Changes in Plant Hardiness Zones Caused by Climate Change, Says OSU Extension

You visit the nursery, choose a primo plant, go home, put it in the ground and wait for it to grow into a beautiful tree, shrub or perennial. Then spring comes and he’s dead or floundering.

What happened?

In some cases, cold temperatures are to blame. It’s all about the right plant in the right place, according to Oregon State University Extension horticulturist Nicole Sanchez. Plants survive down to a certain low temperature, a characteristic that they evolved through evolution in their native habitat. You need to know the hardiness zones to be sure your plants will avoid cold-related death. Plants purchased from nurseries must have the hardiness zone on the label.

Zones are determined by the U.S. Department of Agriculture, in conjunction with OSU PRISM Climate Group, and based on a 30-year period of low mean minimum temperatures. the latest hardiness map was released in 2012 and is based on data collected during the years 1976-2005. Zones are ranked from 1 (-60 degrees) to 13 (70 degrees), although extremes are rare. To find your hardiness zone, enter your postal code.

Oregon’s hardiness zones range from 6a (-20 to -15) to 8b (15 to 20), with pockets of 5b (-15 to -10) in eastern Oregon. The map does not reflect the coldest it has ever been or will ever be at a specific location, but simply the lowest average winter temperature for the location over a specified time period.

But with climate change, areas are shifting, Sanchez said. For example, areas in the 2012 map have changed to many regions 5 degrees – or half an area warmer – than the previous map. Already, there is talk of the Portland area going from zone 8b to 9a, which has a minimum of 20-25 degrees.

“We won’t know the details of how the zones are changing until we have another 30 years of data, but that doesn’t mean we’ll wait until 2035,” Sanchez said. “They can use an earlier period. Then we can watch and see if there are any long-term changes that would be reflected in new areas for particular areas. So we wait and see.

“I always hear people say, ‘This is the hottest spring ever’ or ‘I’ve never seen such a hot summer.’ Usually they are wrong. Weather is a short term phenomenon and climate is a long term pattern.

The map is a guide, not an absolute document. There may be winters when lows drop below your zone’s minimum, she said. Nature can throw a curve ball; nothing is guaranteed. For now, gardeners should base their plant choices on the current map.

What happens in your garden may very well be different from your neighbor’s due to what are called microclimates. A microclimate can have an effect similar to moving to a colder or warmer area. They can be influenced by structures such as the orientation of your house in relation to the sun. The south will be warmer than the north, the west warmer than the east. Other microclimates can be related to the slope: pools of cold air at the bottom and the high points are cold, too, because of the wind and the exposure. Thus, the middle of a slope is the sweet spot for borderline hardy plants. Microclimates can be created with rock faces and mulches, buildings and fences, and windbreaks created with plants.

“I’m in zone 6,” Sanchez said. “If I wanted to plant dahlias, which are zone 7 plants, I would plant them on the south side with a gravel mulch that would store the heat.”

Zones are essential when choosing a plant. To be successful, plants must adapt to your zone, which makes selection easier. If you have five plants that you like and there are only two in your area, that narrows down your selection. You can’t push the envelope; just be prepared to lose plants periodically.

“Some gardeners love to grow something that wouldn’t grow in their zone,” Sanchez said. “Sometimes a plant will survive for several years because the weather doesn’t go down to your minimum zone. But, then a very cold winter arrives and zaps them. Be careful with woody plants like shrubs and trees. These are investments. You can pay $350 for a tree or $15 for a dish of impatiens.