Gardening Maps for Plant Hardiness and Heat Tolerance
On my prepping blog, I often share information about home gardening because I believe it’s an essential skill that everyone should learn. For preppers, having a garden means a reliable food source that’s not dependent on grocery stores.
It also teaches valuable skills like problem-solving, patience, and planning, all of which are beneficial in a survival scenario.
Plus, gardening can be adapted to any space—whether you have a large backyard or just a small balcony—making it accessible for everyone, no matter their living situation or physical abilities.
Why Zone Maps Matter: Avoid Costly Planting Mistakes in Your Garden
Gardening success often starts with selecting the right plants for your climate, and two tools can make all the difference: the USDA Hardiness Zone Map and the AHS Heat Zone Map. These maps serve as a gardener’s best friend by offering valuable guidance on which plants will thrive in your specific location.
I learned this lesson the hard way when I moved from Ft. Thomas, Kentucky, then Indianapolis, Indiana, to Florida.
Eager to create a beautiful garden, I filled a flower bed with plants that weren’t equipped to handle the intense Florida sun, even though they were sold at a local store.
Unsurprisingly, despite my efforts, they withered away. If I had relied on these zone maps, I would have chosen more suitable varieties and saved myself both time and money.
In this post, I’ll explain how using these maps can help you avoid similar mistakes and ensure your plants are well-suited to your growing conditions.
USDA Plant Hardiness Zone Map
The United States Department of Agriculture introduced the first plant hardiness zone map in 1960 to help gardeners and farmers choose plants best suited for different regions of the United States.
The map is based on average minimum winter temperatures, dividing the country into 13 zones that range from the coldest to the warmest. Over the years, it has been updated multiple times to reflect changes in climate data, with the latest version released on November 15, 2023.
This new version, developed by the USDA's Agricultural Research Service (ARS) and Oregon State University’s PRISM Climate Group, is the first update since 2012. It now uses data from 13,412 weather stations, compared to 7,983 in the previous edition, resulting in more precise zone boundaries and better accuracy.
Each zone is divided into "a" and "b" subzones, with about a 5-degree difference between them. A lower zone number means a colder climate, while a higher number signifies a warmer region.
Understanding these zones helps gardeners select plants that can withstand local winter temperatures, making it a key tool for successful gardening and landscaping.
How Should I Use This Map?
Plant Hardiness Zone Maps offer general guidelines for growing perennial plants. They focus on the average lowest temperatures in a region, not the coldest temperatures ever recorded.
The current USDA PHZM is based on weather data from 1991 to 2020, reflecting average minimum winter temperatures over that 30-year period. However, a plant labeled as hardy in a particular zone may still be at risk during an extreme cold spell.
For instance, a plant growing near the colder limit of its hardiness zone might thrive for several years but could be lost during an unusually harsh winter.
So how do you find out which zone a plant prefers?
If you start your garden from seeds, check the back of the seed packet. Most packets list the hardiness zones suitable for that particular plant.
If you prefer to buy established plants, visiting a local nursery that specializes in native species is your best bet. Native plants are naturally adapted to your region’s climate, soil, and growing conditions, making them a smart choice.
Knowing your zone helps you choose plants that will thrive, saving you from the disappointment of watching them struggle and fail in the wrong environment.
Additional Tips for Selecting the Right Plants
- Read Labels: Check plant tags for zone, sunlight, and water needs before buying.
- Consult Local Experts: Seek advice from your local extension service, gardening clubs, or Master Gardeners. They can provide insight on what grows best in your specific microclimate.
- Use Online Resources: Websites like the USDA Plant Hardiness Zone Map offer detailed information about different plant species. You can enter your zip code on their web page to find your zone and print a map specific to your area.
Beyond hardiness zones, many other factors influence a plant’s survival. Wind, soil type, soil moisture, humidity, pollution, and winter sunlight all play a role. Even how a plant is positioned in the landscape or the way it’s planted can impact its health.
