lettuce Jul 26 – Aug 23; sow radishes Aug 23 – Sep 20. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 21. How the math works: succession and fall planting.
Did Broomfield's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Broomfield 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.
Is all of Broomfield in zone 6a?
No. Zone 6a covers about 89% of the city's land; the rest is in 5b (11%), measured across the city's 4 ZIP codes. Check your own ZIP code in the table above or with the ZIP lookup.