Longmont, Colorado Frost Dates, Growing Season & Hardiness Zone
Frost odds and planting zone for Longmont for the 2027 season, based on 1991–2020 normals.
Last frost date
May 7
typical (50% chance of a later 32°F night)
Safer date: May 22 (10% risk)
earlyMay 7
typicalMay 22
safer
First fall frost
Oct 5
typical (50% chance it has happened by then)
Early-risk date: Sep 23 (10%)
Growing season
151 days
typical frost-free (32°F) season
Short year: 131 days (1 year in 10 is shorter)
USDA hardiness zone
5b–6a
2023 map: -15 to -5°F average coldest night
2012 map: 5b (most of the city)
Stations around the metro don't all agree. The typical last frost is as early as May 2 at Northglenn and as late as May 12 at Waterdale.
By May 7, half of years have seen their last 32°F night. A later freeze, after May 22, happens in 1 year out of 10.
Based on NOAA station Longmont 2 ESE (USC00055116), 1.2 mi E, elevation 4,950 ft (−31 ft vs. city center). Confidence: Medium.
When is the last frost in Longmont?
The typical last 32°F night is May 7, and in 1 year out of 10 a freeze still comes after May 22. Cold 36°F nights (frost possible on plants) typically last longer, to around May 18. The last hard freeze (28°F) typically comes earlier, around Apr 28.
In Longmont, late frosts can come well after the typical date. The safer 1-in-10 date (May 22) is 15 days after the typical date (May 7). Tender plants set out early will need cover on cold nights.
Cold nights don't end with the last freeze. The typical last 36°F night (frost possible on plants in low spots) comes around May 18, 11 days after the typical last 32°F night. On clear, calm nights, frost can form on leaves while the thermometer still reads above freezing.
| Threshold | 10% (safer) | 30% | 50% (typical) | 90% (early) |
|---|---|---|---|---|
| 36°F (frost possible on plants) | Jun 2 | May 24 | May 18 | May 5 |
| 32°F freeze | May 22 | May 13 | May 7 | Apr 24 |
| 28°F hard freeze | May 12 | May 2 | Apr 28 | Apr 11 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | Jun 2 | May 27 | May 24 | May 21 | May 18 | May 16 | May 13 | May 10 | May 5 |
| 32°F freeze | May 22 | May 16 | May 13 | May 10 | May 7 | May 4 | May 1 | Apr 29 | Apr 24 |
| 28°F hard freeze | May 12 | May 6 | May 2 | Apr 30 | Apr 28 | Apr 24 | Apr 20 | Apr 16 | Apr 11 |
Is it safe to plant after Mother's Day in Longmont?
Around Longmont, many gardeners use Mother's Day as their cue to plant (CSU PlantTalk). It falls on May 9 in 2027. From then on, the chance of another 32°F night is between 40% and 50%; for a 36°F night (frost possible on plants) it's between 80% and 90% (NOAA 1991–2020, Longmont 2 ESE).
Is it too early to plant in Longmont?
Before May 7 the odds of another 32°F night are better than even; after May 22 they are about 1 in 10 or less.
Today's risk is computed in your browser from the same NOAA probability table, shown as the range between the two nearest NOAA probability levels.
When is the first frost in Longmont?
By Oct 5, half of years have had their first 32°F night. In 1 year out of 10, it comes as early as Sep 23. In a typical year, look for the first 36°F night (frost possible on plants) around Sep 28 and the first 28°F hard freeze around Oct 12.
For fall crops, count back from Oct 5, or from Sep 23 to be safe. Our planting calendar does the math for each crop.
| Threshold | 10% (early) | 30% | 50% (typical) | 90% (late) |
|---|---|---|---|---|
| 36°F (frost possible on plants) | Sep 14 | Sep 23 | Sep 28 | Oct 8 |
| 32°F freeze | Sep 23 | Oct 1 | Oct 5 | Oct 16 |
| 28°F hard freeze | Sep 30 | Oct 6 | Oct 12 | Oct 25 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | Sep 14 | Sep 19 | Sep 23 | Sep 25 | Sep 28 | Sep 30 | Oct 2 | Oct 4 | Oct 8 |
| 32°F freeze | Sep 23 | Sep 27 | Oct 1 | Oct 2 | Oct 5 | Oct 6 | Oct 9 | Oct 12 | Oct 16 |
| 28°F hard freeze | Sep 30 | Oct 4 | Oct 6 | Oct 9 | Oct 12 | Oct 15 | Oct 18 | Oct 21 | Oct 25 |
How long is Longmont's growing season?
