Thornton, Colorado Frost Dates, Growing Season & Hardiness Zone
Frost odds and planting zone for Thornton for the 2027 season, based on 1991–2020 normals.
Last frost date
May 2
typical (50% chance of a later 32°F night)
Safer date: May 14 (10% risk)
earlyMay 2
typicalMay 14
safer
First fall frost
Oct 11
typical (50% chance it has happened by then)
Early-risk date: Sep 26 (10%)
Growing season
160 days
typical frost-free (32°F) season
Short year: 142 days (1 year in 10 is shorter)
USDA hardiness zone
6a
2023 map: -10 to -5°F average coldest night
2012 map: 5b (most of the city)
Where you live in the metro matters. The typical last frost ranges from Apr 30 at Denver Water Dept. to May 7 at Longmont 2 ESE.
Half of years have their last 32°F night by May 2. In 1 year out of 10, a freeze still comes after May 14.
Based on NOAA station Northglenn (USC00055984), 4.1 mi WSW, elevation 5,407 ft (+212 ft vs. city center). Confidence: High.
When is the last frost in Thornton?
The typical last 32°F night is May 2, and in 1 year out of 10 a freeze still comes after May 14. Cold 36°F nights (frost possible on plants) typically last longer, to around May 12. The last hard freeze (28°F) typically comes earlier, around Apr 20.
There are 12 days between the typical date (May 2) and the safer 1-in-10 date (May 14). Planting in that stretch is a gamble, so keep an eye on the 10-day forecast.
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 12, 10 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) | May 26 | May 17 | May 12 | Apr 30 |
| 32°F freeze | May 14 | May 7 | May 2 | Apr 18 |
| 28°F hard freeze | May 5 | Apr 28 | Apr 20 | Apr 4 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | May 26 | May 20 | May 17 | May 14 | May 12 | May 9 | May 7 | May 4 | Apr 30 |
| 32°F freeze | May 14 | May 9 | May 7 | May 4 | May 2 | Apr 30 | Apr 27 | Apr 23 | Apr 18 |
| 28°F hard freeze | May 5 | May 1 | Apr 28 | Apr 24 | Apr 20 | Apr 18 | Apr 14 | Apr 9 | Apr 4 |
Is it safe to plant after Mother's Day in Thornton?
Around Thornton, 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 20%; for a 36°F night (frost possible on plants) it's about 60% (NOAA 1991–2020, Northglenn).
Is it too early to plant in Thornton?
Before May 2 the odds of another 32°F night are better than even; after May 14 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 Thornton?
The typical first fall freeze (32°F) comes by Oct 11, but 1 year in 10 it shows up as early as Sep 26. Cold 36°F nights (frost possible on plants) typically start around Oct 1, and the first hard freeze (28°F) typically comes around Oct 21.
For fall crops, count back from Oct 11, or from Sep 26 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 18 | Sep 27 | Oct 1 | Oct 14 |
| 32°F freeze | Sep 26 | Oct 4 | Oct 11 | Oct 24 |
| 28°F hard freeze | Oct 7 | Oct 15 | Oct 21 | Nov 4 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | Sep 18 | Sep 23 | Sep 27 | Sep 29 | Oct 1 | Oct 4 | Oct 7 | Oct 10 | Oct 14 |
| 32°F freeze | Sep 26 | Oct 1 | Oct 4 | Oct 7 | Oct 11 | Oct 13 | Oct 16 | Oct 19 | Oct 24 |
| 28°F hard freeze | Oct 7 | Oct 11 | Oct 15 | Oct 18 | Oct 21 | Oct 24 | Oct 27 | Oct 30 | Nov 4 |
How long is Thornton's growing season?
In a typical year, Thornton gets 160 days between the last and first 32°F nights. About 1 year in 10, the season is shorter than 142 days.
Most warm-season vegetables have time to finish if you plant on schedule. For long-season crops, transplants do best.
