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What a USDA hardiness zone is and why it's crucial for keeping your 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.

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Aloysius Low/CNET

At a glance, gardening might seem 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.

What are hardiness zones?

Hardiness zones are geographical areas divided up by climate that can be used to determine where different plants will grow best.

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The USDA 2012 Plant Hardiness Zone Map.

USDA

The USDA Plant Hardiness Zone Map divides the United States and Canada into 13 zones, based on the average annual minimum winter temperature. Each zone is, on average, 10 degrees warmer or colder in the winter than the zone next to it, with Zone 1 being the coldest and Zone 13 the hottest. 

This broadly corresponds with a North to South pattern, but that's not always the case. Things like mountains, lakes, desert topography and urban heat island effects can result in different zones in the same latitude and within just miles of each other.

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.

Many plants, fruits and vegetables sold at your local nursery or home improvement store include a hardiness zone suggestion on the label that is based on this map. The 2012 USDA Plant Hardiness Zone Map is the most current version, developed by the USDA's Agricultural Research Service and Oregon State University's PRISM Climate Group. There are similar maps available for the UK and Australia.  

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. 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.

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 so they can avoid planting something that would never survive the winter or a spring frost.

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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 necessarily 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 anytime your area has a frost warning. 

Inconsistencies and exceptions

The USDA map can be a good guideline, but it isn't a set of strict rules to be followed. The National Gardening Association notes that while the map does a good job of addressing climates of the eastern half of North America, it has a few shortcomings, especially in the west, where climates are much more varied thanks to mountains and deserts. For example, though Seattle and Tuscon are both in Zone 8 for temperature, there's a large difference between the coastal, rain-heavy climate of Seattle and drier, inland Tuscon. 

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Hardiness zones can vary between towns and even within the same city. Denver is in a different zone than the suburban areas immediately surrounding it. 

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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, down to the ZIP code. Remember, hardiness zones can differ across town. A heat island effect in downtown areas might warrant a warmer zone than a less-populated suburban area. Elevation change can affect average temperatures, too. 

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. In my area, vegetables like radishes, lettuce, peas and tomatoes are all likely to thrive when cared for correctly.  

You should also consider the direction and amount of sunlight your garden receives during the day. Your garden's orientation to the sun and its shade play a huge role in the life cycle of your plants. 

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.