How-to, Planning and Design, Planting Tipsgail
Have you ever returned from a trip, determined to recreate that tropical vacation vibe in your northern backyard? Or maybe you hope to grow lilacs, like those that filled the vase on your grandmother’s kitchen table—but aren’t sure they’ll flourish in your southern garden. While it’s easy to drool over the latest, greatest plants featured in magazines, catalogs and websites, how do you know which plants will thrive in your area—and which ones will struggle to live up to your expectations?
Get in the (hardiness) zone!
What’s a Hardiness Zone Map?
You’ve probably heard about a plant’s hardiness zone—but what exactly does that mean? And how do you determine your gardening zone? A hardiness zone refers to a geographic area with a certain average minimum temperature, a factor that predicts the survival of many plants. Growers typically include a plant’s hardiness zone on the plant’s tag, as well as listing its hardiness zone on the company’s website or in catalogs.
But how do you know the hardiness zone for where you garden? The USDA Plant Hardiness Zone Map (PHZM) shows the 13 hardiness zones throughout the United States (zones 12 and 13 are relatively new additions for Puerto Rico and Hawaii), broken into subzones—for instance, zones 7a and 7b. Data is collected for a 30-year period from 7,983 temperature stations to create the hardiness zone map. The data is reviewed by a team of climatologists, agricultural meteorologists, and horticultural experts, double-checking the data for errors or biases. There’s a lot of science and expertise behind a map we often take for granted. By entering your zip code here, you’ll find your hardiness zone.
Easy peasy, right?
But is your hardiness zone absolute? Are there any factors that might make it hotter or colder than indicated on the USDA PHZM—and can you cheat the system, by adding plants outside your zone? And if you abide by the USDA Plant Hardiness Zone Map, are your plants guaranteed to grow and flourish?
To answer these questions, let’s play a little game called “Two Truths and a Lie”—about hardiness zones! Read each statement, make your guess, and then find out if it’s a truth or a lie regarding hardiness zones.
Truth or Lie? A Hardiness Zone May Contain Hotter or Colder Areas Within It
Based on the average annual extreme minimum temperatures during a 30-year period in the past—not just the lowest temperature—hardiness zones provide a good indicator for plant survival. However, in every hardiness zone, hotter or colder temperatures may appear, meaning that plants listed for other zones might grow well within these areas.
Higher and lower temperatures in a zone
Areas within zones can sport higher or lower temperatures, along with “microclimates” that can be found in individual gardens. For instance, cities tend to be slightly warmer than the surrounding zone, as they hold more heat due to the large amounts of blacktop and concrete. A city might even be listed as a warmer zone than the surrounding countryside. Likewise, higher elevations tend to be colder than surrounding lower-lying areas, so the top of a mountain may be a colder zone than the town just below it.
Large bodies of water can change the zone
Even large, unfrozen bodies of water can impact an area’s hardiness zone. Areas adjacent to bodies of water stay a bit warmer in winter and cooler in summer because water moderates the air temperatures.
Microclimates in your garden
Along with zone deviations within a region, you might find microclimates in your garden—pockets that are warmer or cooler than the general zone for your area or the rest of your yard.
A microclimate is an area with fine-scale climate variations, like a warmer pocket caused by nearby blacktop and concrete, or a cooler spot in a hill or valley. An entire yard can be slightly warmer or cooler than the surrounding area because it is sheltered or enclosed. Microclimates can also differ from the surrounding area in terms of light exposure, moisture levels, wind, and soil.
A sheltered area in front of a south-facing wall or a low spot where cold air collects—these microclimates offer you the chance to “push” your zone a bit. Zone pushing is an effort to grow plants in your garden that shouldn’t survive in your climate based on their hardiness zone.
However, if you’re dying to grow a crape myrtle in your zone 5 garden, finding the warmest microclimate might yield a happy payout for your gamble. But just like throwing the dice at the craps table, zone pushing requires strong nerves, because not all gambles win. (Still, it’s a sweet reward when they do!)
So, if you guessed that hardiness zones can vary in temperature, you’re correct!
When determining microclimates in your garden, look for areas that are slightly different than the surrounding landscape. Cold air pools in depressions, making low-lying areas of a yard colder than level areas. Areas next to houses or other buildings tend to be slightly warmer, as the structures absorb heat during the day and radiate it back out at night. Walls, driveways, and patios also absorb heat, making the spaces near them slightly warmer.
Truth or Lie? It’s OK to Ignore Hardiness Zones
If you’re dying to create a tropical staycation on your patio but live in zone 3a, don’t fret: you can enjoy lush, lovely foliage and fabulous flowers in your serene space by breaking the hardiness zone rules.
Out-of-zone goodies
Before you load your cart with out-of-zone goodies, however, make sure you’ve thought through your gardening goals. If you’re OK treating warm-zone plants as annuals in your cold-zone garden, load up! Enjoy a summer-long treat of a gorgeous, tropical oasis. Just keep in mind that those lovelies will most likely be a “one-and-done.” It’s a rare tropical plant that survives grown in-ground where low temperatures average -40 degrees in winter.
Treat as annuals
If you’re enamored with plants that aren’t hardy in your zone, enjoy them as annuals. Or, if you’re looking to add plants that are just slightly outside your zone—maybe a 6b plant in a 7a garden—take advantage of your garden’s microclimates. Positioning plants in sheltered spots or areas with radiant warmth may help a “zone-pushed” plant survive and thrive.
