Denver CO Frost Dates — When to Plant & Protect Your Garden
Denver’s frost dates define your planting schedule and protection needs. Based on NOAA’s 30-year climate normals from station USW00023062, here’s what the data shows for your USDA zone 6a garden.
Your Last Spring Frost
Your spring planting decisions depend on frost probability. Denver’s last spring frost dates vary by confidence level:
| Confidence Level | Date | Risk Assessment |
|---|---|---|
| 90% chance frost is over | May 15 | Safest for heat-loving plants |
| 80% chance frost is over | May 10 | Good for most warm-season crops |
| 70% chance frost is over | May 7 | Moderate risk acceptable |
| 60% chance frost is over | May 5 | Higher risk tolerance needed |
| 50% chance frost is over | May 3 | Coin flip - prepare protection |
| 40% chance frost is over | May 1 | Significant frost risk remains |
| 30% chance frost is over | April 29 | High probability of future frost |
| 20% chance frost is over | April 25 | Very likely more frost coming |
| 10% chance frost is over | April 21 | Almost certain frost ahead |
Source: NOAA Climate Normals 1991-2020, station USW00023062
The 50% probability date (May 3) represents the median last frost. Half of all years see their final frost before this date, half after.
Your First Fall Frost
Fall frost arrives progressively through autumn in Denver:
| Confidence Level | Date | Garden Impact |
|---|---|---|
| 10% chance frost has arrived | September 22 | First risk for sensitive plants |
| 20% chance frost has arrived | September 27 | Moderate early frost risk |
| 30% chance frost has arrived | September 30 | Growing risk level |
| 40% chance frost has arrived | October 3 | Nearly even odds |
| 50% chance frost has arrived | October 6 | Median first frost date |
| 60% chance frost has arrived | October 9 | More likely than not |
| 70% chance frost has arrived | October 12 | High probability |
| 80% chance frost has arrived | October 15 | Very likely |
| 90% chance frost has arrived | October 19 | Near certainty |
Source: NOAA Climate Normals 1991-2020, station USW00023062
October 6 marks Denver’s median first fall frost. This is when you have a 50-50 chance of seeing your first killing frost.
Your Growing Season
Denver’s frost-free season spans 156 days based on the 50% probability dates (May 3 to October 6). This calculation uses NOAA’s climate normals from 1991-2020.
Your zone 6a classification means average annual minimum temperatures fall between -10°F and -5°F, defining which perennial plants can survive Denver winters without protection.
What This Means for Your Garden
Spring Planting Strategy
- High-risk, early planting (April 21-May 2): Be ready with row covers, cold frames, or other protection
- Moderate-risk planting (May 3-May 9): Good compromise for experienced gardeners who can react to forecasts
- Conservative planting (May 10+): Safest approach for heat-sensitive crops
Fall Garden Management
- Early September: Begin monitoring frost forecasts closely
- Late September: Have protection materials ready for tender plants
- Early October: Expect first killing frost around the median date of October 6
This Week in Denver
Current conditions (April 2, 2026 at 3:00 PM): 71.5°F, 12% humidity, winds at 11.8 mph. Soil temperature at 6 inches: 67.1°F.
7-day frost outlook:
| Date | High/Low | Frost Risk | Precipitation |
|---|---|---|---|
| Apr 2 | 73°F / 35°F | Yes | 3% chance |
| Apr 3 | 55°F / 37°F | No | 5% chance |
| Apr 4 | 65°F / 34°F | Yes | 5% chance |
| Apr 5 | 66°F / 45°F | No | 2% chance |
| Apr 6 | 65°F / 48°F | No | 15% chance |
| Apr 7 | 71°F / 46°F | No | 33% chance |
| Apr 8 | 72°F / 54°F | No | 28% chance |
Frost alerts: April 2 and April 4 show frost risk with lows near freezing. Protect sensitive plants or delay planting.
Forecast data from Open-Meteo, updated hourly
Your Growing Season: How’s It Going?
Growing Degree Day Accumulation
Denver’s monthly growing degree day (GDD) accumulation with base 50°F:
| Month | Average GDD | Heating Days | Cooling Days |
|---|---|---|---|
| January | 1 | 1,056 | 0 |
| February | 2 | 900 | 0 |
| March | 18 | 775 | 0 |
| April | 79 | 527 | 2 |
| May | 255 | 261 | 18 |
| June | 511 | 62 | 122 |
| July | 731 | 6 | 272 |
| August | 668 | 11 | 214 |
| September | 384 | 137 | 58 |
| October | 118 | 457 | 2 |
| November | 19 | 792 | 0 |
| December | 1 | 1,085 | 0 |
Source: NOAA CDO Climate Normals 1991-2020, station USW00023062
Peak growing degree day accumulation occurs in July (731 GDD), followed by August (668 GDD). This data helps time plantings for crops with specific heat requirements.
Zone 6a Chill Hours
Denver typically receives 1,100 chill hours annually, with a range of 900-1,400 hours. This meets the requirements for most temperate fruit trees and berry bushes that need winter chill for proper spring flowering and fruit set.
About This Data
Frost date source: NOAA Climate Normals 1991-2020 from weather station USW00023062, the primary climate station serving Denver. This station provides the 30-year average frost probabilities used by agricultural extensions and commercial growers.
Zone classification: USDA Plant Hardiness Zone Map 2023 places Denver in zone 6a, with average annual minimum temperatures of -10°F to -5°F.
Geographic coverage: Data applies to Denver ZIP codes 80201, 80202, 80203, 80204, 80205, 80206, 80207, 80208, and 80209 in Denver County (FIPS 08031), serving approximately 710,800 residents with 159,483 owner-occupied housing units.
Current weather and forecasts: Open-Meteo API provides real-time conditions and 7-day forecasts updated hourly. Growing degree day and chill hour estimates compiled from extension service data.
All temperature and date data reflects historical climate patterns. Individual years will vary from these averages, sometimes significantly.
Frost Date Planner Calculator
Interactive planting date calculator coming soon. Will use pre-computed frost probability data to recommend optimal planting windows for your specific crops.
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