The United States Department of Agriculture updated its map of the country's planting zones two years ago — the first update since 1990.
The map, originally published in 1960, looks at every state, including Hawaii, Alaska and the territory of Puerto Rico, and classifies them into planting zones by 10 degree differences.
It ranges from Zone 1a where the average annual extreme low temperature is minus 60 degrees F to Zone 13b where the average lowest temperature experienced there is 70 degrees F. These averages have been calculated throughout a 30-year period.
Deb Babcock's gardening column appears Thursdays in Steamboat Today.
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For more information about which perennials are great ones to plant here in the Yampa Valley and fall into the appropriate zones for our area, stop by the Master Gardener booth at the downtown Farmers Market on Saturday. Master Gardeners will be on hand to answer your plant questions.
In Colorado, our zones range from from 3a to 7a with Routt County ranging between 4a and 5b. (For zip code 80487 in downtown Steamboat Springs, it is categorized as 4a, meaning our average lowest temperature is around minus 30 degrees.) In Hayden, the plant hardiness zone is 5a (minus 20 degrees average low) and in Oak Creek, the zone is rated as 4b (minus 25 degrees average low). You now can type in your zip code on the website at www.planthardiness.ars.usda.gov and get the zone for your area.
What the plant hardiness map doesn't do is map the microclimates within each of the zones. That means although your garden may be rated as best for plants designated as 4a, for example, a Zone 5 plant might thrive because of a warm pocket you've created through the use of berms, rocks, depressions or structures in your garden. And vice versa, a Zone 4 plant may not survive because of a microclimate in part of your garden that creates colder pockets of air.
The plant hardiness zone map is simply a guide for gardeners to follow when choosing plants for their home garden. You'll note that in gardening catalogs and garden centers, most plants are labeled with the USDA plant hardiness zone to help you select those most likely to survive and thrive in your garden.
Obviously, there are other factors that account for the success of a plant in your garden, and that includes the nutrient make-up of your soil, soil composition, the amount of water the plants receive through either rainfall or irrigation, wind, heat, hours of daylight versus shade, and how well we care for the plants by clearing away debris, trimming them when needed and thinning them when the plants become too big for their planting spot.
Another factor that the USDA doesn't address but that the American Horticulture Society tracks is the heat index for each area of the country since some plants cannot handle certain high temperatures.
While extreme cold will kill a plant instantly, heat and drought can bring a slow death to a plant first causing foliage to wilt or wither, later causing stunted growth, and finally death.
The heat zone map published by the AHS shows 12 zones where an area experiences what they call "heat days," or number of days that the temperatures exceeded 86 degrees F.
Zone 1 has less than one heat day while Zone 12 experiences 210 or more heat days.
In Steamboat Springs downtown zip code, the heat zones are 2 to 5; in Hayden, it's 3 to 5; in Oak Creek, 1 to 3.
You can learn more about heat zones by logging on to the AH website at www.ahs.org.
Deb Babcock is a Master Gardener with the CSU Extension Routt County. Call 970-879-0825 or email CSUMGProgram@co.routt.co.us with questions.