Each year the station logs an average of 3,168 growing degree days (base 50°F), a measure of the heat available for crop growth.
| Threshold | 10% (long year) | 50% (typical) | 90% (short year) |
|---|---|---|---|
| 36°F (frost possible on plants) | 159 days | 142 days | 122 days |
| 32°F freeze | 180 days | 160 days | 142 days |
| 28°F hard freeze | 204 days | 182 days | 160 days |
How does Thornton's season compare?
Among the 20 Colorado cities with frost dates on this site, the typical frost-free season in Thornton (160 days) is tied for the 4th longest.
Nationwide, across the 56 cities with frost dates on this site, the typical frost-free season here outlasts the one in 13 of them and falls short of the one in 39.
Growing degree days (base 50°F)
Annual normal: 3,168 GDD (NOAA flag R, 21 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 | 49 | 152 | 443 | 1,025 | 1,799 | 2,514 | 2,977 | 3,131 | 3,165 | 3,168 |
What growing zone is Thornton? (USDA 2023)
The 2023 USDA map puts Thornton in zone 6a (-10 to -5°F average coldest winter night). On the 2012 map, most of the city was 5b. Comparing ZIPs listed in both tables, 4 of 5 moved up half a zone.
| ZIP | 2023 | 2012 | Change | Share of ZIP in Thornton |
|---|---|---|---|---|
| 80221 | 6a | 5b | +1 half zone (warmer) | 0% |
| 80229 mostly in city | 6a | 5b | +1 half zone (warmer) | 55% |
| 80233 | 6a | 5b | +1 half zone (warmer) | 50% |
| 80241 mostly in city | 6a | 5b | +1 half zone (warmer) | 97% |
| 80260 | 6a | 6a | unchanged | 44% |
Other ZIPs touching Thornton
ZIPs outside the 802 prefix that overlap the city boundary:
- 80023: 2023 6a, 2012 6a (14.1% of the ZIP is in Thornton)
- 80602: 2023 6a, 2012 5b (40.8% of the ZIP is in Thornton)
- 80603: 2023 5b, 2012 5b (0.0% of the ZIP is in Thornton)
- 80640: 2023 5b, 2012 5b (6.9% of the ZIP is in Thornton)
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 Thornton's frost dates?
No. Zones describe only how cold the coldest winter night gets on average. They don't tell you when frost starts or stops. The frost dates for Thornton come from NOAA's 1991–2020 frost probabilities, which the 2023 zone map left untouched. Choose winter-hardy perennials by zone, and schedule planting by frost dates.
Why do Thornton frost dates differ between websites?
If you've seen May 7 listed as Thornton'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 2, the 50% (median) date.
May 14 is the 10%-risk, or 'safer', date. Only about 1 year in 10 has a 32°F night after it.
May 12 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.
The choice of weather station matters too. The 11 stations within 25 miles of the city center that pass our quality rules give typical (50%) last 32°F frost dates from Apr 30 to May 7; see the table below.
| Date | What it is (NOAA 1991–2020) |
|---|---|
| May 7 | 32°F last frost, 30% risk (a 'balanced' date) |
| May 12 | 36°F last cold night, 50% (frost possible on plants) |
| May 14 | 32°F last frost, 10% risk (the 'safer' date) |
| Apr 20 | 28°F last hard freeze, 50% |
| May 3 | 32°F last frost, 50%, at Brighton 3 SE (nearest station with NOAA flag S) |
| Station | Distance | Elevation | Typical last frost |
|---|---|---|---|
| Denver Water Dept. | 13.7 mi SSW | 5,228 ft | Apr 30 |
| Northglenn used here | 4.1 mi WSW | 5,407 ft | May 2 |
| Marston Filter Plant | 21.7 mi SSW | 5,610 ft | May 2 |
| Brighton 3 SE | 5.8 mi ENE | 5,016 ft | May 3 |
| Lakewood | 15.2 mi SW | 5,640 ft | May 3 |
| Denver-Stapleton | 11.5 mi SSE | 5,286 ft | May 4 |
| Wheat Ridge 2 | 13.7 mi SW | 5,398 ft | May 5 |
| Denver International Airport | 16.2 mi ESE | 5,414 ft | May 5 |
| Boulder | 17.9 mi WNW | 5,484 ft | May 6 |
| Longmont 2 ESE | 18.6 mi NNW | 4,950 ft | May 7 |
| Cherry Creek Dam | 21.1 mi SSE | 5,647 ft | May 7 |
Which weather station is this based on?