Grow out-of-zone plants in containers
Growing out-of-zone plants in containers seems like an easy way to enjoy your favorites without fuss. And, if you own a greenhouse or can provide warm shelter over winter for the containers, your plants may continue to thrive for years. However, many gardeners don’t realize that soil in pots typically drops to the same temperature as the surrounding air, while in-ground plants benefit from “bottom heat”—the geothermal heat below the soil. The temperature of the ground soil just a few inches deep can be more than 20 degrees warmer than the air temperature on a blustery, cold day. So, the roots of potted plants are less protected from cold than you might expect. Make sure to place container-grown, out-of-zone plants in a warm, sheltered spot over winter to help them survive.
If you’re a risk-taker, an eternal optimist, or you’re willing to spend your gardening budget on perennials that act like annuals in your zone, then yes: it’s OK to ignore hardiness zones!
Newer cultivars for other hardiness zones
Or, if you’re giddy for gardenias or crazy for camellias, look for newer cultivars developed for colder zones. Plant breeders continuously strive to push zones, creating hardy plants that flourish outside of their traditionally grown regions. While most gardenias, for example, are known for hardiness in zones 8-11, newer introductions push the border to zones 6 and 7.
And, for plants that typically need a long winter chill to bloom beautifully, like lilacs, newer cultivars are developed that thrive in warmer zones. If there’s a plant you covet, you just might find a new, hardy solution to your wishes!
Do you know that National Garden Bureau members include the very best plant breeders and growers, who develop a wide range of cultivars that can push zones for traditionally grown plants? Newer cultivars may be bred for better cold or heat tolerance, allowing you to grow plants in your zone that previously weren’t an option due to extreme cold or heat. Thank goodness for our brilliant members!
Truth or Lie? All Plants Listed for My Hardiness Zone Will Grow Perfectly in My Garden
You’ve made a list, checked it twice…and limited your plant selections to those perfect for your hardiness zone. Your garden is guaranteed to grow beautifully, right?
Well…
Remember that hardiness zones are based on the average annual extreme minimum temperatures, not the coldest. And even plants perfectly defined for your hardiness zone can be injured by a sudden bout of extremely cold weather in fall before going dormant. Or a heat wave in mid-winter that tricks plants into breaking dormancy, followed by normal seasonably cold weather. Extreme, unusual temperatures make gardening a challenge, even when you play by the hardiness zone rules.
Many factors affect the success of your plants
Even without temperature fluctuations, many other factors contribute to the success or demise of plants. Wind, soil quality, moisture, pollution, humidity, light, competing plants—all of these environmental factors contribute to your garden’s success.
Knowing your garden = success!
The reality is: no hardiness zone map can take the place of knowing your garden—its microclimates, soil challenges, wind breaks, radiant heat, and competing plants. Hardiness zone maps provide a perfect start to help you make good plant selections, but hands-on experience gives you the best knowledge of what will survive—and thrive—in your garden.
About National Garden Bureau
Founded more than 100 years ago, the National Garden Bureau educates, inspires, and motivates people to grow home gardens. National Garden Bureau members are horticultural experts, and the information shared with you comes directly from these experts to ensure your gardening success.
“This post is provided as an educational/inspirational service of the National Garden Bureau and our members. Please credit and link to National Garden Bureau when using all or parts of this article.”
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7 comments. Leave new
For Felder and others who might be interested, the Sustainable Gardening Library’s interactive EcoRegions app combines the USDA Plant Hardiness Zones with the EPA’s Level III North American Ecoregions. The map layers toggle on and off, so you can look at the two maps separately or together. We recommend zooming in to a location before activating the popup, in order to narrow your results. The EPA Ecoregions popups are linked to descriptive information for about 200 different regions and includes climate info, such as average annual precipitation, number of frost-free days, mean annual temperature, etc. It also includes information on soils, hydrology, terrain, vegetation, wildlife, and prior land use.
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Karin
March 26, 2023 4:52 pm
I also wonder if a plant grown from seed, that can acclimatize from day 1 (in a zone that it might not ideally be matched for), and may survive more readily than the same plant purchased from a garden centre/nursery. Possibly another option for pushing zones.
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Nancy Baxter
March 21, 2023 12:53 pm
Good information, I will share with my herb Society-we are always looking for informative, sound information.
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What a great post. Good advice for beginners as well as us “experienced “ gardeners..
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Paul
March 17, 2023 8:43 pm
Great article! Another aspect to consider is the amount of snow cover which acts like a blanket. LOVE my frost proof gardenia…it lives happily in the garage during the winter here in south coast Maine and so worth the effort when in blooms in the summer!
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Climate change is having an impact on gardening zones, with many areas experiencing warmer winters and shifts in their hardiness zone. For instance, here in Oregon, we have been seeing hotter and drier summers. Some of our typical go-to plants are under a lot of stress. Going forward, we will have to consider plants that can tolerate more heat and drought than in the past. You mention using a greenhouse in your article for winter shelter, but a greenhouse offers protection from the direct intense heat in the summer too (if using a greenhouse that provides diffuse light, like Solexx ). A greenhouse is a pretty fabulous tool for expanding your growing zone and adding a new microclimate in your backyard. Thank you for an informative and engaging article that all gardening enthusiasts will appreciate!
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felder rushing
March 17, 2023 8:15 pm
good insights. most folks aren’t aware that based on just average low winter temperature, most of england – which is on the same latitude as nova scotia – is the same “zone” as mississippi (i garden in both). we need a hardiness map that includes average low temps, sudden temp drops, early and late freezes, hot humid summer nights, rainfall patterns (too much, too little), and clay soils!
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