Fort Collins, Colorado: planting zone & frost dates
Fort Collins 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 7 and the first fall frost around Oct 2.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 14 | Sep 24 |
| Typical planning date (30% risk) | May 7 | Oct 2 |
| Median (50%) | May 2 | Oct 7 |
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 FT COLLINS, CO (USC00053005), 2.2 mi (3.6 km) from the city center and 10 ft higher than the city center (+3 m). Record used for the 1991–2020 normals: 29 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Fort Collins
All 5 ZIP codes covering Fort Collins are in zone 5b.
Vegetable planting calendar for Fort Collins
Dates are counted from the 30%-risk frost dates above (last frost May 7, first frost Oct 2). 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 24 – Aug 21 |
| Green Beans | — | — | May 14 – Jun 18 | Jul 10 – Jul 24 |
| Sweet Corn | — | — | May 14 – Jun 11 | — |
| Zucchini & Summer Squash | Apr 9 – Apr 16 | May 14 – May 28 | May 14 – Jun 4 | Jul 10 – Jul 24 |
| Cucumber | Apr 9 – Apr 16 | May 21 – May 28 | May 14 – Jun 4 | Jul 10 – Jul 24 |
| Carrot | — | — | Apr 16 – May 21 | Jul 10 – Jul 24 |
| 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 7 – Sep 4 |
| Kale | Mar 12 – Mar 26 | Apr 9 – Apr 30 | Apr 9 – Apr 30 | Jul 24 – Aug 21 |
| Broccoli | Mar 12 – Mar 26 | Apr 23 – May 14 | — | Jul 10 – Jul 24 |
| Peas | — | — | Mar 26 – Apr 16 | Jul 24 – Aug 7 |
| Eggplant | Feb 26 – Mar 12 | May 21 – Jun 4 | — | — |
| Cabbage | Mar 12 – Mar 26 | Apr 9 – Apr 23 | — | Jul 10 – Jul 24 |
| Radish | — | — | Apr 2 – May 21 | Aug 21 – Sep 18 |
| Strawberry | — | Apr 9 – Apr 30 | — | — |
| Beets | — | — | Apr 16 – May 28 | Jul 24 – Aug 7 |
Nearby cities in Colorado
- Loveland — zone 5b, last frost May 15, first frost Sep 25
- Greeley — zone 5b, last frost May 7, first frost Oct 3
- Longmont — zone 5b, last frost May 13, first frost Oct 1
- Boulder — zone 6a, last frost May 11, first frost Sep 30
- Broomfield — zone 6a, last frost May 7, first frost Oct 4
- Thornton — zone 6a, last frost May 7, first frost Oct 4
More: all Colorado cities · other zone 5b cities: Madison, WI, Des Moines, IA, Aurora, IL, Naperville, IL, Rockford, IL, Elgin, IL, Greeley, CO, Green Bay, WI
Fort Collins gardening questions
When is the last frost in Fort Collins, CO?
Plan around May 7. At NOAA's Ft Collins, CO station, 2.2 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 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 Fort Collins, 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 148 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 Fort Collins?
About 157 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 139 days, and in 1 year in 10 it runs 175 days or more. In fall there is a 10% chance of frost by Sep 24 and a 30% chance by Oct 2. That ranks 7th longest of 16 Colorado cities on this site.
When should I plant a fall garden in Fort Collins?
Count back from the first fall frost (Oct 2): transplant broccoli Jul 10 – Jul 24; sow kale Jul 24 – Aug 21; sow lettuce Jul 24 – Aug 21; sow radishes Aug 21 – Sep 18. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 17. How the math works: succession and fall planting.
Did Fort Collins's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Fort Collins 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.
How does Fort Collins compare with Loveland, CO?
Both are in zone 5b. The typical last spring frost in Fort Collins (May 7) comes about 8 days earlier than in Loveland, CO (May 15), 9 miles away, and the first fall frost is Oct 2 versus Sep 25. The two cities use different NOAA stations.