Gardening in Colorado

Hardiness zones in Colorado

Colorado spans USDA hardiness zones 4a through 7a. That range comes from data gathered at 211 weather stations across the state. A hardiness zone marks the average coldest night your area sees each winter.

It says nothing about summer heat, rainfall, or the soil in your yard. A plant rated for zone 6 may not survive outdoors in a zone 4 winter. Mountain towns in Colorado often sit in zone 4a or 4b, where winters run coldest. Lower elevations tend toward the warmer end, zone 6a to 7a.

Two gardens in the same zone can still differ in heat, wind, and soil type. Use your zone as a starting point, then check each plant's own notes on sun, water, and soil before you plant it. Confirm your exact zone with local station data rather than guessing from the state's full range.

• Zone 4a
• Zone 4b
• Zone 5a
• Zone 5b
• Zone 6a
• Zone 6b
• Zone 7a

Frost dates

Frost dates in Colorado vary widely depending on where you garden. The earliest spots see their last spring frost around April 14. The latest spots hold on to frost risk until June 21.

In fall, the first frost can arrive as early as September 2 in the coldest areas. In the mildest areas, it can hold off until October 26. These are the dates when there is a 50 percent chance of frost, not a guarantee. Treat them as a planning guide, not a fixed calendar.

Wait until after your local last frost date to set out tender seedlings and warm-season crops. Watch for your first fall frost date as a signal to bring in houseplants and harvest the last tomatoes and peppers. Colorado's frost dates span more than two months in both spring and fall. That means your own station's numbers matter more than a single statewide date. Check the frost data for the station closest to your garden rather than relying on an average for the whole state.

Climate

Colorado's climate is not one climate at all; it changes sharply with elevation and location. The growing season length proves this. Some parts of the state get as few as 31 frost-free days a year. Others get as many as 190. That is more than six times the difference between the shortest and longest growing season in the same state.

A gardener in a 31-day season has to choose fast-maturing crops and may need row cover or a greenhouse to grow much at all. A gardener in a 190-day season has room to grow long-season vegetables. They can often start a second round of fast crops after the first harvest. This spread lines up with the state's hardiness zones, which run from 4a in the coldest spots to 7a in the mildest.

The data behind these figures comes from 211 weather stations spread across Colorado. Local conditions are well represented, even though a single state average hides a lot of detail. Because of this range, a planting plan that works in one part of Colorado can fail completely in another. Always check the zone, frost dates, and season length for your own location instead of treating the state as a single climate. Rainfall, humidity, and soil type are not part of the zone or frost figures. Look to your own local conditions for those details instead.

Season length

Growing season length is the clearest way to see how much Colorado varies from place to place. Some areas get only 31 frost-free days between the last spring frost and the first fall frost. Others get as many as 190 days, enough time for a full range of vegetables and flowers.

A short season calls for quick-maturing varieties, transplants instead of seeds, and season-extending covers. A long season gives you room to succession-plant and to grow crops that need a full summer to mature.

Check your own last and first frost dates before you count on either end of this range. Your actual season length depends on your elevation and local station data, not on the state as a whole.