U.S. Frost Dates by Weather Station

NOAA station USC00053006 · Colorado

Frost Dates for Fort Collins 4 E, CO

NOAA weather station Fort Collins 4 E (USC00053006), Colorado · elevation 4920 ft (1500 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 15only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 650% chance of a frost before it
  • Frost-free season154 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Fort Collins 4 E, COSpring: a frost after April 21 in 9 years out of 10, after May 3 in half of the years, after May 15 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 6 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 154 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 6Frost season at Fort Collins 4 E, COSpring: a frost after April 21 in 9 years out of 10, after May 3 in half of the years, after May 15 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 6 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 154 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 6
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 15September 22
50% (typical)May 3October 6
90% (likely)April 21October 17
Frost-free season (32°F, 50% chance)
154 days
Hard frost (28°F), last in spring, 50% chance after
April 23
Hard frost (28°F), first in fall, 50% chance before
October 14

The last 15 years (2006–2020 normals)

Typical last frost May 4 (1 day later than the 30-year normal), typical first fall frost October 10 (4 days later). Frost-free season 157 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Fort Collins 4 E: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 15, 20% May 11, 30% May 8, 40% May 6, 50% May 3, 60% May 1, 70% April 29, 80% April 26, 90% April 21.Spring: last frostChance of a frost after this date10%50%90%May 1May 15May 3Apr 21Fall at Fort Collins 4 E: chance of a frost before this dateFall, chance of a frost before the date: 10% September 22, 20% September 27, 30% October 1, 40% October 3, 50% October 6, 60% October 9, 70% October 11, 80% October 14, 90% October 17.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 22Oct 6Oct 17
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there.

What these dates mean

The 154 days between the 50% light-frost dates of May 3 and October 6 in Fort Collins 4 E define the core growing window. The May 3 date marks the point at which there is an even chance a light frost will occur later in spring, while October 6 marks the point at which there is an even chance a light frost will have occurred earlier in fall. Gardeners can use these median dates to gauge the broad season for planting and harvesting.

In a cautious plan, the spring tender-crop window begins after the May 15 date, which carries only a 10% chance of a light frost later, and the fall window ends by September 22, which carries only a 10% chance of a light frost earlier. Planting on April 21 or keeping crops until October 17 would be a gamble, as each carries a 90% chance of frost. The 50% hard-frost dates are April 23 and October 14, and a hard frost at 28 °F can damage plants more severely than a light frost at 32 °F.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after May 15" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before September 22" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Fort Collins 4 E, CONumbered nearest stations: 1 — Fort Collins, CO — 3 mi; 2 — Loveland, CO — 10 mi; 3 — near Greeley, CO — 21 mi; 4 — Rustic, CO — 37 mi; 5 — Cheyenne, WY — 41 mi.WYNE25 mi · 40 km1. Fort Collins, CO — 3 mi12. Loveland, CO — 10 mi23. near Greeley, CO — 21 mi34. Rustic, CO — 37 mi45. Cheyenne, WY — 41 mi5
  1. Fort Collins, CO 3 mi (5 km)
  2. Loveland, CO 10 mi (16 km)
  3. near Greeley, CO 21 mi (34 km)
  4. Rustic, CO 37 mi (60 km)
  5. Cheyenne, WY 41 mi (67 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 40.5756, -105.0236.