What are USDA Zones?

What are USDA Zones?

You may see that in many of my plant care guides, I talk about USDA Zones. In this post, I will give a brief overview of USDA Zones and what it means for your garden.

What are USDA Zones?

The United States Department of Agriculture (USDA) provides gardeners around North America with a hardiness zone map which can help determine what sorts of plants are most likely to thrive. These zones are standardized categories, broken up by average temperatures. Each zone is 10 degrees Fahrenheit (approximately five degrees Celsius) colder than the zone below it on the map, and that much warmer than the zone above it. 

For example, knowing your zone will help you determine if, based on your average annual minimum winter temperature, a plant can survive the winter.

How are temperatures tracked?

The USDA zones incorporate data from the 1930s until today. Since its relase in the 1960s, these zones have been updated from the original ten to eleven. Zones two through ten have also been divided into more nuanced zones of 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b, 10a, 10b, and 11. 

Limitations of USDA Zones

While USDA Zones are very useful for taking into account average temperatures, they cannot tell you about other factors that impact a plant's health, such as humidity, elevation, soil quality, rainfall, indigenous species, and extreme weather events. Talking to specialists in your area can usually be a more accurate way to know if a plant will succeed. 

USDA Zones in the US

 

USDA Zones in Canada

USDA Zones in France

How is climate change impacting USDA Zones?

As temperatures rise, and droughts and other extreme weather events become more common, specialists are predicting the need to make changes to these maps.

The USDA Office of Sustainability and Climate has published interactive maps which show how climate change is expected to change the landscape of the US thorughout this century. 

Plant Hardiness Zones: Left Side (1980–2009) ↔ Right Side: 2070–2099).