What Zone Are You In? It's Important to Know

Snowy forest
Heavy snow in 2023 and prolonged cold temperatures, a little unusual for our area

What zone are you in? Plant zones let you know if a plant will live where you live.

It all has to do with plant hardiness. Hardiness doesn’t mean how tough a plant is or how easy it is to grow. No, it’s something very different.

It’s all about how cold it gets where you are.

What the average low temperatures your area gets in the winter determines your zone. The US plant zones with the coldest temperatures have the lowest numbers, and the warmest zones have the highest numbers, ranging from 1 to 13. The zones represent 10 degree increments.  They’re further broken down to half zones, zone a and zone b, with a difference of 5 degrees. These are called plant hardiness zones.

How do you find your zone?

Luckily in the US we have the USDA Plant Zone Map*. You can enter your zip code and get your zone.

The map is general so if your area is mountainous and variable, it may be hard to get yours exactly right. But you can go to Map Downloads to zero in on your area better.

The hardiness zone is based on an average of 30 years of low temperatures, not the lowest temperature possible. Weather is always variable and it is becoming even more variable these days with our changing climate.

There are plant zone maps for all over the world. If you live outside the US you can find the one for your area from an agricultural commissioner or a local nursery or gardening group.

What do you do with your zone?

When you’re buying plants, ordering plants online, or seeds for perennials, or visiting a nursery in another area, you can use the zone to know if the plant will survive the winters where you are. It’s usually on the plant label.

There are some tricks you can do to overwinter plants you really want to grow. Like growing them in pots and bringing them indoors to a cool, sunny room. Or draping Christmas lights around them for warmth. But this is extra work.

Black mulberry plant with its label showing US plant zones 5-11
Black mullberry, label shows it grows in US plant zones 5-11zones, a wide range of growing opportunity.
Peony plant label with USDA plant zones for the plant
Peony plant label showing the US plant zones it grows in.

But there's more

There are other factors that influence whether a plant will grow in an area. Microclimates, prevailing winds, health of the plants, and their water stress level will affect whether a plant will survive a winter or thrive where you are.

Microclimates can include things like being near a blacktop or paved area that soaks up heat, or on top of a windy slope, or in a warm sheltered spot, or if you’re in a city or out in the country. So give your zone some wiggle room because it may be a little off for your spot.

Sunset zones for if you live in the western states

The no-longer published (sadly!) Sunset Western Garden Book took zones a major step forward. Since much of the western states are mountainous and/or influenced by the Pacific Ocean or the deep broad valleys, the people at Sunset broke the zones much further into many sub zones to account for the large variables that occur all over the western states.

Sunset’s zones  range from A2, in Alaska, the coldest to H2, in Hawaii, the mildest. Los Angeles ranges from 24 at the coast to 22 inland from the beach. Where I am in northern California in the Sierra foothills is Sunset zone 7.

You can see the old maps on the Sunset website to get an idea of how much terrain can affect plant hardiness.

It’s such a helpful zoning system but if you know your Sunset Zone it most likely won’t do you any good anymore. It’s rare to see Sunset zones on plant labels anymore. But it does go to show how broad the USDA map can be and how much your local weather can vary and influence what can grow.

Zones are changing

Zones are changing

Climate mappers and scientists have been observing warming in various climate zones. When the USDA updated its  plant hardiness map in 2012, nearly half the country was bumped up to a half zone higher than it was in 1990.  They say this was partly due to improved mapping techniques. (Based on Redrawing the Map: How the World’s Climate Zones Are Shifting, Nicola Jones, Yale Environment 360)

Researchers have found plant hardiness zones in the US to be moving north in many areas by up to 13 miles per decade. This movement has many consequences in changing weather and rain patterns, too.

But I think we all can attest to the fact that things are getting warmer. But as the climate changes, so do the vagaries of the weather. We’re experiencing more weather extremes.

In my area we got an unprecedented amount of snow in the northern Sierra mountains. After a historic prolonged drought.

Last spring we had a hard late frost that killed back new spring growth and killed fruit tree flowers, resulting in no fruit harvest.

So when using the plant hardiness zone to guide you, it can be tempting to think you’re in a warmer zone now. But the winter may be mild or it may end up being extremely harsh. So it’s best to err on the side of caution and stay with your zone. For now.

Keep in mind...

Remember that the hardiness of a plant is for perennials, trees, and shrubs. Annuals grow during the warmer growing season. Some tolerate frost, like breadseed poppies, nigella, and Agrostemma, but many do not. And different areas have longer or shorter growing seasons.

Plant label on a potted plant that shows no plant zone, but a hardiness temperature instead
Sometimes a plant will have a label like this one. It shows only the lowest temperatures it can live through. This can be enough information. This was for sale at a local nursery and so I can feel sure that it's pre-qualified that it's going to thrive in my area.

*Credit: The USDA Plant Hardiness Zone Map, 2012, is from the Agricultural Research Service, U.S. Department of Agriculture. 

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