GoGardenNow - The Gardening Blog: May 2019

How to find and use the USDA climate zone map

Will those plants survive the winter where you live?

Nearly every plant catalog and label comes with valuable information for each plant to help gardeners like you enjoy growing success. One of those info bits might read something like this, “Zone: 3 to 8”. But what does it mean for you? You'd better check out the climate map.

Making sense of local climates

Climatologists and horticulturists have long tried to make sense of the earth’s geography and climate to better understand how plants relate to their environments, and how they might perform in them. Such knowledge would help farmers and gardeners enjoy success. So climate zone maps were developed, sort of like road maps to benefit just about anyone who worked with plants. There was a lot riding on it, too, not only for gardeners, but because entire industries relied on the data for their businesses.

They had to gather and study climate data collected over many years. Imagine the many locations throughout the U.S. where weather statistics had to be recorded, then collated. That was a huge undertaking.

A short history of climate map-making

In 1990, the U.S. Department of Agriculture (USDA) published a climate zone map based upon data spanning the years from 1974 to 1986. The American Horticultural Society produced a map in 2003 from info gathered from 1987 to 2002. The Arbor Day Foundation released a map in 2006 from statistics taken from 1990 to 2005.

As you might expect, the climate zone delineations differed, which made matters rather confusing. In some cases, a location on one map would be in a different climate zone than on another. The spans of years differed. The locations of data gathering differed. So did their conclusions. Frankly, there were some areas of the country where climate data wasn’t gathered at all. Perhaps it was because they were inaccessible, or there were just too many places to actually study.

Then, the USDA published another map in 2012 based on a larger sampling of data from 1976 to 2005. It’s called The USDA Plant Hardiness Zone Map, and is the one in use today. It used an algorithm developed by the University of Oregon PRISM Climate Group. The method involved estimating climate conditions, which seemed like a pretty good idea.

How to read the map

The map shows a series of lines winding in and out of various parts of the U.S. Sometimes you’ll see enclosed patches or circles. Some of the spaces are very wide. Other spaces are quite narrow. The spaces between the lines are color-coded. These represent separate climate zones. There are 26 of them. You’ll see 1a and 1b, 2a and 2b, 3a and 3b, and so forth, all the way through 13a and 13b. Each of those zones represents a 5-degree Fahrenheit temperature range, and a corresponding range in Celsius.

Here’s an important point. The map is based upon the average annual minimum temperature. It says nothing about heat, humidity, or any other climate condition. What that means for any particular plant is that it is likely to survive the winter in whatever zone it’s rated for. If the plant is rated for zone 6, it will likely survive a minimum winter temperature of minus 10-degrees F. At any rate, the map only suggests the likelihood of survival; it’s not a guarantee.

The times they are a-changin'

I used to keep a large printed copy of the 1990 USDA map in my garden shop to display to shoppers, with a bright red push pin to show where we were located. If they were from another county or state, we’d find it on the map. “There---it---is---right-----THERE.” Those days are over. Though less nostalgic, the new map is downloadable to your computer, or you can use the interactive map on the USDA website. All you do is type your zip code into the field provided, press “enter" and Voilà! You have your zone.

I don’t have the map on my wall now, and even if I did, you wouldn’t be able to see it. So, I’ve progressed with the times. I’ve included a link on every description page of my online plant catalog. If you’re considering whether a plant is right for your climate zone, just click the link. You’ll be taken to the USDA site under a separate tab (so you don’t lose your place in my catalog).

See for yourself

Check it out. Go to the plant catalog at GoGardenNow.com, and see the USDA Plant Hardiness Zone Map in action.

While you're there, be sure to sign up for our newsletter so you can be among the first to receive GoGardenNow news and gardening tips.

Follow me to see what grows behind the garden wall.

The Red Hills Desert Garden is aptly named, as you can see. Situated on about 5 acres located above the city of St. George, Utah, the garden is described as "beautiful and smart." "Beautiful" because of its attractive layout and enchanting desert plant collection; "smart" because the garden "uses an average of five million gallons of water less per year than a traditional turf landscape. That’s enough water to support 50 average American homes for a year."

Red Hills Desert Garden - opened in 2015 - is Utah’s first desert conservation garden, a collaboration of Washington County Water Conservancy District, City of St. George and Virgin River Program. It was established to showcase the beauty of a water-wise landscape, and to provide information to homeowners and businesses about designing, installing and maintaining one. It helps that the garden signage is well-placed and clear.

Simple fascinations

You'd think that such an exhibit would be a dull place to visit, but it's not. We were there on a Sunday afternoon. There were many families strolling about, picnicking nearby, and kids having the time of their lives. Certain features captivated the children most: dinosaur tracks, the meandering stream, and areas off the designated paths.

These attractions, of course, kept their parents occupied with scolding, "Get off the dinosaur tracks", "¡Sal del agua!", and "Come back here RIGHT NOW! NOW! I'LL COUNT TO THREE!" Then they'd demonstrate their arithmetical skills, and threaten to do it again.

Chilopsis linearis

I had a child-like fascination with the plants. Some, like the Red Yucca (Hesperaloe parviflora), Desert Willow (Chilopsis linearis), Prickly Pears (Opuntia spp), and Chollas (Cylindropuntia spp) I soon realized are ubiquitous in the environment. But they interested me, nevertheless.

Recurring questions

I wondered about those dinosaur tracks. What were they doing around here?

More than that, though, I wondered about the plants. Many of the landscapes in the American southwest feature plants are well-known to me - Agave, Lantana, Oleander, Yucca, Barrel Cactus, Prickly Pear Cactus, Parkinsonia and Vitex.

Agave americana var. marginata

Parkinsonia florida

Vitex agnus-castus

The recurring question in my mind was whether many of the native or otherwise unfamiliar plants would thrive in the hot, humid coastal Georgia climate. I already knew that some do - Prickly Pear cactus, for instance. What about some others in this garden?

Problem solving

About the dinosaur tracks. Seen from the air, the area around and above the Red Hills Desert Garden looks like a dried up mud flat. I'm guessing those lizards were wandering around when conditions were more clement and left their tracks in the muck. Or, perhaps they were looking for water when it was becoming scarce. The climate was changing - as usual - and the region becoming more arid. Eventually, the mud dried, and the impressions turned to stone.

We're not in such dire straits, but Lord knows we coastal plain gardeners would have fewer worries and fatter pocketbooks if we didn't have to irrigate our plots so much. During some summers, the crispy grass under our feet is an immediate concern.

Not only does summer heat and occasional drought cause problems, some of us have poor, sandy soil. There are deer to contend with, and even some 2-legged trespassers. I asked myself, "Self, are there any plants commonly used in the southwest - native or otherwise - that might solve some of our problems back home?"

Here are some plants I saw at Red Hills Desert Garden that I think would be most likely to adapt to the climate at the southeastern edge of our continent. In addition to being drought tolerant, some might be deer resistant, and those armed with spines might deter trespassers.