Hardiness Zones for Plants

You’re meandering through your local lawn and garden center, in quite a daze because you’re uncertain which plants will thrive in your landscape. This becomes especially tricky when you begin looking at exotic or tropical plants. Sure, the answer may simply be to plant only native plants, but many homeowners looking for a varied and interesting landscape also want to include a few ornamental plants, trees or shrubs into their overall landscape design.

How can I choose the right plants?

One of the easiest ways to determine if a new plant will thrive in your particular area of the country is to check its hardiness zone rating. This is basically a rating system, first developed in the 1960s by the United States Department of Agriculture (USDA) to help large-scale agricultural producers and backyard gardeners alike determine which plants will thrive best in any given area of the country.

This USDA Official Hardiness Zone Map was revised in 1990 and used as a guide for selecting annual plant varieties. The American Horticultural Society also produced a new map which showed an even finer level of detail than the original USDA map.

The degree to which a plant can withstand weather and temperature changes varies widely between plants. The most common measure of a plant is its ability to withstand harsh, winter climates. Although plants can adapt somewhat to changing weather conditions (many horticulturalists work to “harden” certain plant varieties to better accept changes in climate), their ability to withstand cold temperatures is a vital piece of information every gardener should be armed with when choosing flowers and plants.

Should I follow the hardiness zone map?

The hardiness zone map should not be taken verbatim, though. Although this map, which clearly separates the areas of the country into 11 different hardiness zones, is ideal for gauging a plant’s overall hardiness, it should not be used as an end-all to choosing plants. What is of most interest to many horticulturalists is that although the hardiness zone map addresses cold temperatures, it does not address hot temperatures, which can often take as much of a toll on plants as cold weather.

In other words, even though the plant may survive cold weather, it may suffer in the height of summer.

What other factors should I consider?

Other factors within any particular climate zone which affect the health and hardiness of a plant include topography (different climates and frost dates within the same zone) - lakes, rivers, canyons and mountains. Altered air flows can raise or lower air temperatures in any given climate, thereby changing the zone rating. Other factors to consider include soil, exposure, altitude, rainfall, humidity, sunlight levels and wind chills factors.

The bottom line: Use the hardiness zone rating of your plant only as a general guideline.  In addition, take into account the many different weather conditions of your particular area and research the plant – especially if it’s tropical or exotic – before purchasing it.  Don’t forget to remember the many different factors that make up your particular zone – wind, rainfall amounts, humidity and topography – when mapping out your landscape.