Plant Hardiness Ratings

Have you ever been confused about how to choose perennials? Perhaps you’ve seen a photo of a plant that you simply can’t resist -- but how do you know if it will thrive where you live?

To help gardeners choose plants, various systems of rating hardiness have been developed. A plant is considered hardy in a region if it can grow and thrive there without requiring special protective measures such as an insulating straw mulch.

The most common rating system is the USDA Plant Hardiness Zone Map, and this is the map they’re referring to when a plant description says "Hardy to Zone 5". The USDA map breaks the country into 20 regions based on the average minimum winter temperature. If a plant’s hardiness is rated to USDA Zone 5, then the plant will usually survive in regions where winter temperatures get no colder than minus 20F. A plant rated to USDA Zone 9, on the other hand, will overwinter only in regions where the temperatures get no colder than 20^o  above zero.

Though this map is helpful in determining a plant’s winter hardiness, it does have its limitations, because the minimum winter temperature is not the only factor that determines whether or not a plant will survive.

To illustrate this, let’s look at two places with two very different climates: Portland, Oregon and Tucson, Arizona. Both of these cities are rated in USDA Zone 8b, with an average minimum winter temperature of 15^o F above zero. Yet if you’ve ever visited these cities, you know that they have very different climates! Portland’s weather is greatly influenced by the Pacific Ocean. Mild winters and cool summers, along with plenty of moisture, make it an ideal climate for cool weather, moisture-loving plants like heather, bleeding heart, and primroses. Tucson’s dry heat, on the other hand, favors desert plants, like aloe, evening primrose, and gaillardia (blanket flower).

To help gardeners better determine what plants are suitable for their region, Sunset Publishing developed their own series of hardiness maps. These maps divide the U.S. and southern Canada into 45 climate zones, based on various factors including rainfall, summer and winter temperatures, length of growing season, humidity, elevation, and a number of others. These maps are especially useful to gardeners west of the Mississippi, where climate varies greatly across mountain ranges, deserts, and coastal regions.

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