Environmental Factors to Consider
- Light: Plants need the right amount of sunlight. For example, plants needing partial shade may suffer winter damage if exposed to too much sun, causing rapid temperature changes in their tissues.
- Soil Moisture: Plants have varying moisture needs, and this can shift seasonally. Hardy plants may still be damaged if the soil is too dry in late autumn, as they enter dormancy already stressed by lack of moisture.
- Temperature: Every plant has a temperature range it prefers. Some varieties can tolerate wider temperature swings, while others have a much narrower tolerance.
- Cold Exposure Duration: Some plants may survive brief cold spells but cannot handle prolonged exposure to freezing temperatures.
- Humidity: High humidity can reduce cold damage by limiting moisture loss from leaves and buds. On the other hand, low humidity can worsen cold damage, especially for evergreen species.
Recognizing these variables will help gardeners make more informed decisions, improving their chances of success in their specific climates.
Back when I lived in northern Kentucky and Indianapolis, I didn’t have to change much about my gardening routine because both places were in Zone 6b. Plants that thrived in one location did well in the other.
But when I moved to Florida, I quickly realized I couldn’t use the same plants. My new zone, now labeled 10A (it used to be 9b before the most recent update), presented a completely different environment—one that many of my old favorites just couldn’t handle.
Using the USDA Plant Hardiness Zone Map could have saved me time, money, and a lot of frustration.
American Horticultural Society's Heat Zone Map
The American Horticultural Society's (AHS) Heat Zone Map was created by H. Marc Cathey in 1997 to serve as a companion to the USDA Plant Hardiness Zone Map.
While the USDA map measures cold tolerance based on the average minimum winter temperatures, the Heat Zone Map focuses on how plants handle high temperatures, specifically the number of “heat days” they experience each year.
A “heat day” is defined as any day when the temperature exceeds 86℉ (30℃). The Heat Zone Map is divided into 12 zones, ranging from Zone 1, which has no heat days, to Zone 12, which has more than 210 heat days per year.
This is slightly different from the USDA Plant Hardiness Zone Map, which has 13 zones. Each plant in catalogs or nurseries that uses the AHS system is given a two-part code: one for its USDA hardiness zone and the other for its AHS heat zone, helping gardeners choose plants that can withstand both cold and heat.
Why 86℉?
At 86℉, many plants start to experience heat stress, making it a key marker in the Heat Zone Map. High heat can make cell membranes unstable, leading to stunted growth, failure to bloom, or even death.
How Heat Affects Plants
When temperatures rise above 86℉, plants lose water faster through transpiration. If water is not readily available, they dehydrate, wilt, and lose their ability to photosynthesize effectively. Prolonged exposure can result in stunted growth, loss of fruit, or even death.
To make the most informed decisions when selecting plants, gardeners should use the Heat Zone Map alongside the USDA Plant Hardiness Zone Map. Each map serves a specific purpose: the Plant Hardiness Map focuses on a plant's ability to withstand cold temperatures, while the Heat Zone Map helps identify a plant’s tolerance to prolonged high temperatures.
Using both maps together provides a more comprehensive view of which plants will thrive in a given location. However, it’s essential to use these maps as general guidelines rather than strict rules. Many plants can survive short periods of heat above their designated zones if they receive enough water and proper care.
Ultimately, the Heat Zone Map helps determine a plant’s tolerance to heat stress, while the Hardiness Zone Map focuses on cold tolerance. By using both, gardeners can select plants that are well-suited to their climate year-round, ensuring better growth and fewer issues with extreme temperatures.
Final Thoughts
By using both the USDA Hardiness and AHS Heat Zone maps, you can make informed choices about the plants in your garden. Whether you’re a seasoned gardener or just starting out, knowing your zone is the first step to a thriving garden. Check your zone today and start planning!
- MarthaStewart.com: How To Find Your Gardening Zone - Plus the Best Plants to Grow in Your Region
- United States Department of Agriculture: How to Use the Maps
- University of Wisconsin Horticulture: Hardiness Maps