- Station
- Northglenn (NOAA USC00055984, COOP)
- Distance
- 4.1 mi WSW of the city center
- Elevation
- 5,407 ft; city center 5,195 ft (USGS), difference +212 ft
- Record
- NOAA completeness flag S, 26 of 30 years (1991–2020)
- Confidence
- High (rule)
Confidence: High. The station is close (4.1 miles from the city center), its elevation differs by just 212 ft, 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 |
|---|---|---|---|---|---|---|---|
| Northglenn | USC00055984 | 4.1 mi WSW | 5,407 ft (+212) | May 2 | Oct 11 | 160 d | S / 26 |
| Brighton 3 SE | USC00050950 | 5.8 mi ENE | 5,016 ft (-179) | May 3 | Oct 7 | 155 d | S / 25 |
| Denver-Stapleton | USW00023062 | 11.5 mi SSE | 5,286 ft (+91) | May 4 | Oct 8 | 156 d | S / 28 |
| Denver Water Dept. | USC00052223 | 13.7 mi SSW | 5,228 ft (+33) | Apr 30 | Oct 12 | 165 d | R / 22 |
| Wheat Ridge 2 | USC00058995 | 13.7 mi SW | 5,398 ft (+203) | May 5 | Oct 5 | 151 d | S / 28 |
| Lakewood | USC00054762 | 15.2 mi SW | 5,640 ft (+445) | May 3 | Oct 7 | 155 d | R / 24 |
| Denver International Airport | USW00003017 | 16.2 mi ESE | 5,414 ft (+219) | May 5 | Oct 9 | 156 d | R / 25 |
| Boulder | USC00050848 | 17.9 mi WNW | 5,484 ft (+289) | May 6 | Oct 5 | 151 d | S / 26 |
| Longmont 2 ESE | USC00055116 | 18.6 mi NNW | 4,950 ft (-245) | May 7 | Oct 5 | 151 d | R / 17 |
| Cherry Creek Dam | USC00051547 | 21.1 mi SSE | 5,647 ft (+452) | May 7 | Oct 8 | 152 d | R / 13 |
| Marston Filter Plant | USC00055402 | 21.7 mi SSW | 5,610 ft (+415) | May 2 | Oct 11 | 160 d | R / 18 |
Other stations within 25 mi (not used)
- Ralston Reservoir (USC00056816), 16.8 mi WSW, 5,900 ft: typical last frost May 3. Not used: |elev diff| 705 ft > 500 ft.
- Gross Reservoir (USC00053629), 21.7 mi W, 7,970 ft: typical last frost May 24. Not used: |elev diff| 2775 ft > 500 ft.
- Coal Creek Canyon (USC00051681), 23.6 mi W, 8,950 ft: typical last frost Jun 5. Not used: |elev diff| 3755 ft > 500 ft.
- Denver Centennial Airport (USW00093067), 24.7 mi SSE, 5,883 ft: typical last frost May 5. Not used: |elev diff| 688 ft > 500 ft.
84 other NOAA normals stations within 40 km (mostly rain gauges) have no frost statistics.
