Boulder, CO Frost Dates
Average frost dates, USDA hardiness zone, and growing season length for Boulder, Colorado.
Gardening in Boulder
Colorado's creative, outdoor-obsessed city applies the same intensity to food growing that it applies to trail running and climbing. Pearl Street's food scene starts in backyard gardens.
Zone 5b at 5,430 feet — intense sun, hail risk, and 148 frost-free days. Boulder's foothills location creates wind patterns and microclimates that vary from block to block. The city's water rights come from mountain snowmelt.
Boulder's food culture is nationally recognized, and the path from backyard to table is shorter here than in most cities. The Boulder County Farmers Market is one of the most vibrant in Colorado. The city's environmental ethos means composting, water conservation, and organic growing aren't trends — they're expectations.
What This Means for Boulder Gardeners
The average last spring frost in Boulder is around May 5, and the average first fall frost arrives around September 30. That gives you approximately 148 frost-free days to work with.
At 148 days, you're working with a compressed but productive window. Choose varieties by their days-to-maturity number — anything under 75 days is safe, 75-90 requires indoor starting, and 90+ is a calculated risk. The tradeoff: your cool, moderate summers are excellent for crops that heat-zone gardeners struggle with. Your lettuce doesn't bolt in June. Your peas produce for weeks longer. Cool-season crops are your superpower.
These dates are based on NOAA 30-year Climate Normal data for the Boulder area. Your actual frost dates could shift 2-3 weeks in either direction in any given year. Learn more about our data sources.
What to Grow in Boulder
With 148 frost-free days, Boulder gardeners need to plan strategically — start warm-season crops indoors and choose short-season varieties. Cool-season crops are your strength, thriving in the moderate temperatures that define your growing window. Recommended starting points: kale, lettuce, peas, carrots, potatoes, radishes, garlic, and short-season tomatoes.
See the full Colorado planting guide for all 40 plants: Colorado Planting Calendar. Or enter your zip code for exact planting dates personalized to Boulder.
More About Zone 5B
Boulder is in USDA Hardiness Zone 5B, which means average annual extreme minimum temperatures between -15°F to -10°F. View the full Zone 5B planting guide.
See the complete planting calendar for Colorado: Colorado Planting Calendar.
Other Cities in Colorado
Frequently Asked Questions
These dates are based on NOAA's 30-year Climate Normal data for the Boulder area. They represent historical averages, not predictions. In any given year, the actual last frost could be 2-3 weeks earlier or later. Microclimates within Boulder (urban heat islands, hilltops, low-lying valleys) can also shift your local frost dates by a week or more.
Start everything possible indoors — your 148-day season doesn't leave room for a slow start. Direct sow only the fastest, hardiest crops (radishes, lettuce, peas) 3-4 weeks before last frost (May 5). Choose short-season varieties for warm crops. Enter your zip code for exact dates.