Frost dates · USDA zone 6a
Frost dates for Thornton, CO
Thornton, CO sees its last 32 °F night of spring around May 2 and its first of fall around October 11 — a 160-day freeze-free season in a typical year.
Pick a sauce; the Plan Builder runs this station's dates and hands back the calendar, plant count and gear list.
Freeze odds, by date
| Odds | Last spring freeze after | First fall freeze before |
|---|---|---|
| 9 years in 10 | Apr 18 | Oct 24 |
| Median — 1 year in 2 | May 2 | Oct 11 |
| 1 year in 10 | May 14 | Sep 26 |
The top row is the risky edge: in 9 years of 10 a spring freeze still comes after it, so tender transplants set out then need cover. The bottom row is the cautious date — only 1 year in 10 freezes after it — and the one to plan uncovered tender transplants against. Freeze-free season: 160 days median, at least 142 in 9 years of 10, at least 180 days in 1 year of 10. NOAA ranks season length year by year, so it is not the gap between two rows above. A hard freeze (28 °F) has a median last date of Apr 20 and first of Oct 21.
Thornton planting calendar, 2027
Every row runs from the median last frost (May 2) and median first frost (Oct 11) above, using each cultivar's days to maturity. Same engine as the Plan Builder.
| Crop | Start indoors | Transplant / plant | First harvest | Days |
|---|---|---|---|---|
| Hardneck GarlicSiberian (porcelain hardneck) | — | Oct 15prior fall | Jun 12 | 240 from sow |
| Spanish OnionYellow Sweet Spanish (long-day) | Feb 7 | Apr 4 | Jul 23 | 110 from transplant |
| Mustard greensGarnet Giant (Brassica juncea) | Direct sow | May 2 | Jun 1 | 30 from sow |
| CilantroSlow Bolt (Coriandrum sativum) | Apr 18 | Apr 18 | Jun 2 | 45 from sow |
| Genovese BasilGenovese | Apr 4 | May 16 | Jul 15 | 60 from transplant |
| CayenneLong Slim Cayenne (Capsicum annuum) | Mar 7 | May 9 | Jul 23 | 75 from transplant |
| HabaneroOrange Habanero (Capsicum chinense) | Feb 21 | May 16 | Aug 19 | 95 from transplant |
| JalapeñoEarly Jalapeño (Capsicum annuum) | Mar 7 | May 9 | Jul 23 | 75 from transplant |
| Marconi Red PepperMarconi Red (Capsicum annuum) | Mar 7 | May 9 | Jul 28 | 80 from transplant |
| SerranoSerrano Tampiqueño (Capsicum annuum) | Mar 7 | May 9 | Jul 28 | 80 from transplant |
| Nantes CarrotScarlet Nantes | Apr 18 | Apr 18 | Jun 27 | 70 from sow |
| TomatilloToma Verde | Mar 21 | May 9 | Jul 18 | 70 from transplant |
| Roma TomatoRoma VF (determinate paste) | Mar 21 | May 9 | Jul 23 | 75 from transplant |
| San Marzano TomatoSan Marzano (indeterminate paste) | Mar 21 | May 9 | Jul 28 | 80 from transplant |
Tight means first harvest lands within two weeks of the median first fall frost: start that crop indoors on time or cover it in fall.
What sauce can Thornton grow?
Each sauce, scheduled for a Thornton container garden: when the first seeds go in and when enough fruit has ripened for one batch.
Thornton frost questions
When is the last frost in Thornton, CO?
The median last 32 °F freeze at NORTHGLENN, CO is May 2: half of years see a later one. One year in ten, a freeze still comes after May 14; nine years in ten, it comes after April 18. Source: NOAA NCEI 1991–2020 U.S. Climate Normals.
When is the first frost in Thornton, CO?
The median first 32 °F freeze of fall is October 11. One year in ten it arrives before September 26; nine years in ten, before October 24.
How long is the growing season in Thornton, CO?
160 days between the last spring and first fall 32 °F freeze in a median year. Nine years in ten the season runs at least 142 days; one year in ten, at least 180 days.
What growing zone is Thornton, CO?
USDA hardiness zone 6a, per the 2023 USDA Plant Hardiness Zone Map for ZIP 80241. A zone describes the coldest winter night, not when frost ends, so it is the wrong number for a planting calendar — the frost dates on this page are the right one.
When should I plant tomatoes in Thornton, CO?
Start Roma VF (determinate paste) seeds indoors around Mar 21 and transplant around May 9, 1 week after the median last frost. To cut the freeze risk to one year in ten, wait for May 14. First ripe fruit lands around Jul 23.
When should I plant peppers in Thornton, CO?
Sow jalapeño indoors on a heat mat around Mar 7; transplant no earlier than May 9, and wait past it until nights hold above 50 °F — that can run weeks later. First green pods around Jul 23.
Nearby in Colorado
- Broomfieldlast frost May 2
- Westminsterlast frost May 2
- Commerce Citylast frost May 3
- Brightonlast frost May 3
- Denverlast frost May 4
- Arvadalast frost May 5
All Colorado frost dates → · How frost-date math drives a sowing calendar →