Longmont's frost-free season (32°F) typically runs 151 days. In about 1 year out of 10, it's shorter than 131 days.
Most warm-season vegetables have time to finish if you plant on schedule. For long-season crops, transplants do best.
The station averages 2,764 growing degree days (base 50°F) a year. That number measures how much heat crops have to grow on.
| Threshold | 10% (long year) | 50% (typical) | 90% (short year) |
|---|---|---|---|
| 36°F (frost possible on plants) | 151 days | 132 days | 111 days |
| 32°F freeze | 169 days | 151 days | 131 days |
| 28°F hard freeze | 189 days | 169 days | 151 days |
How does Longmont's season compare?
Among the 20 Colorado cities with frost dates on this site, the typical frost-free season in Longmont (151 days) is tied for the 15th longest.
Nationwide, across the 56 cities with frost dates on this site, the typical frost-free season here outlasts the one in 3 of them and falls short of the one in 50.
Growing degree days (base 50°F)
Annual normal: 2,764 GDD (NOAA flag R, 16 years). Running total by the end of each month (sum of NOAA daily GDD normals, so the year-end total can differ slightly from the annual normal):
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 1 | 25 | 103 | 354 | 872 | 1,587 | 2,230 | 2,634 | 2,744 | 2,764 | 2,764 |
What growing zone is Longmont? (USDA 2023)
Longmont is in USDA zone 5b–6a on the 2023 map, which means an average coldest winter night of -15 to -5°F. Most of the city was 5b on the 2012 map, and 1 of 3 city ZIPs found in both tables moved up half a zone.
| ZIP | 2023 | 2012 | Change | Share of ZIP in Longmont |
|---|---|---|---|---|
| 80501 mostly in city | 5b | 5b | unchanged | 84% |
| 80503 | 6a | 5b | +1 half zone (warmer) | 9% |
| 80504 | 5b | 5b | unchanged | 11% |
Other ZIPs touching Longmont
ZIPs outside the 805 prefix that overlap the city boundary:
- none
Zones: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed 2026-10-08. Shown as published in the ZIP tables; this page does not show a zone map. Not the official USDA Plant Hardiness Zone Map (official map: planthardiness.ars.usda.gov).
Does the new zone change Longmont's frost dates?
No. Hardiness zones are based only on the average coldest winter temperature. They say nothing about when spring and fall frosts happen. Longmont's frost dates come from NOAA's 1991–2020 frost probabilities, which did not change with the 2023 zone map. Use your zone to pick perennials that will survive winter, and your frost dates to time planting.
Why do Longmont frost dates differ between websites?
If you've seen May 13 listed as Longmont's last frost date, that's the 30%-risk date: about 3 years in 10 still get a 32°F night after it. We lead with May 7, the 50% (median) date.
May 22 is the 10%-risk, or 'safer', date. Only about 1 year in 10 has a 32°F night after it.
May 18 is the typical (50%) date for a 36°F night (frost possible on plants), not a 32°F freeze. Any site that draws the line at 36°F will show later dates.
The choice of weather station matters too. The 5 stations within 25 miles of the city center that pass our quality rules give typical (50%) last 32°F frost dates from May 2 to May 12; see the table below.
| Date | What it is (NOAA 1991–2020) |
|---|---|
| May 13 | 32°F last frost, 30% risk (a 'balanced' date) |
| May 18 | 36°F last cold night, 50% (frost possible on plants) |
| May 22 | 32°F last frost, 10% risk (the 'safer' date) |
| Apr 28 | 28°F last hard freeze, 50% |
| May 10 | 32°F last frost, 50%, at Loveland 2 N (nearest station with NOAA flag S) |
| Station | Distance | Elevation | Typical last frost |
|---|---|---|---|
| Northglenn | 19.2 mi SSE | 5,407 ft | May 2 |
| Brighton 3 SE | 20.9 mi SE | 5,016 ft | May 3 |
| Longmont 2 ESE used here | 1.2 mi E | 4,950 ft | May 7 |
| Loveland 2 N | 18.4 mi N | 5,080 ft | May 10 |
| Waterdale | 18.7 mi NNW | 5,230 ft | May 12 |
Which weather station is this based on?
