Alamosa, CO Frost Dates
Average frost dates, USDA hardiness zone, and growing season length for Alamosa, Colorado.
Gardening in Alamosa
The San Luis Valley sits at 7,500 feet — the highest valley large enough for commercial agriculture in North America. Gardening here is genuinely extreme.
Zone 4a with only 108 frost-free days. The high-altitude cold and UV intensity are serious challenges. But the valley's artesian water and intense sun produce exceptional potatoes, and the cool nights develop remarkable flavor.
The San Luis Valley's potato and barley farms prove agriculture works at 7,500 feet. Home gardeners share the extreme conditions — and the extreme satisfaction of harvesting at altitude.
What This Means for Alamosa Gardeners
The average last spring frost in Alamosa is around May 25, and the average first fall frost arrives around September 10. That gives you approximately 108 frost-free days to work with.
108 frost-free days means every warm day counts. Focus on cold-hardy crops that thrive in your moderate temperatures: kale, lettuce, peas, radishes, potatoes, and root vegetables. For warm-season crops, choose only the shortest-season varieties (sub-65-day tomatoes, 55-day beans) and start everything indoors. Season extension — row covers, cold frames, plastic mulch — isn't a luxury here, it's essential infrastructure.
These dates are based on NOAA 30-year Climate Normal data for the Alamosa 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 Alamosa
With 108 frost-free days, Alamosa 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 Alamosa.
More About Zone 4A
Alamosa is in USDA Hardiness Zone 4A, which means average annual extreme minimum temperatures between -30°F to -25°F. View the full Zone 4A 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 Alamosa 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 Alamosa (urban heat islands, hilltops, low-lying valleys) can also shift your local frost dates by a week or more.
Start everything possible indoors — your 108-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 25). Choose short-season varieties for warm crops. Enter your zip code for exact dates.