The U.S. Department of Agriculture recently issued a new Plant Hardiness Zone Map that has shifted many areas in our region a half a zone higher. This is the map that most gardeners use to determine if a plant is hardy enough to live in their garden. The last update of the map was in 2012.
The biggest changes for Eastern Washington and North Idaho came at the higher elevations of the mountains along the Canadian border. Here the hardiness zones went from Zone 5 to Zone 6. Southwest of Spokane into Lincoln County and the higher ground around Cheney, the zones seemed to have shifted a half a zone from Zone 6a to 6b. Around Spokane and Coeur d’ Alene there doesn’t seem to be much change to the Zone 6 we were put in on the 2012 map.
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Does this mean we can now grow tropical plants in northeast Washington? Not necessarily. The hardiness map in all its iterations over the decades has always been a guideline, not a hard and fast rule. The zones are established by using the lowest average winter temperature and no other data like topography, snow cover, wind or average summer high temperatures. In this region, our mountainous topography makes the map harder to use. Cool air is heavy and when it forms at the tops of mountains and in areas like the Palouse and Columbia Plateau, it will flow down the valleys and canyons and create cold pockets where it pools at the bottom of valleys. Where I live in the Painted Hills, we get cold air drainage off the Palouse that puts my garden two weeks behind areas out on the Spokane Valley floor. Some of these pockets are small and cover only a few acres, others are large and cover whole valleys.
Other factors can influence the zone you are in but are not reflected in the map’s designation. If your garden is on a south-facing slope, it will warm up quicker in the spring and stay warmer in the winter. Snow cover insulates plants from the cold, so they aren’t as prone to take the brunt of winter. Many gardeners in the region garden around large basalt outcroppings that tend to hold enough of the sun’s heat to warm the area immediately around them. Windy areas can be affected by wind chill. None of this is measured by the hardiness map.
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How will we see some of these changes beyond being able to grow new plants? We might see longer growing seasons over time, weeds germinating earlier and an increase in the amount and type of insects.
Are these changes caused by global climate change? Not necessarily, say the researchers who built the new map. Much of the change is attributed to better technology and the collection of more data points. The 2023 map is based on almost twice the number of weather stations as the 2012 map; 13,412 weather stations in 2023 compared to 7,983 for the 2012 map.
Source: https://t-tees.com
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