Many of our gardening decisions involve knowing the "zone" of our gardens. The USDA has divided North America into plant hardiness zones based on average annual minimum temperatures. The United States, Canada, and Mexico, are divided into 11 zones with each zone number representing a range of 10 degrees F. The averages were determined by recording the lowest temperatures between 1974 and 1986 (1971-1984 in Mexico). This map below from the National Arboretum shows the color-coded zones.
Zones 2 through 10 are further subdivided into light and dark variations of the same color, representing five-degree spans within the 10-degree zone. The lighter color is the colder half and the darker color is the warmer half. For example, Zone 5 represents average minimum temperatures of -10F to -20F. Zone 5A is -15F to -20F; 5B is -10F to -15F.
Plants are identified and labeled by zone to help match them to appropriate landscapes. The concept is to help identify the probable hardiness and survivability of plants based on temperature and geography. If horticulturists know that a plant will not survive below a certain temperature, it's beneficial for gardeners to avoid planting it if they know they live in an area where temperatures regularly drop below that threshold. It's a great guideline for you to use, but it's not foolproof.
Zone 5 runs through the middle of the United States. It includes my area of Colorado, Kansas, Missouri, Iowa, Illinois and over to Pennsylvania. To the west, the Rocky Mountains interrupt the horizontal pattern, but Zone 5 drops into New Mexico and Arizona before swinging back up into Utah, Nevada, Oregon, and Washington. And that's why there's a deficiency in the system.
North America is not the same from one coast to the other. The Eastern United States with relatively flat topography and stable weather patterns differs greatly from the Mountain Southwest and Pacific Coast. You might be surprised to know that Portland, Oregon, Tucson, Arizona, and Shreveport, Louisiana, are all in USDA Zone 8A. Few gardeners would argue that you can grow the same garden in those three areas.
You can find out what zone you fall into at a number of online sites. You can go to the National Arboretum site and look where your city falls on the map. The National Gardening Association offers a zone finder by entering your zip code. The Arbor Day Foundation offers a similar site too.
The USDA Hardiness Zone Map only takes into account the average minimum temperatures of an area. It doesn't look at average high temperatures, average elevation, average rainfall, average days of sun, or any of the other factors critical to gardening success. It's up to you to recognize these differences when selecting plants for your garden.
Also be aware of variations within zones. The color scheme was painted with a fairly broad brush. My Colorado Springs city garden fell squarely within Zone 5B, meaning the average minimum low temperature falls between -10F and -15F. A few years ago I recorded an overnight low of -35F. I lost a number of perennial plants that winter. My new garden northeast of town still falls within Zone 5B, but now I live 1,000 feet higher at 7,500 feet elevation. Local gardeners know that it is more difficult to garden in my new area than in my last. I'm well aware of that too.
If you live on the edge of a zone or at a higher elevation like mine, it's best to default to a colder zone when selecting plants. That's why I prefer to focus on Zone 4 when I can. That takes me and my plants down to an average low of -30F, providing a survival buffer when the temperature occasionally drops to unusual extremes.
Remember that the USDA Plant Hardiness Zones should be used as a guideline in selecting perennial plants, shrubs, and trees. If a plant label lists something like "Zone 4-9", it means the plant is suitable for and should survive in any of those zones. Below the minimum zone and it may not survive the winter, above the zone and it may need colder winter weather to grow and propagate properly (cold stratification; a term I've discussed before). You still need to determine if you have the appropriate soil, sun, and water requirements for the plants.
Understanding what the zones are and how they affect you is an important step in successful gardening. You can always choose plants that don't match your zone as long as you know they may not live past one season. Many of the annuals we plant are perennials when planted in a higher zone. You can also modify your landscape and create microclimates to simulate different zones. I'll talk about that in future blogs.
Being aware of what your zone is officially and what it is actually can make the difference in plant survival. If you don't already know, start tracking the minimum temperatures in your garden to find your own average. That way you avoid the broad brush stroke and garden with more precision.