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Why knowing your USDA hardiness zone is crucial for keeping plants alive

Also called a "gardening zone" or "planting zone," this detailed map will help you pick the plants that can thrive in your climate.

- 03:38
Chris Monroe/CNET

At a glance, gardening seems simple. Dig a hole, plant a seed and water it. Still, a little bit of planning goes a long way, and you should consider the size of your garden, the richness of your soil and the hardiness zone of the region where you live.

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What are hardiness zones?

A hardiness zone is a geographical zone where certain plants grow best in that specific climate. The USDA Hardiness Zone Map divides the United States and Canada into 13 zones, with Zone 1 being the most northern and Zone 13 the most southern. Each zone is, on average, 10 degrees warmer or colder in the winter than the zone next to it. 

Most zones have very different weather conditions from the others. For instance zones 4a and 4b in the US will almost always get snow every year, while zone 8a and 8b will likely never see snow.

The hardiness of a plant is largely measured by how well it can withstand cold winter temperatures, so something that can survive the minimum temperature in Zone 5 would find Zone 4 to be too cold.


The USDA 2012 Plant Hardiness Zone Map


Many plants, fruits and vegetables sold at your local nursery or home improvement store include a hardiness zone suggestion on their labels that is based on this map. The 2012 USDA Plant Hardiness Zone Map is the most current version, developed by USDA's Agricultural Research Service (ARS) and Oregon State University's (OSU) PRISM Climate Group.

There are maps available for the UK and Australia, but they aren't as current as the ones available for the US. Still, they can be a handy resource for deciding what to plant in your garden.

Why do I need it?

You can adjust for a lot of factors in your garden, like shade, water drainage or soil quality. What you can't control is the weather.

That's where the Plant Hardiness Zone Map comes in. It helps expert and amateur gardeners know which plants have the highest chance of success in their climate and avoid planting something that would never survive the winter or a spring frost. If you want a plant, vegetable or tree to survive and grow year after year, it must be able to tolerate conditions in your area.

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The map is also a good way to compare your climate with the ideal climate for a particular plant you're interested in growing. It doesn't always mean you can't grow that plant in your area. It just means you should take extra care to address its sensitivities. For example, if a vegetable grows better in a warmer climate, you should cover it any time your area has a frost warning. 

Inconsistencies and exceptions

The USDA map is a good guideline, but it isn't a strict rule to be followed. The National Gardening Association notes that while the map does a good job addressing climates of the eastern half of North America, it has a few shortcomings, especially in the west, where climates are much more varied.

For example, Seattle and Tuscon are are both in Zone 8, though there's a large difference between the coastal, rain-heavy climate of Seattle and drier, inland Tuscon.

Zones are a helpful guide, but you should also take into consideration the direction and amount of sunlight your garden receives during the day. Your garden's orientation to the sun and shading plays a huge role in the life cycle of your plants.

Hardiness zones can vary between towns and even within the same city.

Screenshot by Sarah Mitroff / CNET

If you'd like to view the zone for your area, you can use the USDA's interactive map to find the plant hardiness zone in your exact location. And yes, hardiness zones can differ across town.

My neighborhood sits in Zone 6b, but the part of town just a little farther west is categorized as Zone 7a. While you might not notice big differences in which plants can survive in two zones so similar, it's still important to know where your garden falls on the map. For my area, vegetables like radishes, lettuce, peas and tomatoes are all likely to thrive when cared for correctly.  

Armed with the knowledge of your home's hardiness zone, you can confidently sow, grow and harvest the plants most suited for your part of the country this season.

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