Like many gardeners, I enjoy browsing through the latest garden seed catalogs. It seems every year they come out with even more bizarre and exotic looking plants and flowers, and I can’t help but fantasize about creating the next Hanging Gardens of Babylon. But my dreams always come crashing down when I notice the hardiness zone of many of these plants. In my high mountain desert climate, my plant selection is limited by our severe winters. Based on the average annual minimum temperatures of a particular region, plant hardiness maps have been one of the most basic guides used by gardeners, farmers and anyone else involved in plant agriculture. That’s why when the USDA released the latest plant hardiness zone map last year, it was big news. 

The Plant Hardiness Zone Map

The latest plant hardiness zone map was released in January 2012. The old map, and the one I grew up studying, was over 20 years old. In order to reflect the most current climate conditions, hardiness zones for the new map are based on data gathered from the past 30 years. The new updated map reflects the climate changes that have taken place within these last few decades. Those familiar with the old map will notice how the new map indicates a northward warming trend throughout much of the country. I was surprised to find my zone, formally zone 4b, is now a 5b pushing a 6a. The new map is also more detailed than the older version, taking into account elevations and bodies of water which influence climate. 

Average Low Temperatures, Not Lowest Ever Temperatures

But just because your zone has changed doesn’t mean you need to start altering your garden. If you’ve been growing something successfully for the last 20 years, it will most likely continue to survive just fine. Remember, your plant hardiness zone is a measurement of your average coldest temperatures. If anything, you will probably be able to grow a wider range of plants that were formerly off limits. However, be aware that these are average low temperatures, not the lowest ever recorded temperatures. Experimenting with plants on the edge of their hardiness zone is still risky as one below average winter, or even a few below average cold days, could damage or kill them. 

Microclimates

Homeowners and gardeners should also take into consideration their microclimates. Microclimates are small, localized climate variations that heat or cool a particular area. For example, I know of a saucer magnolia tree thriving in a low zone 4. This particular variety of magnolia prefers a zone 5 and up, so what’s it doing defying its hardiness zone? This magnolia is surrounded on two sides by large brick walls. These walls protect it from wind and raise the temperature enough to bump the area around the tree to a zone 5. Other microclimates could be low valleys where cold air gets trapped, or even parking lots that produce heat for nearby plants.  

Other Factors To Consider

While hardiness is a major factor in determining the success or failure of a particular plant, there are other important elements to consider. Light, humidity, average high temperatures, soil moisture and soil types all play into the equation. Even our own gardening practices and maintenance methods can influence a plant’s survivability. The USDA hardiness zone map is a great resource for general climate information, but it’s impossible to predict exactly how a plant will react in a particular area. Use the map, be aware of the risks, but at the end of the day get out there in your garden and find out for yourself.