- Station
- Longmont 2 ESE (NOAA USC00055116, COOP)
- Distance
- 1.2 mi E of the city center
- Elevation
- 4,950 ft; city center 4,981 ft (USGS), difference −31 ft
- Record
- NOAA completeness flag R, 17 of 30 years (1991–2020)
- Confidence
- Medium (rule)
Confidence: Medium. The station passes our quality rules, but it's farther away or at a different elevation than ideal (1.2 mi, −31 ft; NOAA flag R, 17 of 30 years).
Caveat: NOAA flags this station's frost statistics 'R' (representative). They rest on 17 of the 30 years in 1991–2020, with missing months filled in from surrounding stations. The nearest station with a full 'S' (standard) record is Loveland 2 N, 18.4 mi away; its typical last frost is May 10.
| Station | ID | Distance | Elev. (Δ) | Last frost 50% | First frost 50% | Season 50% | NOAA flag / years |
|---|---|---|---|---|---|---|---|
| Longmont 2 ESE | USC00055116 | 1.2 mi E | 4,950 ft (-31) | May 7 | Oct 5 | 151 d | R / 17 |
| Loveland 2 N | USC00055236 | 18.4 mi N | 5,080 ft (+99) | May 10 | Oct 1 | 142 d | S / 25 |
| Waterdale | USC00058839 | 18.7 mi NNW | 5,230 ft (+249) | May 12 | Oct 1 | 141 d | R / 23 |
| Northglenn | USC00055984 | 19.2 mi SSE | 5,407 ft (+426) | May 2 | Oct 11 | 160 d | S / 26 |
| Brighton 3 SE | USC00050950 | 20.9 mi SE | 5,016 ft (+35) | May 3 | Oct 7 | 155 d | S / 25 |
Other stations within 25 mi (not used)
- Boulder (USC00050848), 15.0 mi SW, 5,484 ft: typical last frost May 6. Not used: |elev diff| 503 ft > 500 ft.
- Gross Reservoir (USC00053629), 20.8 mi SW, 7,970 ft: typical last frost May 24. Not used: |elev diff| 2989 ft > 500 ft.
- Allenspark 3 NW (USC00050185), 22.4 mi W, 8,500 ft: typical last frost Jun 14. Not used: frost normals incomplete or flagged (needs all 81 values with S/R flags, or values missing only because NOAA's probability of occurrence is below 10%); |elev diff| 3519 ft > 500 ft.
- Coal Creek Canyon (USC00051681), 24.1 mi SW, 8,950 ft: typical last frost Jun 5. Not used: |elev diff| 3969 ft > 500 ft.
- Ralston Reservoir (USC00056816), 24.4 mi SSW, 5,900 ft: typical last frost May 3. Not used: |elev diff| 919 ft > 500 ft.
82 other NOAA normals stations within 40 km (mostly rain gauges) have no frost statistics.
Longmont planting dates at a glance
Balanced dates use the 30%-risk last-frost date as the anchor. The Longmont planting calendar has typical and safe dates, fall planting and a month-by-month list for 27 vegetables.
