Deb Babcock's gardening column appears Thursdays in Steamboat Today.
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Steamboat Springs The U.S. Department of Agriculture recently updated its map of the country’s planting zones — 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, to Zone 13B, where the average lowest temperature is 70 degrees. These averages have been calculated during a 30-year period.
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 is the average lowest temperature), and in Oak Creek, the zone is rated as 4B (minus 25 degrees average lowest temperature). You can now type in your zip code 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 that although your garden may be rated as best for plants designated as 4A, 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. Similarly, a Zone 4 plant may not survive because of a microclimate in a 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 makeup 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. 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, then causing stunted growth, and finally causing death.
The heat zone map published by the AHS shows 12 zones where an area experiences what they call “heat days,” or the number of days that the temperatures exceed 86 degrees. Zone 1 has less than one heat day, while Zone 12 experiences 210 or more heat days. In downtown Steamboat Springs, the heat zones are 2 to 5; in Hayden, it’s 3 to 5; and in Oak Creek, it’s 1 to 3. You can learn more about heat zones by visiting www.ahs.org.
Deb Babcock is a master gardener with the Routt County Extension Service. Call 970-879-0825 with questions.