Boulder, Colorado Frost Dates, Growing Season & Hardiness Zone
Frost odds and planting zone for Boulder for the 2027 season, based on 1991–2020 normals.
Last frost date
May 6
typical (50% chance of a later 32°F night)
Safer date: May 18 (10% risk)
earlyMay 6
typicalMay 18
safer
First fall frost
Oct 5
typical (50% chance it has happened by then)
Early-risk date: Sep 21 (10%)
Growing season
151 days
typical frost-free (32°F) season
Short year: 131 days (1 year in 10 is shorter)
USDA hardiness zone
6a
2023 map: -10 to -5°F average coldest night
2012 map: 6a (most of the city)
Across the metro, the typical last frost runs from Apr 30 (Denver Water Dept.) to May 7 (Longmont 2 ESE), depending on where you live.
Typical last freeze (32°F): May 6. In 1 year out of 10, the last one comes after May 18.
Based on NOAA station Boulder (USC00050848), 2.4 mi SSW, elevation 5,484 ft (+233 ft vs. city center). Confidence: High.
When is the last frost in Boulder?
In half of years, the last 32°F night has passed by May 6. In 1 year out of 10, one still comes after May 18. The last 36°F night (frost possible on plants) typically comes later, around May 17, and the last 28°F hard freeze earlier, around Apr 27.
There are 12 days between the typical date (May 6) and the safer 1-in-10 date (May 18). Planting in that stretch is a gamble, so keep an eye on the 10-day forecast.
Cool nights hang on after the last freeze: the typical last 36°F night (frost possible on plants in low spots) is around May 17, 11 days later than the typical last 32°F night. Frost can form on leaves on clear, calm nights even with the thermometer above freezing.
| Threshold | 10% (safer) | 30% | 50% (typical) | 90% (early) |
|---|---|---|---|---|
| 36°F (frost possible on plants) | Jun 2 | May 22 | May 17 | May 5 |
| 32°F freeze | May 18 | May 11 | May 6 | Apr 24 |
| 28°F hard freeze | May 10 | May 2 | Apr 27 | Apr 10 |
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 22 | May 20 | May 17 | May 15 | May 12 | May 9 | May 5 |
| 32°F freeze | May 18 | May 14 | May 11 | May 9 | May 6 | May 4 | May 1 | Apr 29 | Apr 24 |
| 28°F hard freeze | May 10 | May 5 | May 2 | Apr 30 | Apr 27 | Apr 23 | Apr 20 | Apr 16 | Apr 10 |
Is it safe to plant after Mother's Day in Boulder?
Around Boulder, 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 about 40%; for a 36°F night (frost possible on plants) it's about 80% (NOAA 1991–2020, Boulder).
Is it too early to plant in Boulder?
Before May 6 the odds of another 32°F night are better than even; after May 18 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 Boulder?
The typical first fall freeze (32°F) comes by Oct 5, but 1 year in 10 it shows up as early as Sep 21. Cold 36°F nights (frost possible on plants) typically start around Sep 28, and the first hard freeze (28°F) typically comes around Oct 15.
For fall crops, count back from Oct 5, or from Sep 21 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 13 | Sep 23 | Sep 28 | Oct 10 |
| 32°F freeze | Sep 21 | Sep 30 | Oct 5 | Oct 18 |
| 28°F hard freeze | Sep 30 | Oct 8 | Oct 15 | Oct 30 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | Sep 13 | Sep 19 | Sep 23 | Sep 26 | Sep 28 | Oct 1 | Oct 3 | Oct 5 | Oct 10 |
| 32°F freeze | Sep 21 | Sep 27 | Sep 30 | Oct 3 | Oct 5 | Oct 8 | Oct 11 | Oct 14 | Oct 18 |
| 28°F hard freeze | Sep 30 | Oct 5 | Oct 8 | Oct 12 | Oct 15 | Oct 18 | Oct 21 | Oct 25 | Oct 30 |
How long is Boulder's growing season?
In a typical year, Boulder gets 151 days between the last and first 32°F nights. About 1 year in 10, the season is shorter than 131 days.
That's enough time for most warm-season vegetables if they go in on time. Long-season crops do best started from transplants.
The station averages 2,812 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) | 150 days | 130 days | 109 days |
| 32°F freeze | 170 days | 151 days | 131 days |
| 28°F hard freeze | 193 days | 171 days | 149 days |
How does Boulder's season compare?
Among the 20 Colorado cities with frost dates on this site, the typical frost-free season in Boulder (151 days) is tied for the 15th longest.
Compared with the 56 U.S. cities with frost dates on this site, this typical frost-free season is longer than in 3 and shorter than in 50.
