The United States (and all other countries) has different climates that range from hot and tropical to cold and icy. Even within the same state the terrain can go from sea level and a sandy, sunny beach to tall mountainous, shady, tree-covered land. The terrain change also changes the climate of the area. That directly impacts the plants that can be grown in each climate.
This is why the USDA Hardiness Zones were created – to help people understand what will, and what will not, thrive in their particular climates. These zones are also called Growing Zones, Planting Zones, or Heat Zones and they can vary within a state or province.
How Hardiness Zones Are Determined
The USDA Hardiness Zone system is the system that represents the minimum average winter temperatures across the country. The numbers go from 1-11 and the lower the USDA Zone number, the colder the climate. Factors other than temperatures, like terrain, rainfall, and soil conditions, affect the ability of a plant to survive. However, the zone system is a good starting point for gardeners to select the types of plant to grow in their home landscape.
The USDA Hardiness Zone system divides the country into 11 different zones. Each growing zone is 10°F (-12℃) colder in an average winter than the adjacent zone. This explains why the same state, province, or territory may have several different Hardiness Zones and be able to grow many different types of plants.
Not only are there 11 different zones, but those zones may also have an ‘a’ or ‘b’ after the number, like zone 7a or 7b. The climate with a number followed by the letter ‘a’ will be about 5°F degrees (-15℃) colder than the same number with a letter ‘b’ following. The temperature change is not a full 10°F degrees (-12℃) so the region did not qualify as a different growing zone. The ‘a’ and ‘b’ letter lets the gardener know which area will receive the first plant-killing frost in fall (autumn).
How To Use Hardiness Zones
If you see a hardiness zone in a gardening catalog or plant description, chances are it refers to this USDA map. Use your zip code on a USDA Hardiness Zone map to discover what zone you live in. In this way, you can select plants that will thrive in your climate.
Australia is divided into growing zones 7-12 and the UK is divided into zones 6-9 with each having some variations across the country.
A Worldwide Hardiness Zone map can be found online to help determine the growing zone of your exact location. However, you can also check your USDA Zone here.
Why Use Hardiness Zones?
Plants and seeds bought online or from a local nursery will contain a label with growing information. It will include which Hardiness Zone the plant will grow best in. Knowing which Zone you live in will help you decide which plants will give you the best results.
Knowing your Hardiness Zone often means the difference between life and death of a plant. Many people fall into the trap of bringing home plants or seeds after enjoying a vacation (holiday) far away from their homes. They see lovely plants in a far away location and want to try growing the plant at home as a reminder of their vacation. That usually does not work.
The seeds fail to germinate or the plant dies because the climate is different from the native home of the plant. The vacation (holiday) destination does not even have to be a long distance away to have a significant change in climate and cause a plant to die.
The USDA Hardiness Zone map does a good job of helping gardeners to understand the best plants to grow in their climates, but it’s not perfect. The delineation of the Zones should be used as a starting point. However, other specifics should be taken into consideration when deciding what and what not to plant in your landscape.
In parts of the country, the USDA Hardiness Zone map doesn’t account for the beneficial effect of snow for perennial plants, the freeze-thaw cycles in certain areas, soil drainage during cold periods, and arid or humid air.
Factors besides winter lows, like elevation and precipitation, determine growing climates across the world. And even in regions that have the same weather patterns, temperatures, terrain, etc, there are microclimates to be considered.
A microclimate is a small section of land that has a different climate than the area surrounding it. There are typically several different microclimates within an acre of land. It can range from low-lying, moist, and shady to sunny with hot, dry soil and poor water retention.
The USDA Hardiness Zone is the starting point for planting. Yet, understanding the exact climate within your landscape is also a key factor in growing the right plants. This can help you understand why your neighbor has a gorgeous blue-flowering hydrangea and you have a stunted hydrangea that only produces a few small pink blooms. With everything else being equal, the climate between your property and the neighbors’ is slightly different. The Zone map does not account for that difference.
For articles organized by season check out Gardening by season. You might be interested in reading them for your garden. You can also access these pages from the top menu.