USDA Hardiness Zones — The Great Misconception About Climate and Growing Conditions

Good For Winter — Totally Useless In Summer

🌿 Garden Talk | by Guy Saldiveri | May 17, 2026

And here we are again, everyone wondering what bee is in my bonnet today…

Well, here's the thing: you get into any gardening group, online, in person, doesn't matter — the talk will always come around to USDA hardiness zones and which one a particular person is in. 

Generally, when this is happening, we are well into spring or even into summer. I can't help but think, hardiness zones? Why are we discussing that now? Hardiness zones do have meaning — and quite a bit — but not during the prime growing season. They come into play for one reason, and one reason only — to let you know how cold it will get in your area.

For example, you're wanting to grow a beautiful tropical plant like a Majesty Palm, but you're in zone 7b. You are about to spend $30 on a gorgeous specimen you just found at your local big box store, but should you? 

USDA Plant Hardiness Zone Map

To be completely honest? Absolutely not. 

These palms are tropical and don't like the cold. They prefer to be in temperatures above 40°F and will freeze back (and die) below 30°F. 

Looking at the map, zone 7b on average reaches a minimum temperature of 5–10°F. That plant will not survive unless kept indoors all winter.

And honestly, that's the gist of the hardiness map. That's the information I can glean from it. How cold it will most likely get in my area. 

That tells me what plants I can reasonably expect to survive winter where I am.

What doesn't this map tell me? 

Well, everything else that actually matters about the climate. Let's get out the spade and dig into that a little to drive the point home. Pay attention to the climate information and you'll see just how wild this gets.

I'm in zone 9a — Southern Mississippi — and here's a list of other areas in the same zone, along with details about their local climates.

• Phoenix, AZ — technically 9a in some pockets, but bone‑dry, blazing sun, huge day-night swings.
• Orlando, FL — humid subtropical, warm nights, long rainy season, tropical pests.
• Houston, TX — swamp‑humid, hurricane‑prone, long heat season, mild winters.
• Savannah, GA — humid, coastal, long growing season, frequent storms.
• New Orleans, LA — hot, humid, wet, tropical disease pressure.
• San Antonio, TX — hot, semi‑arid tendencies, alkaline soils.
• Tucson, AZ — desert heat, low humidity, cool nights, monsoon season.
• Redding, CA — dry heat, Mediterranean pattern, wet winters, bone‑dry summers.
• Perth, Australia — Mediterranean climate, mild wet winters, hot dry summers.
• Lisbon, Portugal — mild winters, dry summers, ocean‑moderated temps.
• Tokyo, Japan — humid subtropical, rainy season, typhoons, warm nights.
• Auckland, New Zealand — maritime mildness, cool summers, no extreme heat.
• Valencia, Spain — Mediterranean, hot dry summers, mild winters.
• Cape Town, South Africa — Mediterranean, cool wet winters, warm dry summers.

Now, if we are talking about a plant that can handle temperatures down to say 10°F in the winter, it will work just fine for every area listed. 9a has an average low temperature of 20–25°F. 

But what if it says it needs full afternoon sun, 6-8 hours per day, and 1-2 inches of water per week?

Looking at that list, I can tell you that plant is going to have a lot harder time thriving in Phoenix, AZ than it would in, say, New Zealand. Same zone number — completely different growing reality.

Zones matter for cold — local climates are a whole different story.

So is there anything else we can use to help us out? Yes, there is an abundance of information out there. 

Heat Zones

One thing I like to look at is this map from the American Horticultural Society. It shows heat zones and how many days on average you can expect in your area to be above 86°F. 

 

American Horticulture Society - Heat Zone Map

This is where the heat zone map earns its keep.

So, what's the importance of 86°F?

Plants, like any other living thing, have strong reactions to temperature. Whether it's cold or hot, plants react and attempt to protect themselves. While most folks are aware that freezing temperatures can damage plants fairly quickly, heat damage is more variable and often more subtle. 

One of the first signs of heat stress in plants is wilting.

As temperatures increase, the plant loses more water through the process of transpiration. Transpiration is the process where plants release water vapor into the atmosphere, primarily through pores called stomata on their leaves. If the plant's water uptake doesn't keep pace, it becomes dehydrated and wilts.

Plants may continue to survive in this dehydrated state, but their growth rate and overall health decline. Curled leaves are the plant's way of hiding from the sun, but that also slows down photosynthesis — limiting the sugars and nutrients the plant needs to stay healthy.

If prolonged, heat stress can cause irreversible damage to the plant.

Most heat stress can be managed until temperatures reach about 86°F. At this point, stress becomes more severe.  

Many vegetables, such as tomatoes, peppers, squash, and eggplant, shut down around this stage: flowers stop forming, buds drop, and fruit production halts.

While you can't control ambient air temperature, you can help plants cope by ensuring they're adequately watered, moving them into more shaded areas, or providing a protective cover, such as shade cloth.

Frost Dates

Another tool I use is this map indicating first and last frost dates by area. This one helps me plan when I can start planting and when I should expect to be able to sow my last round of seeds. 

This is a great tool, but keep in mind it's showing you air temp, not soil temp, so you'll still have to pay attention to that as well — at least coming up on spring. 

Frost Date Map
Image courtesy of FastGrowingTrees.com

Seeds and roots don’t care what the air is doing — they care what the soil is doing. 

That's why I said it matters more in the early part of the season. From summer into fall, it won't matter much. The soil is already as warm as it's going to get. 

Soil Temperature Matters More Than You Think

One last piece of advice: get yourself a cheap soil thermometer. Don't just rely on the NWS or what your local forecast is saying about next week's warm-up. Air temp is one thing, soil temp is another. 

Perfect example of this:

• Air temp is 70°F or 75°F
• Seeds need 70°F to germinate
• Soil is still sitting at 60°F

Those seeds will most likely take forever to pop or they'll simply rot.

Tip: A cheap meat thermometer works just fine for checking soil temperature — and they're usually in the $5 price range instead of the $20 range for soil thermometers.

Using all three maps gives you a valuable head start. Combined, they'll help you understand problems, resolve issues, and select plants. You'll still need to pay attention to current and short-term local conditions in order to be successful.

Knowing how cold it generally gets in your area is a big part of gardening, but it's only one part of it. Knowing your full climate — and learning how to work with it and adjust to it — is what makes a gardener worth their dirty nails. 

Keep digging, planting, and researching. Gardening is all about growth — knowledge is one of the best things you can grow — well, besides peppers, eggplant, and tomatoes — maybe some okra…

Happy Gardening 🌱