What Are Plant Hardiness Zones?

What Are Planting Zones?

Obviously, not every perennial, shrub, or tree grows and thrives in every climate.  When choosing plants for your garden, it’s important to select varieties that can survive and thrive year-round in your area, especially in regions where extreme winter temperatures are normal.  The plant must tolerate year-round conditions, such as the lowest and highest temperatures and the amount of rainfall.

The two most commonly referenced hardiness zone maps are those produced by the U.S. Department of Agriculture (USDA) and Natural Resources Canada (NRC). Different measures are used to create each country’s map, as explained below.

Note that planting zones are a guide, not absolute, especially if you live in a microclimate. These are tiny “pockets” which most commonly occur in areas with steep elevation changes, a body of water, or urbanization. They may be warmer or cooler than the surrounding zone. Learn more about microclimates.

Find Your USDA Planting Zone 

The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on average annual minimum winter temperatures of each region and divided into thirteen distinct 10ºF zones, which are further divided into sub-zones of 5°F.

Most plants that you buy are marked with a hardiness zone number. The label will identify the zones in the U.S. where the plant can thrive. 

The USDA map is color coded to make it even easier to see where your area falls.  A sample map of the USDA Plant Hardiness Zones is below. Click here to go directly to the USDA website and see a map of your state’s zones.

How to Use Your Planting Zone

Planting zones are most useful to gardeners growing perennial plants, since perennials are meant to live beyond just one growing season. Perennials need to be able to survive winter in your area, so it’s important to know how cold it typically gets in your area and whether a particular plant is hardy enough to survive those temperatures. 

Perennial flowers, shrubs, and trees grow best when planted in the appropriate zone. You’ll find that winter damage occurs most often when plants are out of their range or “comfort zone.” When you choose plants for a garden or landscape, avoid selecting plants that are only marginally hardy for your region; that’s when you’ll see winter damage, poor growth, and a reduction in flowering.

Planting native species is a surefire way to achieve a stable garden. Native plants are which occur naturally where you live! So, naturally, they will thrive in their habitat. See our article on natural landscaping.

For annual plants, like most vegetables and some flowers, it’s far more important to pay attention to things like the length of your growing season and the typical dates of your first and last frosts. (See local frost dates here.) Because annuals are only meant to last the length of one growing season, planting zones don’t necessarily factor into the equation.

NRC Canadian Planting Zones Map

Unlike the USDA map, which is based only on minimum winter temperatures, the planting zones map produced by Natural Resources Canada (NRC) considers a wider range of climatic variables, including maximum temperatures and the length of the frost-free period. Because of this, the zones listed in the Canadian and U.S. maps are not on the same scale, so keep that in mind before following one or the other!

NRC also produces a map that shows plant hardiness zones for Canada based on the USDA extreme minimum temperature approach. Click here to see Canadian planting zone maps.

Since some Canadians will buy plants from the U.S., it may also be helpful to understand how to use USDA zones in Canada. As a rule of thumb, add one zone to the designated USDA zone. For example, USDA zone 4 is roughly comparable to zone 5 in Canada. If you’re on the fence about a plant, always go one zone higher to be on the safe side and avoid disappointment!

Learn More

Another key part of successful gardening is knowing when your frost dates are. Find your local frost dates here.

What are your thoughts on planting zones? Are they accurate? Let us know in the comments below!