Hardiness Zones vs Ecoregions (and Why it Matters)

"What should I plant in my growing zone?" 

A question we get more than any other is "what plant should I use in X hardiness zone?"

The short answer is that I rarely think about hardiness zones at all, and if your plants are native to you, you really don't have to worry about it either! 

USDA Plant Hardiness Zones are a concept rooted in agriculture and more "old school" gardening. In order to plant annual food crops, or non-native ornamentals that aren't used to growing in your particular region. Therefore, the hardiness zone tells you when (and where) to plant to ensure a plant's survival. 

Native plants, on the other hand, are adapted to grow right where you are. The most important thing to identify is your ecoregion, and ensure you're planting native to that ecoregion!  

What's the difference between hardiness zones and ecoregions, anyway? And why on earth do they even matter? Fear not, we'll explain both and show you how having a solid knowledge of your ecoregion can make a huge difference in how you approach your landscape and garden design choices!

Hardiness Zones: a Brief History

About a hundred years ago, researchers working at the Arnold Arboretum in Boston attempted to design a map that split up the geography of the United States by average tempuratures, hoping to help gardeners and farmers determine what plants would best survive in their area. The first map was published in 1927, and adapted heavily before being put into use by the United States Department of Agriculture in 1960. 

Since then, the USDA map has been amended and updated multiple times, ultimately becoming the tool that is still widely used today.

The USDA hardiness zone map splits the country up by 10 °F ranges, with a change adapted in 1990 that added five-degree subdivisions to each zone, which is where the "a" and "b" subdivisions came in to common use.

Problems with Hardiness Zones

Issues with hardiness zones became apparent shortly after adoption, with some agricultural and horticultural groups noting that hardiness zones tended to skip over ecological variations that naturally exist within each zone. Other issues became apparent, including the omission of urban heat islands that occur within cities. Hardiness zones also don't take duration of winter conditions, snow coverage, humidity, storms, monsoon conditions, or excessive sun into consideration.

Hardiness zones are also primarily an agricultural tool, heavily used by farmers to determine what crops can be planted in their area and have a good chance of success. Landscaping and home gardening has a lot more variables baked into it than large scale monocropping.

The last big issue with hardiness zones that we would like to discuss is the impact of climate change. Here in Salt Lake City, we have seen a drastic change in our winters in the last decade, resulting in milder temperatures, less snowfall, and fewer freezes. This change hasn't been reflected in the USDA hardiness zone maps, but anyone who has lived in the area for more than a decade will tell you that winters here are nowhere near as severe as they used to be.

And that's not even getting into how extreme our summers are becoming!

The USDA hardiness zone maps have seen little revision over the years to reflect the changes to climates around the country, which can result in folks installing landscaping that was optimal to that zone prior to changes brought about by the climate crisis.

Ecoregions: Another History Lesson!

We really don't have an exact date of when the term "ecoregion" was coined, but the concept has been in use to some degree for at least sixty years. Some terms that had been used in the past include "biome classifications", "biogeographic classifications", and "forest classifications". The actual term "ecoregion" (short for ecological region) was first used in print in 1976, and has been in wide use since then.

Ecoregions are defined first by a predominant biome, then further classified by other defining features of that region. 

For instance, the Northwestern Forested Mountains ecoregion is split into the Boreal Cordillera, which is largely in Canada and Alaska, and the Western Cordillera, which range from the Cascades in Oregon to the Uintas of Utah. We can further define the ecoregion into narrower classifications, like alpine, subalpine, mountainous slopes, rolling plans, and interior grasslands.

With that knowledge we can plan our landscape and garden projects accordingly. Once again, we'll use the Salt Lake City area as an example; all of the Salt Lake Valley is in the Central Basin and Range ecoregion, with the west side being 13d Woodland and Shrub-Covered Low Mountains, the central valley is 13f: Moist Wasatch Front Footslopes, whereas the eastern side of the valley is 19f (Semiarid Foothills). 

Keep in mind that the Salt Lake Valley is less than 30 miles wide, and those ecoregion variations tell us that something that grows well in the typically more temperate area of Salt Lake City might struggle in the more arid growing conditions of suburban Magna, which is only a 20 minute drive away. Go another 20 minutes west of Magna, and you'll find yourself smack dab in the middle of 13a, which is a salt desert that very little vegetation can grow in!

Using our ecoregion as the foundation for our landscaping and gardening choices, we can ensure that we are taking our local climate, soil conditions, and native plant species into consideration. Grama grasses grow natively in our region, for example, so a Blue Grama grass is going to adapt and grow incredibly well here. And since it's a native plant, it's going to use a lot less water and take very little time and energy to maintain!

We can also use our ecoregion knowledge to support native wildlife, which will flourish if their preferred habitat is more readily available. This can help bring more balance to citified ecosystems, which are incredibly inhospitable to local wildlife that struggle to adapt to the concrete and asphalt of urban sprawl.

We will also be able to achieve our landscaping goals with less water; these plants are already adapted to our climate, and since they'll be happily established quicker than a non-native plant to that zone, we'll have a lot less landscape maintenance to do as well!

These reasons are why we prefer to work with eco-regions when designing a project. We want to ensure that your landscape provides you with years of beauty and happiness, is low maintenance, doesn't tax limited local resources, and helps restore balance to our precious native ecosystems!