| Crop | Start indoors | Plant outdoors (balanced) | Fall |
|---|---|---|---|
| Tomatoes | May 4–18 | Jun 22 | – |
| Peppers | Apr 1–15 | May 27 | – |
| Potatoes | – | Apr 29 – May 13 | – |
| Onions (dry, from sets) | – | Apr 15–29 | – |
| Peas | – | Apr 15–29 | sow by Jul 18 |
| Lettuce (leaf) | – | Apr 15–29 | sow by Jul 23 |
| Carrots | – | Apr 29 – May 13 | sow by Jul 13 |
| Beans (snap) | – | May 13 | sow by Jul 9 |
| Summer squash | – | May 13 | sow by Jul 19 |
| Broccoli | Feb 25 – Mar 11 | Apr 15–29 | transplant by Jul 18 |
| Cabbage | Feb 25 – Mar 11 | Apr 15–29 | transplant by Jun 28 |
| Cauliflower | Mar 11–25 | Apr 29 – May 13 | – |
| Kohlrabi | – | Apr 15–29 | sow by Aug 2 |
| Green onions | – | Apr 15–29 | sow by Jul 23 |
| Radishes | – | Apr 15–29 | sow by Aug 22 |
| Spinach | – | Apr 15–29 | sow by Aug 12 |
| Turnips | – | Apr 15–29 | sow by Aug 2 |
| Beets | – | Apr 29 – May 13 | sow by Jul 23 |
| Swiss chard | – | Apr 29 – May 13 | sow by Jul 23 |
| Parsnips | – | Apr 29 – May 13 | – |
| Sweet corn | – | May 13 | – |
| Cucumbers | – | May 13 | sow by Jul 14 |
| Eggplant | Mar 25 – Apr 15 | May 27 | – |
| Cantaloupe | – | May 27 | – |
| Watermelon | – | May 27 | – |
| Winter squash | – | May 27 | – |
| Garlic | – | – | plant Oct 1–14 |
When to plant tomatoes in Longmont · Fall planting dates for Longmont
Frost dates near Longmont
| City | Distance | Last frost | First frost | Season | Zone 2023 |
|---|---|---|---|---|---|
| Boulder frost dates | 13 mi | May 6 | Oct 5 | 151 days | 6a |
| Loveland frost dates | 17 mi | May 10 | Oct 1 | 142 days | 5b |
| Thornton frost dates | 19 mi | May 2 | Oct 11 | 160 days | 6a |
| Arvada frost dates | 23 mi | May 5 | Oct 5 | 151 days | 6a |
| Greeley frost dates | 24 mi | May 2 | Oct 8 | 159 days | 5a–5b |
| Commerce City frost dates | 26 mi | May 3 | Oct 7 | 155 days | 5b |
Frequently asked questions
What is the average last frost date in Longmont?
May 7 is the median (50%) date for Longmont's last 32°F night, from NOAA 1991–2020 normals at Longmont 2 ESE. To play it safe, use May 22: only about 1 year in 10 has a freeze after it.
When is the first frost in Longmont?
The first 32°F night typically arrives by Oct 5. About 1 year in 10, it comes as early as Sep 23. A 36°F night (frost possible on plants) typically shows up around Sep 28.
What growing zone is Longmont, Colorado?
Longmont is in USDA hardiness zone 5b–6a on the 2023 map. Most of the city was already 5b on the 2012 map, though 1 of 3 city ZIPs changed zone.
Is it too early to plant in Longmont?
Before May 7, yes, for anything frost-tender. Until then, the odds of another 32°F night are better than even. The risk falls from 50% to 10% between May 7 and May 22. Peas, lettuce and other cool-season crops can go in weeks earlier; the planting calendar has dates.
What temperature is a killing frost?
It depends on the crop. Tender plants such as tomatoes are killed by a light frost, which CSU Extension describes as 28–32°F; below 28°F is a heavy frost. We show NOAA odds for 36°F (frost possible on plants), 32°F (freeze, our headline) and 28°F (hard freeze).
Is first frost the same as first freeze?
Not exactly. A freeze is about the air reaching 32°F. Frost is ice on surfaces, which can form on calm, clear nights while the air is still a few degrees above freezing. So in Longmont, the first 36°F night (frost possible on plants) typically comes around Sep 28, ahead of the first 32°F night (Oct 5).
Download & sources
- Add Longmont frost & planting dates to your calendar (.ics, 2027)
- Printable planting calendar (PDF)
- How these numbers are calculated
Data: NOAA NCEI U.S. Climate Normals 1991–2020 (annual/seasonal and daily normals, station USC00055116); USDA Plant Hardiness Zone ZIP tables 2023 and 2012 by the PRISM Group, Oregon State University; Census 2020 ZIP/place relationship file; USGS elevation. Long-term odds, not a forecast. Figures and sentences are generated from these data by fixed rules; see the methodology. Last updated 2026-10-08.