Thornton planting dates at a glance
Balanced dates use the 30%-risk last-frost date as the anchor. The Thornton 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 | Apr 22 – May 6 | Jun 10 | – |
| Peppers | Mar 26 – Apr 9 | May 21 | – |
| Potatoes | – | Apr 23 – May 7 | – |
| Onions (dry, from sets) | – | Apr 9–23 | – |
| Peas | – | Apr 9–23 | sow by Jul 24 |
| Lettuce (leaf) | – | Apr 9–23 | sow by Jul 29 |
| Carrots | – | Apr 23 – May 7 | sow by Jul 19 |
| Beans (snap) | – | May 7 | sow by Jul 15 |
| Summer squash | – | May 7 | sow by Jul 25 |
| Broccoli | Feb 19 – Mar 5 | Apr 9–23 | transplant by Jul 24 |
| Cabbage | Feb 19 – Mar 5 | Apr 9–23 | transplant by Jul 4 |
| Cauliflower | Mar 5–19 | Apr 23 – May 7 | – |
| Kohlrabi | – | Apr 9–23 | sow by Aug 8 |
| Green onions | – | Apr 9–23 | sow by Jul 29 |
| Radishes | – | Apr 9–23 | sow by Aug 28 |
| Spinach | – | Apr 9–23 | sow by Aug 18 |
| Turnips | – | Apr 9–23 | sow by Aug 8 |
| Beets | – | Apr 23 – May 7 | sow by Jul 29 |
| Swiss chard | – | Apr 23 – May 7 | sow by Jul 29 |
| Parsnips | – | Apr 23 – May 7 | – |
| Sweet corn | – | May 7 | – |
| Cucumbers | – | May 7 | sow by Jul 20 |
| Eggplant | Mar 19 – Apr 9 | May 21 | – |
| Cantaloupe | – | May 21 | – |
| Watermelon | – | May 21 | – |
| Winter squash | – | May 21 | – |
| Garlic | – | – | plant Oct 1–14 |
When to plant tomatoes in Thornton · Fall planting dates for Thornton
Frost dates near Thornton
| City | Distance | Last frost | First frost | Season | Zone 2023 |
|---|---|---|---|---|---|
| Commerce City frost dates | 8 mi | May 3 | Oct 7 | 155 days | 5b |
| Arvada frost dates | 13 mi | May 5 | Oct 5 | 151 days | 6a |
| Denver frost dates | 13 mi | Apr 30 | Oct 12 | 165 days | 6a |
| Boulder frost dates | 18 mi | May 6 | Oct 5 | 151 days | 6a |
| Lakewood frost dates | 18 mi | May 3 | Oct 7 | 155 days | 6a |
| Aurora frost dates | 19 mi | May 7 | Oct 8 | 152 days | 6a |
Frequently asked questions
What is the average last frost date in Thornton?
Based on NOAA 1991–2020 normals at Northglenn, Thornton's typical last 32°F night is May 2. That's the median (50%) date. For a safer target, go with May 14, after which a freeze comes in only about 1 year in 10.
When is the first frost in Thornton?
The first 32°F night typically arrives by Oct 11. About 1 year in 10, it comes as early as Sep 26. A 36°F night (frost possible on plants) typically shows up around Oct 1.
What growing zone is Thornton, Colorado? Is Thornton zone 5 or 6?
Thornton is in USDA hardiness zone 6a on the current 2023 map (zone 6 for short). On the older 2012 map, most of the city was 5b. That's why older references still say 'zone 5'.
Is it too early to plant in Thornton?
For frost-tender plants, yes if it's before May 2: until then, the odds of another 32°F night are better than even. Between May 2 and May 14, the risk falls from 50% to 10%. Cool-season crops like peas and lettuce can go in 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' means the air reached 32°F. 'Frost' is ice on surfaces, and it can form on calm, clear nights even when the air is a few degrees above freezing. So in Thornton, the first 36°F night (frost possible on plants) typically comes around Oct 1, before the first 32°F night (Oct 11).
Download & sources
- Add Thornton 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 USC00055984); 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.