What zone 6a means for Boulder
Hardiness zones are based on the average of the coldest night of each winter over 30 years (1991 to 2020). In zone 6a, that average is -10 to -5°F (-23 to -21°C). Plants labeled hardy to zone 6a or colder should survive a typical Boulder winter. A plant rated only to a warmer zone will need a sheltered spot, a pot you can bring inside, or treating as an annual.
How Boulder compares with the rest of Colorado
Colorado runs from zone 4a to zone 7a on the 2023 map. About 44% of the state's ZIP codes share Boulder's zone 6a. Gardeners in colder parts of the state may need hardier plants, while those in warmer parts can grow some plants that would struggle here.
Frost dates in Boulder
These are the median dates of the last and first 32°F freeze from NOAA's 1991 to 2020 climate normals, using the weather station closest to Boulder (Boulder, about 2 km away). Half of years have a later last frost, so tender plants are safest a week or two after the date. Your own garden can differ with elevation and shelter: check your exact ZIP code above.
Vegetable planting calendar for Boulder
Purdue Extension groups vegetables by how much frost they can take and times planting from the average last spring frost. Applied to Boulder's last frost of May 6:
| Group | Plant outdoors | Crops |
|---|---|---|
| Hardy crops (tolerate a hard frost) 4 to 6 weeks before the average last spring frost | March 25 to April 8 | Asparagus, broccoli, Brussels sprouts, cabbage, collards, horseradish, kale, kohlrabi, leek, mustard, onion, peas, potato, radish, rhubarb, spinach, turnip |
| Semi-hardy crops (tolerate a light frost) 2 to 4 weeks before the average last spring frost | April 8 to April 22 | Beet, carrot, cauliflower, celery, chard, Chinese cabbage, lettuce, mustard, parsnip |
| Tender crops (damaged by frost) after the average last spring frost, once nights stay above 50°F | From May 6 | Bean, sweet corn, tomato |
| Very tender crops (warm season) at least 2 weeks after the average last spring frost, once nights stay above 60 to 65°F | From May 20 | Cucumber, eggplant, melons, okra, pepper, pumpkin, squash, sweet potato |
Spring planting for direct seeding or transplants. A late cold snap can still happen, so watch the forecast and protect tender plants. Want today's answer for your ZIP code? Use what to plant now.
Sources: NOAA NCEI U.S. Climate Normals 1991 to 2020; Purdue Extension, Indiana Vegetable Planting Calendar (HO-186-W).
Choosing plants for zone 6a
When you shop, look for the hardiness range on the tag or in the catalog, such as "zones 4 to 9". Pick perennials, shrubs and trees whose range includes 6a. If you are set on a plant rated only to a warmer zone, plant it in a sheltered spot against a south-facing wall, mulch the roots before winter, or grow it in a container you can move into a garage. Zones only cover winter cold, so also check sun, water and soil needs. More in how to read a plant tag.
Boulder ZIP codes and zones
ZIP codes GeoNames assigns to Boulder, which can include PO box ZIP codes.
Other cities in Colorado
Frequently asked questions
When is the last frost in Boulder?
The median last spring frost (32°F) near Boulder is May 6, and the median first fall frost is October 5, based on NOAA 1991 to 2020 climate normals.
What planting zone is Boulder in?
Boulder, CO is in USDA plant hardiness zone 6a. Zone 6a means an average coldest winter night of -10 to -5°F (-23 to -21°C), based on the 2023 USDA map.
Did Boulder's zone change in 2023?
The 2023 USDA map uses weather data from 1991 to 2020. The USDA says about half of the country shifted to the next warmer half zone compared with the 2012 map, while the rest stayed in the same half zone.
Zones: 2023 USDA Plant Hardiness Zone Map ZIP code listings, PRISM Climate Group, Oregon State University. Place names: GeoNames (CC BY 4.0). Not the official USDA map. Our data.