Growing degree days (base 50°F)
Annual normal: 2,812 GDD (NOAA flag S, 26 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 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 10 | 46 | 130 | 380 | 884 | 1,581 | 2,225 | 2,635 | 2,775 | 2,808 | 2,812 |
What growing zone is Boulder? (USDA 2023)
On the 2023 USDA map, Boulder is zone 6a (-10 to -5°F average coldest winter night). On the 2012 map, most of the city was 6a. Of the 6 city ZIPs listed in both tables, 1 moved up half a zone.
| ZIP | 2023 | 2012 | Change | Share of ZIP in Boulder |
|---|---|---|---|---|
| 80301 | 6a | 5b | +1 half zone (warmer) | 23% |
| 80302 | 6a | 6a | unchanged | 5% |
| 80303 | 6a | 6a | unchanged | 18% |
| 80304 mostly in city | 6a | 6a | unchanged | 64% |
| 80305 mostly in city | 6a | 6a | unchanged | 68% |
| 80310 mostly in city | 6a | 6a | unchanged | 100% |
Other ZIPs touching Boulder and PO-box ZIPs
ZIPs outside the 803 prefix that overlap the city boundary:
- 80503: 2023 6a, 2012 5b (0.7% of the ZIP is in Boulder)
PO-box and single-address 803xx ZIPs have no Census area, so we cannot tell which city they serve: 80306 (6a/6a), 80307 (6a/6a), 80308 (6a/6a), 80309 (n/a/6a), 80314 (6a/5b) (2023/2012).
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 Boulder's frost dates?
No. A hardiness zone reflects only the average coldest winter temperature, not the timing of spring or fall frost. Boulder's frost dates come from NOAA's 1991–2020 frost probabilities, and the 2023 zone map didn't change them. Pick perennials that survive winter by zone; time your planting by frost dates.
Why do Boulder frost dates differ between websites?
You may see May 11 given as Boulder's last frost date. That's the 30%-risk date, meaning about 3 years in 10 still have a 32°F night after it. Our headline date is May 6, the 50% (median) date.
May 18 is the 10%-risk, or 'safer', date. Only about 1 year in 10 has a 32°F night after it.
May 17 is the 50% date for a 36°F night (frost possible on plants), not a 32°F freeze. Sites that treat 36°F as the danger line show later dates.
Station choice is another reason dates differ. Our 7 qualifying stations within 25 miles of the city center range from Apr 30 to May 7 for the typical (50%) last 32°F frost (table below).
| Date | What it is (NOAA 1991–2020) |
|---|---|
| May 11 | 32°F last frost, 30% risk (a 'balanced' date) |
| May 17 | 36°F last cold night, 50% (frost possible on plants) |
| May 18 | 32°F last frost, 10% risk (the 'safer' date) |
| Apr 27 | 28°F last hard freeze, 50% |
| May 2 | 32°F last frost, 50%, at Northglenn (nearest station with NOAA flag S) |
| Station | Distance | Elevation | Typical last frost |
|---|---|---|---|
| Denver Water Dept. | 24.1 mi SSE | 5,228 ft | Apr 30 |
| Northglenn | 15.3 mi SE | 5,407 ft | May 2 |
| Lakewood | 20.3 mi SSE | 5,640 ft | May 3 |
| Brighton 3 SE | 22.7 mi ESE | 5,016 ft | May 3 |
| Wheat Ridge 2 | 18.6 mi SSE | 5,398 ft | May 5 |
| Boulder used here | 2.4 mi SSW | 5,484 ft | May 6 |
| Longmont 2 ESE | 13.6 mi NE | 4,950 ft | May 7 |
Which weather station is this based on?
- Station
- Boulder (NOAA USC00050848, COOP)
- Distance
- 2.4 mi SSW of the city center
- Elevation
- 5,484 ft; city center 5,251 ft (USGS), difference +233 ft
- Record
- NOAA completeness flag S, 26 of 30 years (1991–2020)
- Confidence
- High (rule)
Confidence: High. The station is 2.4 miles from the city center, only 233 ft different in elevation, and its frost statistics meet our completeness rule (NOAA flag S, 26 of 30 years).
| Station | ID | Distance | Elev. (Δ) | Last frost 50% | First frost 50% | Season 50% | NOAA flag / years |
|---|---|---|---|---|---|---|---|
| Boulder | USC00050848 | 2.4 mi SSW | 5,484 ft (+233) | May 6 | Oct 5 | 151 d | S / 26 |
| Longmont 2 ESE | USC00055116 | 13.6 mi NE | 4,950 ft (-301) | May 7 | Oct 5 | 151 d | R / 17 |
| Northglenn | USC00055984 | 15.3 mi SE | 5,407 ft (+156) | May 2 | Oct 11 | 160 d | S / 26 |
| Wheat Ridge 2 | USC00058995 | 18.6 mi SSE | 5,398 ft (+147) | May 5 | Oct 5 | 151 d | S / 28 |
| Lakewood | USC00054762 | 20.3 mi SSE | 5,640 ft (+389) | May 3 | Oct 7 | 155 d | R / 24 |
| Brighton 3 SE | USC00050950 | 22.7 mi ESE | 5,016 ft (-235) | May 3 | Oct 7 | 155 d | S / 25 |
| Denver Water Dept. | USC00052223 | 24.1 mi SSE | 5,228 ft (-23) | Apr 30 | Oct 12 | 165 d | R / 22 |
Other stations within 25 mi (not used)
- Gross Reservoir (USC00053629), 8.0 mi SW, 7,970 ft: typical last frost May 24. Not used: |elev diff| 2719 ft > 500 ft.
