USDA Hardiness Zone 5b
Frost Dates for 80521 — Fort Collins, Colorado
NOAA 1991–2020 normals from Fort Collins, CO (5,004 ft), 1 miles southeast.
Updated
In Fort Collins, the last spring frost (32°F) is around May 2 in half of years and has passed by May 14 in nine years out of ten. The first fall frost comes around October 7, as early as September 24 one year in ten, leaving a typical freeze-free season of 157 days (NOAA, Fort Collins, CO).
- Last spring frost
- May 2
- First fall frost
- October 7
- Growing season
- 157 days
- Hardiness zone
- 5b
50% probability
50% probability
freeze-free, 50%
-15 to -10°F (-26.1 to -23.3°C) coldest night
- First snow
- October 25
- Last snow
- April 29
- Winters with snow
- 100%
- Snow per year
- 51.4"
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Fort Collins
Half of springs at Fort Collins, CO see their last 32°F night by May 2; only one in ten is past it by April 18, and one in ten still has one after May 14. Planting tender crops after May 7 leaves a 30% risk of a frost on them, after May 10 20%, and after May 14 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 81% of the 110 Colorado stations behind this site's ZIP pages.
In fall the first 32°F night comes by October 7 in half of years, as early as September 24 one year in ten, and holds off past October 20 one year in ten. A hard freeze of 28°F follows 10 days later, around October 17; in spring the one-in-ten date for 28°F is May 5, 9 days before the 32°F one. The freeze-free season lasts 157 days in a typical year, longer than at 75% of the 110 Colorado stations; it reaches 175 days one year in ten and falls short of 139 one year in ten. On average 150 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 10 days longer, to about May 12, and return around September 29. Severe cold of 16°F or below comes every winter, usually between November 14 and March 14.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 1 | May 4 | May 7 | May 10 | May 12 | May 14 | May 17 | May 21 | May 26 |
| 32°F | Apr 18 | Apr 24 | Apr 28 | Apr 30 | May 2 | May 4 | May 7 | May 10 | May 14 |
| 28°F | Apr 5 | Apr 9 | Apr 13 | Apr 17 | Apr 20 | Apr 23 | Apr 27 | Apr 30 | May 5 |
| 24°F | Mar 21 | Mar 28 | Apr 1 | Apr 3 | Apr 6 | Apr 9 | Apr 14 | Apr 20 | Apr 26 |
| 20°F | Mar 7 | Mar 14 | Mar 18 | Mar 23 | Mar 27 | Mar 31 | Apr 3 | Apr 7 | Apr 16 |
| 16°F | Feb 24 | Mar 1 | Mar 5 | Mar 10 | Mar 14 | Mar 17 | Mar 21 | Mar 26 | Apr 2 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 15 | Sep 21 | Sep 24 | Sep 27 | Sep 29 | Oct 1 | Oct 3 | Oct 6 | Oct 11 |
| 32°F | Sep 24 | Sep 29 | Oct 2 | Oct 5 | Oct 7 | Oct 10 | Oct 13 | Oct 16 | Oct 20 |
| 28°F | Oct 2 | Oct 7 | Oct 11 | Oct 14 | Oct 17 | Oct 19 | Oct 22 | Oct 25 | Oct 28 |
| 24°F | Oct 12 | Oct 17 | Oct 20 | Oct 23 | Oct 26 | Oct 30 | Nov 1 | Nov 3 | Nov 8 |
| 20°F | Oct 19 | Oct 25 | Oct 30 | Nov 2 | Nov 5 | Nov 8 | Nov 11 | Nov 15 | Nov 20 |
| 16°F | Oct 26 | Nov 1 | Nov 7 | Nov 11 | Nov 14 | Nov 18 | Nov 22 | Nov 24 | Nov 28 |
Freeze probabilities 1991–2020 at Fort Collins, CO, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 80521 →Compare Fort Collins with another place →
Weather by month in Fort Collins
July is the warmest month at Fort Collins, CO, averaging 87°F by day and 59°F at night; December is the coldest at 44°F and 18°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 57% of years.
Warmth for growing adds up to 2,747 growing degree days a year (base 50°F), more than at 64% of the 110 Colorado stations. 32% of them (884) have built up by the end of June, and July adds the most, 714. Precipitation totals 15.9 inches a year, most in May (2.7") and least in January (0.4"); May to September bring 57% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 45 | 18 | 0.4 | 100% | 29 | 0 |
| Feb | 47 | 21 | 0.5 | 100% | 26 | 3 |
| Mar | 56 | 29 | 1.3 | 100% | 22 | 24 |
| Apr | 63 | 35 | 2.1 | 100% | 9 | 80 |
| May | 71 | 44 | 2.7 | 57% | 1 | 258 |
| Jun | 82 | 53 | 1.9 | 0% | 0 | 519 |
| Jul | 87 | 59 | 1.6 | 0% | 0 | 714 |
| Aug | 85 | 57 | 1.5 | 0% | 0 | 643 |
| Sep | 77 | 48 | 1.4 | 23% | 1 | 387 |
| Oct | 64 | 36 | 1.3 | 100% | 9 | 106 |
| Nov | 53 | 26 | 0.7 | 100% | 24 | 12 |
| Dec | 44 | 18 | 0.5 | 100% | 30 | 1 |
Monthly normals 1991–2020 at Fort Collins, CO (5,004 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Fort Collins 4 E, CO, 4 miles east of Fort Collins, has a last frost 1 day later (May 3), a first frost 1 day earlier (October 6), a season 3 days shorter.
When it snows in Fort Collins, Colorado
The first measurable snow (0.1 inch or more) in Fort Collins usually arrives around October 25, and the last of the season around April 29. Those are the middle dates across the winters of 1991–2020 at Fort Collins, CO, 1.2 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 3 days after the last snow.
Planting calendar for 80521
| When | What to do |
|---|---|
| February 21 – March 7 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 7 – March 21 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 4 – April 18 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 2 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 15 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 23 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 7 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 80521 is 1 mile from Fort Collins, CO, which stands at 5,004 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 80526 (Fort Collins). The 2023 USDA Plant Hardiness Zone Map lists ZIP 80521 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.