Longmont, Colorado: planting zone & frost dates
Longmont is in USDA plant hardiness zone 5b, where the average coldest winter night reaches -15 to -10°F (-26.1 to -23.3°C). The typical last spring frost is around May 13 and the first fall frost around Oct 1.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 22 | Sep 23 |
| Typical planning date (30% risk) | May 13 | Oct 1 |
| Median (50%) | May 7 | 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 LONGMONT 2 ESE, CO (USC00055116), 1.2 mi (2.0 km) from the city center and 30 ft lower than the city center (-9 m). Record used for the 1991–2020 normals: 17 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Longmont
Longmont spans more than one half-zone. Share of the city's land area by zone (via its 3 ZIP codes): 5b (73%), 6a (27%). The headline zone is the one covering the most land. Look up your ZIP for your exact zone.
Vegetable planting calendar for Longmont
Dates are counted from the 30%-risk frost dates above (last frost May 13, first frost Oct 1). 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 18 – Apr 1 | May 20 – Jun 3 | — | — |
| Pepper | Mar 4 – Mar 18 | May 27 – Jun 10 | — | — |
| Lettuce | Apr 1 – Apr 15 | Apr 29 – May 20 | Apr 15 – May 27 | Jul 23 – Aug 20 |
| Green Beans | — | — | May 20 – Jun 24 | Jul 9 – Jul 23 |
| Sweet Corn | — | — | May 20 – Jun 17 | — |
| Zucchini & Summer Squash | Apr 15 – Apr 22 | May 20 – Jun 3 | May 20 – Jun 10 | Jul 9 – Jul 23 |
| Cucumber | Apr 15 – Apr 22 | May 27 – Jun 3 | May 20 – Jun 10 | Jul 9 – Jul 23 |
| Carrot | — | — | Apr 22 – May 27 | Jul 9 – Jul 23 |
| Onion | Feb 18 – Mar 4 | Apr 15 – Apr 29 | — | — |
| Potato | — | Apr 15 – May 6 | — | — |
| Basil | Apr 1 – Apr 15 | May 27 – Jun 10 | May 27 – Jun 17 | — |
| Spinach | — | — | Apr 1 – Apr 29 | Aug 6 – Sep 3 |
| Kale | Mar 18 – Apr 1 | Apr 15 – May 6 | Apr 15 – May 6 | Jul 23 – Aug 20 |
| Broccoli | Mar 18 – Apr 1 | Apr 29 – May 20 | — | Jul 9 – Jul 23 |
| Peas | — | — | Apr 1 – Apr 22 | Jul 23 – Aug 6 |
| Eggplant | Mar 4 – Mar 18 | May 27 – Jun 10 | — | — |
| Cabbage | Mar 18 – Apr 1 | Apr 15 – Apr 29 | — | Jul 9 – Jul 23 |
| Radish | — | — | Apr 8 – May 27 | Aug 20 – Sep 17 |
| Strawberry | — | Apr 15 – May 6 | — | — |
| Beets | — | — | Apr 22 – Jun 3 | Jul 23 – Aug 6 |
Nearby cities in Colorado
- Boulder — zone 6a, last frost May 11, first frost Sep 30
- Broomfield — zone 6a, last frost May 7, first frost Oct 4
- Loveland — zone 5b, last frost May 15, first frost Sep 25
- 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
More: all Colorado cities · other zone 5b cities: Madison, WI, Des Moines, IA, Aurora, IL, Fort Collins, CO, Naperville, IL, Rockford, IL, Elgin, IL, Greeley, CO
Longmont gardening questions
When is the last frost in Longmont, CO?
Plan around May 13. At NOAA's Longmont 2 ESE, CO station, 1.2 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 7, and after May 22 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around Apr 28. The last frost is fairly predictable here: only 28 days separate the 90% and 10% dates (median for cities on this site: 36 days).
When should I plant tomatoes in Longmont, CO?
Start seeds indoors Mar 18 – Apr 1 and set transplants outside May 20 – Jun 3, once nights stay above 50°F and the soil is near 60°F. That leaves 141 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 Longmont?
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 169 days or more. In fall there is a 10% chance of frost by Sep 23 and a 30% chance by Oct 1. That ranks 14th longest of 16 Colorado cities on this site.
When should I plant a fall garden in Longmont?
Count back from the first fall frost (Oct 1): transplant broccoli Jul 9 – Jul 23; sow kale Jul 23 – Aug 20; sow lettuce Jul 23 – Aug 20; sow radishes Aug 20 – Sep 17. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 12. How the math works: succession and fall planting.
Did Longmont's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Longmont is zone 5b 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 Longmont in zone 5b?
No. Zone 5b covers about 73% of the city's land; the rest is in 6a (27%), measured across the city's 3 ZIP codes. Check your own ZIP code in the table above or with the ZIP lookup.