(ALL RIGHTS) November 2015. The Brightside Organization, The Nature Conservancy, UPS and Brown-Forman partnered to plant 150 trees along West Broadway from 20th Street to the end at Shawnee Park in Louisville, Kentucky. Photo credit: © The Nature Conservancy (Devan King)

Understand Your Planting “Tools”: Plant Hardiness Zones vs. Ecoregions

Photo © The Nature Conservancy (Devan King)

Knowing what plants to put in the ground that–barring predation or other unforeseen setbacks–will grow and thrive, can be tricky business. The goal is deceptively straightforward; but, as we are sure you’ve discovered, challenges often emerge when trying to figure out what plants will actually be successful in your region.

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Photo © CollegeDegrees360

Plant hardiness zones and ecoregions are both tools that gardeners and farmers find useful in answering this–What can I plant that will thrive?–question. Each parses out landscape, but in the service of different objectives. At Habitat Network we believe both plant hardiness zones and ecoregions have their merits and should be used in tandem when adding plants to your gardens. If these tools are confusing to you, hopefully this article will help clarify how to maximize their usefulness.

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Photo © USDA

The Plant Hardiness Zone Map was developed in a joint effort between Harvard’s Arnold Arboretum and the United States Department of Agriculture (USDA) back in the 1960s. Thirty years of temperature data were examined and analyzed to create approximate temperature bands across the United States. Since the original map, several more versions have been updated to reflect more accurate ranges, the most recent map (depicted above) was created in 2012.

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Photo © Brad Smith

The current Plant Hardiness Zone Map shows the average annual minimum temperature in a region in the last 30 years. This does not reflect the highest or lowest temperatures in an area, but rather an average of all temperatures over a year. According to the USDA, the most recent version of the map does take into consideration temperature variation based on elevation, topography, and coastal effects.open_in_new

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Photo © USDA

There are 13 hardiness zones, separated by a 10-degree difference and within each zone is an “a” and “b” separated by a five-degree difference–for example, zone 3a(-40F to -35F) and zone 3b (-35F to -30F). The lower the zone number the colder the region, so zone 1a (-60F to -55F) is the coldest region and zone 13b (65F to 70F) is the warmest.

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Photo © USDA

Plants are often temperature sensitive. Knowing whether a plant can tolerate different annual average temperatures is helpful when planning your gardens. For example, if you live in New York State there are seven different planting zones (ranging from 2b in the highest parts of the Adirondack Mountains in pink, to 7b on Long Island in green-yellow). Thus, if you wanted to plants “native” to New York, simply knowing whether they are native to the state, or not, does not ensure survival of the plant. A native plant that can thrive in New York City may not survive in the Adirondack Mountains, and vice versa.

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Photo © Steve Lyon

Plants, like this recently snow-buried manzanita (Arctostaphylos manzanita), depend on more variables for survival than just temperature. Complex ecological interactions, both biotic and abiotic, also influence the success rate of plants. Here’s where a different way of parsing the landscape, one that takes into account both temperature and other climatic and biotic factors, becomes very helpful.

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Photo © USDA Forest Service

In addition to plant hardiness zones, there are something called ecoregions that break up the map of the U.S. (and other places) a little differently. Originally proposed in 1976, ecoregion maps are produced by the USDA Forest Service. Ecoregions are areas of land and water grouped together by similar temperature, precipitation, soil composition, geography, ecosystem relationships, and biodiversity characteristics. In the contiguous United States, there are 34 different ecoregion provinces. Hawaii and Puerto Rico make 36 and adding 13 for Alaska gives us a grand total of 49 ecoregions (each of these is broken into even smaller regions, but for our purposes, we present them at the “province” gradation). Each province has a descriptive name, like Great Plains-Palouse Dry Steppe Province, or Outer Coastal Plain Mixed Forest Province, hinting at the dominant landscape features found there.

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Photo © plant4wildlife

Each of these ecoregions supports certain plant and animal communities with their own, sometimes unique, ecological relationships. Some species of plants and animals can be found across several ecoregions while others are found only in very specific small areas. These relationships are crucial to consider when trying to create thriving gardens. Plants are the foundation of most food chains, providing food, shelter, and areas for reproduction. Ecoregions, then, offer clues about what we should plant in order to provide biodiverse, native habitat to support the specific, important ecological relationships of our region.

Florida to California

Photo © Julie Jordan Scott, Elias Schewel

Here’s an example of how you might use these tools together. Maybe you live in coastal California in planting zone 10a and notice that parts of Florida are also in planting zone 10a. Could you plant things native to Florida in your California yard? Yes, you could, and they may even survive, but not necessarily thrive, since they certainly won’t be recognized by the native insects, microbes, and animals that also call coastal California home.

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Photo © USDA Planting Zones vs Ecoregion Provinces

Looking at the ecoregion map reveals southern Florida and coastal California to be in very different ecoregions (Everglades Province and California Coastal Chaparral Forest and Shrub Province, respectively) even though they share a planting zone. Using an ecoregion guide you may learn there is a particular pollinator that lives in Florida but does not live in California that this plant needs for successful pollination. So, looking at planting zones alone, you might be led to believe that the Florida plant would do just fine in Coastal California, but the reality for that plant may be that it lives but is unable to reproduce.

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Photo © Pollinator Partnership

Pollinator Partnership has made this work relatively easy for us. Using the ecoregion maps they have created detailed guides of plants and their respective pollinators for each specific ecoregion. These guides can be found in our Local Resources Tool where you enter your zip code and receive information back on your ecoregion, planting zone, and other valuable resources to inform your planting decisions.

Pollinators on Swamp Millkweed

Photo © plant4wildlife

With mounting environmental and habitat pressures on pollinators, birds, and other wildlife, focusing on what we should plant to restore or preserve native habitat needs to be a priority (pictured above are pollinators feeding on Swamp Milkweed (Asclepias incarnata). This doesn’t mean you can’t have that native Florida plant in your California garden, it just means along with that plant, adding native plants to your planting palette ensures your garden is contributing to the ecological underpinnings of your specific plant-animal community.

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The map on the left shows the change in Plant Hardiness Zones calculated from those based on the 1971-2000 climate to those based on the 1981-2010 climate. Even greater changes are projected over the next 30 years (right). (Figure source: NOAA

Photo © NOAA

When we factor in climate change, it becomes more clear how planting zones and ecoregions should be used together. There is strong evidenceopen_in_new in the scientific community that temperature and habitat availability BOTH influence where animals can live. Planting zones, only taking into account temperature, are likely to shift more quickly than ecoregions. Ecoregions, for the most part, formed over thousands of years of evolution and geological change. They can and will shift in response to a changing climate, but not as quickly as planting zones.

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Photo © Michele Truex

We hope you use these tools to plant for the future. If you live in Wisconsin on the edge of zone four and five, you may want to take into account projected changes in climate in your planting decisions. If zone five and six move further north, zone three and four tolerant plants may not thrive as well in ten years.

As gardeners, we know all too well the only thing we can count on is change. Use the plant hardiness zones and ecoregion tools, together, to anticipate, plan, and plant for the landscapes we have now and may have in ten or more years from now.

From a Habitat Network Users: Also, consider referencing your region using the American Horticultural Society: Plant Heat Zone Map. The heat is often responsible when plants fail to thrive.

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