When you’re looking for what varieties of perennial plants will thrive in your local area, one of the best and most widely used indicators is the USDA’s Plant Hardiness Zones. But what does this map mean and how can we use it to help choose our plants?
The zones on this map determine the average annual extreme cold temperatures of an area. The zones start at Zone 1 which has an extreme low of -55°F and go up 10°F with each number. Each zone is divides into subzones of 5°F increments, labeled ‘a’ and ‘b’. Subzone ‘a’ representing the colder half, and subzone ‘b’ representing the warmer half. For example Zone 7a includes temperatures 0°F-5°F, and Zone 7b includes temperatures 5°F-10°F
How to use this map:
The zones represent the average coldest temperatures throughout the last 30 years. The most current map represents the averages from 1991-2020. It is important to note that these hardiness zones are based on the average coldest temperatures. Therefore, the temperatures in one’s area may drop lower than the zone indicates. It is also important to be familiar with the space you are planting in, and any microclimates that exist there. Microclimates are small areas that may experience temperatures different from the larger surrounding area due to elevation, sun exposure, wind-breaks etc.
As stated by the USDA, here are other factors to consider when planning your landscape:
Light: To thrive, plants need to be planted where they will receive the proper amount of light. For example, plants that require partial shade that are at the limits of hardiness in your area might be injured by too much sun during the winter because it might cause rapid changes in the plant’s internal temperature.
Soil moisture: Plants have different requirements for soil moisture, and this might vary seasonally. Plants hardy in your zone can be still injured if soil moisture is too dry in late autumn. Entering dormancy while suffering dehydration stress creates likelihood for failure.
Temperature: Plants grow best within a range of optimal temperatures, both cold and hot. That range may be wide for some varieties and species but narrow for others.
Duration of exposure: Many plants that can survive a short period of exposure to cold. They may not be able to tolerate extended periods of cold weather.
Humidity: High relative humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for evergreens.
How do I find my zone?
Now that you understand what the zones mean, how do you find yours?
- Go to the USDA’s Zone Map website: https://planthardiness.ars.usda.gov/
- Enter your zip code into the search bar at the top of the map. Press enter. Select your city that pops up under the search bar.
- Explore your area using the Zoom buttons on the left side of the map. Use you mouse to move the map.
How to find a plant’s zone on One Green World:
- Find the plant you’re looking for on our website. Use the search bar at the top left, OR select the “Shop” menu and select a category to search through.
- Scroll down to the bottom of each plant page, where you will find in bold “USDA Zone”. You see a range of zones or a single zone listed.
