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Plant Zones and Microclimates

© Keith Muraoka

If you're an avid home gardener, you've probably heard of plant zones where specific plants are supposed to be able to grow. But I would bet a year's supply of Miracle-Gro that the vast majority of home gardeners don't know what plant zone they live in or the fact that there are many different types of plant zones.

For instance, the U.S. Department of Agriculture's plant hardiness zone map has been around for decades. It breaks the U.S. into 11 different climatic zones, providing the average minimum temperatures in different regions of the country. By knowing what zone you are in, you'll know if a particular plant can take the lowest average temperatures of that specific zone.

The area in which I live is a zone 9, according to the USDA plant hardiness zone map. This means the lowest average temperatures in the area is between 20 and 30 degrees. Impatiens, the most popular summer bedding plant, is listed as being able to grow in a zone 11. This means impatiens should not be able to overwinter in our area and is listed as a summer annual rather than a perennial.

Of course, anyone who has lived where I live for any amount of time already knows that impatiens will sometimes overwinter here. It all depends on how harsh a winter we have. I have impatiens in my garden right now that are two or three-years-old.

You may be asking yourself, "Then what good is the USDA plant hardiness zone?" Well, it provides an overall estimate so you'll know going in whether a particular plant can be expected to grow or come back in an area. If you see a plant that you really want, you can always gamble that the experts haven't really tested the limits of its cold hardiness. That's one way to help cure what fellow suite101 columnist Carol Wallace calls, "zone envy."

I've gotten around any zone envy by simply planting specimens that aren't so cold hardy, such as tropical bougainvillea or hibiscus, against the south side of my house. Being on the southern side ensures it gets as must heat as possible. The protection of the house also provides a little extra warmth during cold winter nights.

Further complicating matters are microclimates. I live in an area where there are many thermal belts, including hilltops, swales and canyons. In such locations, your climate may be slightly milder or slightly more severe than that of a neighbor's a half-mile away. A house, a row of trees, a fence or even where you park your car can affect microclimates.

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