Wild Ones   Why Hardiness Zones, Native Ranges, Ecoregions?  

Oak branch with fresh snow.
Copyright: Jim Nachel.
By Mariette Nowak

Can hardiness zones be used by natural landscapers interested in selecting native plants? What about using native ranges or ecoregions for selection? Is there a difference? To help clear up the confusion, these various approaches to selecting plants are described and contrasted below.

Hardiness Zones

Many gardening catalogs give hardiness zones for plants, based on the Plant Hardiness Zone Map published by the U.S. Department of Agriculture. This map divides most of North America into 11 climate zones, based on the average winter minimum temperatures. Subtropical Zone 11 has minimum temperatures above 40 degrees F., while Zone l has minimum temperatures of –50 degrees F. Because temperature is the single most important factor in determining whether a plant will survive, these zones help gardeners determine whether plants that are not native to their areas will be able to survive. Obviously, these Hardiness Zones are not useful for native plant gardeners who want to plant truly native plants on their properties.

Native Ranges

Native Ranges are a better criteria for natural landscapers. Most field guides and taxonomy texts give native ranges for plant species, listing the states in which the species naturally occur.

Ecoregions

Selecting plant species from within your geographical region or ecoregion is the best way to select plant species. This is because, although the same species may be found in several states or regions, the plants may have subtle differences in their genetic make-up in different areas of their native ranges. A locally-derived plant will generally do best in its “home” region due to its unique adaptations to that region’s unique environmental conditions. The ecoregions within the U.S. are best delineated by The Nature Conservancy and in Canada by the Conservation Data Centres.

Resource

The Nature Conservancy’s ecoregion map is the most helpful U.S. guide for helping native plant enthusiasts choose plants from within a specific geographic region. The boundaries are based on a combination of climate, topography, geology, and vegetation.

Mariette is a member of the Milwaukee Southwest-Wehr (WI) Chapter, and former national board member, having served as vice-president and editor-in-chief of the Wild Ones Journal.

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