Become a Member
Facebook
Twitter
YouTube
Instagram
Pinterest
  • New Plants
    • New Plants Search
    • Green Thumb Award Winners
  • Year Of Plants
    • Year of the Amaryllis
      • Amaryllis Pictures
    • Year of the Broccoli
      • Broccoli Pictures
    • Year of the Celosia
      • Celosia Pictures
    • Year of the Orchid
      • Orchid Pictures
    • Year of the Rudbeckia
      • Rudbeckia Pictures
    • Year of the Spirea
      • Spirea Pictures
    • 2022 Year Of Plants
      • Year of the Gladiolus
        • Gladiolus Pictures
      • Year of the Lilac
        • Lilac Pictures
      • Year of the Peperomia
        • Peperomia Pictures
      • Year of the Phlox
        • Phlox Pictures
      • Year of the Salad Greens
        • Salad Greens Pictures
      • Year of the Verbena
        • Verbena Pictures
    • 2021 Year Of Plants
      • Year of the Hardy Hibiscus
        • Hardy Hibiscus Pictures
      • Year of the Garden Bean
        • Garden Bean Pictures
      • Year of the Hyacinth
        • Hyacinth Pictures
      • Year of the Monarda
        • Monarda Pictures
      • Year of the Sunflower
        • Sunflower Pictures
        • #YearoftheSunflower Video Contest Winners
    • 2020 Year Of Plants
      • Year of the Lavender
        • Lavender Pictures
      • Year of the Lantana
        • Lantana Pictures
      • Year of the Hydrangea
        • Hydrangea Pictures
      • Year of the Iris
        • Iris Pictures
      • Year of the Corn
        • Corn Pictures
    • 2019 Year Of Plants
      • Year of the Snapdragon
        • Snapdragon Pictures
      • Year of the Dahlia
        • Dahlia pictures
      • Year of the Pumpkin
        • Pumpkin Pictures
      • Year of the Salvia nemorosa
        • Salvia nemorosa pictures
    • 2018 Year Of Plants
      • Year of the Coreopsis
        • Coreopsis Pictures
      • Year of the Tulip
        • Tulip Pictures
      • Year of the Calibrachoa
        • Calibrachoa Pictures
      • Year of the Beet
        • Beet Pictures
    • 2017 Year Of Plants
      • Year of the Daffodil
        • Daffodil Pictures
      • Year of the Brassica
        • Brassica Pictures
      • Year of the Rose
        • Rose Pictures
      • Year of the Pansy
        • Pansy Pictures
  • Combo Ideas
  • Inspiration
    • NGB Blog
    • FAQs
    • Newsletter
    • Pinterest
    • Promote Gardening
    • #YearoftheSunflower Video Contest Winners
    • Snapdragon Video Winners
  • Education
    • Member Blogs
    • Books
    • Online Courses
    • Podcasts
    • Videos
    • Webinars
  • Products
    • Garden Books
    • Garden Décor
    • Garden Tools
    • Green Thumb Award Winners
  • Shop Members
    • United States
    • Canada
  • Garden Grant
    • Grant Application
    • Therapeutic Grant Program
  • Contact
Jazz Hands Loropetalum is a warm hardiness zone plant - National Garden Bureau

Get in the Zone: Two Truths (and a Lie) about Zone Maps

How-to, Planning and Design, Planting TipsMarch 15, 2023gail
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!

All States Hardiness Zones - National Garden Bureau

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.
Pond Microclimate - National Garden Bureau

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.
Beginner Tip for the New Gardener

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.

Tropical Plants by a wall is a warmer microclimate - National Garden Buurea
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!
Beginner Tip for the New Gardener

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.

Sun Parasol Giant Crimson Mandevilla from NGB member Suntory Flowers

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.
Gardenia Diamond Spire - Plant Development Services - National Garden Bureau

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!
Just Chill Red Tip Camellia from NGB member Proven Winners
Blue Skies Lilac from NGB Member Monrovia

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!
National Garden Bureau Expert Tips

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!

Small Tropical Garden - National Garden Bureau

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.”

Previous post 5 Essential Spring Gardening Tips Next post 10 Best Plants & Flowers to Use as Tea

5 comments. Leave new

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.

Reply
Toni Gattone
March 17, 2023 11:34 pm

What a great post. Good advice for beginners as well as us “experienced “ gardeners..

Reply
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!

Reply
Michelle Torres
March 17, 2023 8:35 pm

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!

Reply
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!

Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Recipe Rating




Follow Blog via Email

Sign-Up to receive notifications of our new posts.

Search

Topics

  • DIY Crafts & Gifts
  • Flower Shows and Festivals
  • Garden Grant Recipients
  • Gardening as Therapy
  • Gardening Books
  • How-to
  • Indoor Gardening
  • Kid's Gardening
  • Planning and Design
    • Backyard Habitat
    • Container Gardening
    • Garden Planning & Design
    • Perennials
    • Plants for Shade
  • Plant Care
    • Pests and Diseases
    • Plant Care & Maintenance
    • Soil, Compost and Mulch
    • Winter Gardening
  • Planting
    • Flower Gardening
    • Foodscaping
    • Herbs
    • Planting Tips
    • Seeds and Seed Starting
    • Vegetables
  • Pollinator Gardening
  • Recipes

Archives

  • March 2023
  • February 2023
  • January 2023
  • October 2022
  • September 2022
  • August 2022
  • July 2022
  • June 2022
  • May 2022
  • April 2022
  • March 2022
  • February 2022
  • January 2022
  • December 2021
  • November 2021
  • October 2021
  • September 2021
  • August 2021
  • July 2021
  • June 2021
  • May 2021
  • April 2021
  • March 2021
  • February 2021
  • January 2021
  • December 2020
  • November 2020
  • October 2020
  • September 2020
  • August 2020
  • July 2020
  • June 2020
  • May 2020
  • April 2020
  • March 2020
  • February 2020
  • January 2020
  • December 2019
  • November 2019
  • October 2019
  • September 2019
  • August 2019
  • July 2019
  • June 2019
  • May 2019
  • April 2019
  • March 2019
  • February 2019
  • January 2019
  • December 2018
  • November 2018
  • October 2018
  • September 2018
  • August 2018
  • July 2018
  • June 2018
  • May 2018
  • April 2018
  • March 2018
  • February 2018
  • January 2018
  • October 2017
  • September 2017
  • August 2017
  • June 2017
  • May 2017
  • April 2017
  • February 2017

Contact

National Garden Bureau
5201 Walnut Ave., Suite 3
Downers Grove, IL 60515
Phone: 630-963-0770
Email

Quick Links

New Plants
2022 Year Of Plants
Inspiration
Shop Our Members
National Promotions
Garden Grant
AAS Winners
Video Contest Winners

About NGB

Who We Are
Become a Member
Member Directory
Member Photo Libraries
NGB in the News

Newsletter Sign-Up

Sign-Up for our Newsletter Here

Copyright © 2023 National Garden Bureau. All rights reserved.