Why the Climate Zone Map Is Nearly Worthless
It seldom reflects what goes on in your garden.
When you're thinking about buying a plant you haven't tried before, what's one of the first things you'll probably do? Check the magazine article or plant tag for the USDA (United states Department of Agriculture) Climate Zone Map rating to see if the plant will succeed where you live. BOOM! Problem solved.
Unfortunately, it isn't that easy. To see why, take an exciting journey with the Grump to discover what that map up above tells you and what it doesn't.
The colored coded map divides the continental U.S. into nine roughly horizontal climate zones with little climate blobs scattered about, usually due to mountaintops. (Forget about Zones 1 and 2, which pertain to Alaska, as well 11, which occurs only in Hawaii.) So we have Zones 3 to 10 spanning the country from Canada to Key West. These zones rely on only one factor – the coldest temperature expected during the year. As you can see, the coldest expected temps in Zone 3 run from -40 degrees F to -30 degrees F. In Zone 10, however, the coldest temps drop to only 30 to 40 degrees above zero. Quite a range, and a good reason for not moving to Zone 3.
Thus, the USDA map decides whether a plant is suited to your area based solely on cold-hardiness. It doesn't even do a great job of that, because how long a spot stays cold is just as important as the absolute low there. Wind and sun exposure can make dramatic differences in survival rates too.
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But here in the South, cold isn't the main burden plants must bear. Heat is. Our summers last much longer and are hotter than those up North. Will a plant that thrives in Wisconsin make it through 100 straight days of near 100 degrees in Dallas? Highly doubtful. Don't look to the map for guidance.
Now let's examine another shortcoming of the map. Notice how central Florida, south Texas, south Louisiana, and much of California sit in Zone 9. Again, this reflects only the coldest expected winter temperatures. If you've visited these places, you know the summers in the Florida and California are nothing alike. California is arid with dry air. Florida is rainy with air so humid that venturing outside feels like being smacked in the face with a warm, wet towel.
The difference is humidity is vital to what grows well in these places, and not just because humidity encourages a raft of plant diseases. Dry air cools off at night, often dropping into the fifties. Plants there get a good night's sleep and a chance to recover from the heat of the day. Moist air retains heat. Right now where I live in Alabama, nightly lows are running around 76 to 77 degrees. Plants don't get to rest and recover. This is why West Coast plants transplanted to the Eastern and Gulf Coasts often croak in the wink of an eye.
What, then, is the average weekend gardener to do? Use the USDA climate zones as a general guide. For example, if you live in Zone 7 and your target plant is rated for Zone 7, you're off to a decent start. Don't stop there, though. Ask someone at a local independent garden center (not a big box) how well the plant does where you live. Scour the surrounding neighborhood to see if anyone has that plant growing in their gardens. If you can't find it anywhere, chances are there's a good reason.
Grumpy will continue to provide USDA Zone ratings in his plant profiles to give you a good starting point. Just realize their limitations and know that the same plant that survives the winter across the street may die in your yard. Gardening gives no guarantees.