- Coal Creek Canyon (USC00051681), 11.4 mi SW, 8,950 ft: typical last frost Jun 5. Not used: |elev diff| 3699 ft > 500 ft.
- Ralston Reservoir (USC00056816), 13.5 mi S, 5,900 ft: typical last frost May 3. Not used: |elev diff| 649 ft > 500 ft.
- Boulder 14 W (USW00094075), 15.3 mi W, 9,828 ft: typical last frost Jun 21. 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| 4577 ft > 500 ft.
- Allenspark 3 NW (USC00050185), 20.0 mi NW, 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| 3249 ft > 500 ft.
79 other NOAA normals stations within 40 km (mostly rain gauges) have no frost statistics.
Boulder planting dates at a glance
Balanced dates use the 30%-risk last-frost date as the anchor. The Boulder 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 2–16 | Jun 20 | – |
| Peppers | Mar 30 – Apr 13 | May 25 | – |
| Potatoes | – | Apr 27 – May 11 | – |
| Onions (dry, from sets) | – | Apr 13–27 | – |
| Peas | – | Apr 13–27 | sow by Jul 18 |
| Lettuce (leaf) | – | Apr 13–27 | sow by Jul 23 |
| Carrots | – | Apr 27 – May 11 | sow by Jul 13 |
| Beans (snap) | – | May 11 | sow by Jul 9 |
| Summer squash | – | May 11 | sow by Jul 19 |
| Broccoli | Feb 23 – Mar 9 | Apr 13–27 | transplant by Jul 18 |
| Cabbage | Feb 23 – Mar 9 | Apr 13–27 | transplant by Jun 28 |
| Cauliflower | Mar 9–23 | Apr 27 – May 11 | – |
| Kohlrabi | – | Apr 13–27 | sow by Aug 2 |
| Green onions | – | Apr 13–27 | sow by Jul 23 |
| Radishes | – | Apr 13–27 | sow by Aug 22 |
| Spinach | – | Apr 13–27 | sow by Aug 12 |
| Turnips | – | Apr 13–27 | sow by Aug 2 |
| Beets | – | Apr 27 – May 11 | sow by Jul 23 |
| Swiss chard | – | Apr 27 – May 11 | sow by Jul 23 |
| Parsnips | – | Apr 27 – May 11 | – |
| Sweet corn | – | May 11 | – |
| Cucumbers | – | May 11 | sow by Jul 14 |
| Eggplant | Mar 23 – Apr 13 | May 25 | – |
| Cantaloupe | – | May 25 | – |
| Watermelon | – | May 25 | – |
| Winter squash | – | May 25 | – |
| Garlic | – | – | plant Oct 1–14 |
When to plant tomatoes in Boulder · Fall planting dates for Boulder
Frost dates near Boulder
| City | Distance | Last frost | First frost | Season | Zone 2023 |
|---|---|---|---|---|---|
| Longmont frost dates | 13 mi | May 7 | Oct 5 | 151 days | 5b–6a |
| Arvada frost dates | 14 mi | May 5 | Oct 5 | 151 days | 6a |
| Thornton frost dates | 18 mi | May 2 | Oct 11 | 160 days | 6a |
| Lakewood frost dates | 24 mi | May 3 | Oct 7 | 155 days | 6a |
| Denver frost dates | 24 mi | Apr 30 | Oct 12 | 165 days | 6a |
| Commerce City frost dates | 26 mi | May 3 | Oct 7 | 155 days | 5b |
Frequently asked questions
What is the average last frost date in Boulder?
The typical (median, 50%) last 32°F night in Boulder is May 6, based on NOAA 1991–2020 normals at Boulder. For a safer target, use May 18; after that, only about 1 year in 10 still has a freeze.
When is the first frost in Boulder?
Typically by Oct 5 for the first 32°F night, though about 1 year in 10 it comes as early as Sep 21. The first 36°F night (frost possible on plants) typically shows up around Sep 28.
What growing zone is Boulder, Colorado?
Boulder is in USDA hardiness zone 6a on the 2023 map. Most of the city was already 6a on the 2012 map, though 1 of 6 city ZIPs changed zone.
Is it too early to plant in Boulder?
If it's before May 6, it's too early for frost-tender plants: another 32°F night is more likely than not. From May 6 to May 18, the risk falls from 50% to 10%. Cool-season crops such as peas and lettuce can be planted weeks earlier (see the planting calendar).
What temperature is a killing frost?
A light frost is enough to kill tender crops such as tomatoes. CSU Extension describes 28–32°F as a light frost and anything below 28°F as a heavy frost. We report NOAA odds at three thresholds: 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 quite. A freeze is when the air reaches 32°F. Frost is ice that forms on surfaces, and on calm, clear nights it can form with the air a few degrees above freezing. So in Boulder, the first 36°F night (frost possible on plants) typically comes around Sep 28, before the first 32°F night (Oct 5).
Download & sources
- Add Boulder 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 USC00050848); 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.