The Importance (& Limitations) of Knowing Your Hardiness Zone

If you are new to the desert southwest or new to gardening in general, one of the first things you should do before planting anything new in your yard is to know your USDA Plant Hardiness Zone.

Growing a plant in the wrong zone can significantly reduce its chances of surviving.

Once you know your zone, you can look for plants that are rated to grow in your zone when you shop.

And if you haven’t checked your zone for a while, you might want to do it again, since the USDA Plant Hardiness Zones were revised in 2023.

USDA Plant Hardiness Zones

The USDA Plant Hardiness Zone is the main zoning system in the US.

The USDA Hardiness Zone map tells you the minimum temperature you can expect in your area based on recent 30-year averages. This will help you choose plants that are likely to survive winters in your area.

Here’s a zone map of the area considered the US Southwest.

The map is not very precise but you can find your exact zone by entering your zip code here. I find it interesting to see how the zones can vary, even from neighborhood to neighborhood.

Limitations of the USDA Zones

While you should know your zone, it only covers one variable — cold. So obviously the USDA Hardiness Zone system has severe limitations.

While it’s important not to grow plants that will die from cold, for most of us in the southwest HEAT is a much bigger concern. The USDA Zone system frustratingly does not address this crucial variable.

Other Zoning Systems

You may be wondering if there are other zoning systems that take more variables into account. There are, but they have their limitations, too.

One you may have heard of is the Sunset Climate Zones. It takes into account several variables including length of growing season, amount of rainfall, summer highs, wind, and humidity. Which all sounds pretty good! But currently, it’s of limited value since very few nurseries label their plants with the Sunset Zones.

Both the American Horticultural Society and the Arbor Day Foundation have developed heat zone maps which also sounds promising. But until nurseries get on board and label their plants with this information, these zone systems aren’t all that helpful, either.

(Update: The AHS’s heat map is no longer mentioned on their website, which makes me think they no longer support this system. You can still find the map here.)

Microclimates: More Important Than Zones

Hardiness zones are guidelines, but they aren’t the last word. You are. Most yards have microclimates that can be very different, and you know your yard better than anyone.

Factors that can affect the microclimate of your yard include:

• Areas near your house or a block wall will be warmer.
• Areas that get full sun will be warmer than areas shaded by a tree, shade sail, or porch roof.
• Gravel mulch holds the heat and radiates it back. This can make your yard hotter on summer nights but also warmer on winter nights.
• Elevation matters. If you have a low area in your yard, it will be cooler.
• Living near a wash or river will make your yard cooler.

I have two backyard thermometers and there is usually a 5-degree difference, even on winter’s coldest nights. That’s enough of a difference to change the zone!

Cold Work-Arounds

If you really want to grow a plant that’s rated as too cold for your zone, you can work around them.

You can extend a plant’s zone if you’re willing to cover it during cold snaps or grow it in a pot that can be moved to a protected area.

And you can always grow a plant and treat it like an annual – enjoy it for one season and accept that it will not survive the winter.

Which Zones Southwest Gardener Covers

You may be wondering what zones the information on Southwest Gardener covers and whether it’s applicable to you.

I live in Tucson in zone 9b, but the bulk of the information on this site is to help gardeners in arid regions of zones 9 through 11.

This includes most of the population of Arizona, since both Phoenix and Tucson are in zone 9. It also covers Las Vegas (zone 9) and southern California (zones 9 to 10). I know that I have readers and newsletter subscribers who live in pockets of Utah, New Mexico, Texas, and even Colorado, too.

Hardiness Zones: The Bottom Line

So is knowing your hardiness zone useful? Yes. Every gardener should know and respect their hardiness zone, but it’s only a guideline and definitely not the last word on what will grow in your yard.

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Image Credit

USDA