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Explaining The USDA Hardiness Zone Map

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Emma Gallagher is a Brit living in North Carolina. She grows organic gardens and... Read More

If you are an avid gardener or just a beginner, you have probably noticed websites, seed packets, and plant tags referring to USDA hardiness zones.

It might not be all that obvious to a new gardener as to what this is referring to, but this bit of information is actually a great deal of help. Paying attention to the hardiness zone recommendations is the difference between sheer disappointment when your plants fail to pop the next spring and elation at seeing your perennials sprouting spring after spring.

The USDA Hardiness Zone Map Explained

The USDA hardiness zone map is a map of the USA that has been divided into 13 zones based on the average lowest winter temperature for that area. There is about a 10-degree difference between each zone. Each zone is divided again into zone a and b to account for significant temperature differences within zones.

When viewing a USDA hardiness zone map, zone 1 refers to the coldest areas and zone 13 the warmest. For example, zone one 1a has an average minimum temperature of -60 to -55°F, while zone 13 has an average minimum temperature of 65 to 70°F.

The zones roughly relate to north and south, with the northern states being lower-numbered zones and the southern states being higher-numbered zones, but it isn’t as cut and dry as that. Geographical and human-made features such as lakes, mountain ranges, deserts, urban settings, and coastlines can dramatically change the average temperatures of an area within just a few miles of another.

Just because your relatively close neighbor is zone 6 doesn’t mean that you are, too! You can plug your zip code into the USDA hardiness zone website to find out what zone you are in.

Annuals and Perrenials

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First of all, it is important to understand the difference between an annual and a perennial plant. An annual plant completes its life cycle within one season. This plant will start to germinate in the warmth of spring, grow throughout the summer and die back at the beginning of fall.

However, many plants labeled annual can, in fact, be perennials in year-round warm climates. More northern gardeners will grow tropical perennials as annuals. An example of this would be the growing of pineapple sage or lemon verbena as annuals in a cold winter climate when they are actually warm-climate perennials.

To be clear, perennials are either evergreen plants that thrive year-round or plants that die back in the fall and reemerge in the spring.

Your Zone Tells You What You Can Grow

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The USDA growing zone helps you to know what plants will survive your coldest winter temperatures. In short, it tells what works as a perennial in your area. Something that can survive the coldest winter temperatures in zone 7b might have a really hard time or no luck at all growing in zone 6a, for example.

When you look at a plant tag, it will likely give you a range of zones that this plant is hardy to. It might be that a particular plant is hardy to zones 3-7. If you plant said plant in zones colder than this, the roots will not likely survive the winter. Likewise, if you plant it in a zone warmer than this, the roots might not get the appropriate cold spell that they require during winter dormancy.

You might find a plant that says it is hardy to zones 10-11. These are the warmer zones. If you live in a colder zone, you can still enjoy these plants as annuals.

The USDA Zone Map Is Not the Be-All and End-All

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Though the USDA hardiness zone map is an incredibly useful tool for knowing what may or may not do well in your area, it is still a very general guide. It does not account for humidity in a certain area, localized geographical features, or micro-climates within a specific area. Even a garden shed casting a little shade can change how well a plant grows.

You might be able to get away with playing around with your allocated hardiness zone a little, but this will be something you have to learn and research yourself.

Talking to local, experienced gardeners would be a great way to learn about your very specific area. Local nurseries, too, will have a wealth of knowledge about what you can and can’t get away with in your area. It would be a terrible business for a nursery to be selling plants that had no hope of surviving locally!

If you find a plant that is on the cusp of being hardy to your zone, you could play around with cutting it back in the fall and very heavily mulching around the roots to protect them from the cold. An example of this would be growing pineapple sage as a perennial in zone 7. This is not a foolproof solution, but might be worth a shot if it saves you from having to buy a new plant the next spring.

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