What Plant Hardiness Zones Don’t Tell You
Plant hardiness zones help determine how long your growing season is and which plants you can grow. Put simply, it’s an average of the lowest temperatures during the year. Though it’s a helpful metric, there’s a lot that a hardiness zone doesn’t tell you!
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Whether you’re new to gardening or have been for decades, you’re probably familiar with hardiness zones. These regions, as defined by the United States Department of Agriculture (USDA), separate states into distinct areas depending on their average winter temperatures.
Using years’ worth of data, the USDA created distinct zones that vary across the country. There are 13 total zones, and each one has subcategories like 1a and 1b. In total, there are 26 zones that cover all of the U.S., including Alaska and Hawaii.
The colder your climate, the lower your zone will be. High zones are often frost-free, meaning they have a year-round growing season. Others, like zone 4, experience long winters and a few months of crop-growing weather.
No matter your zone, consider what this tool doesn’t tell you. There are many more factors in the garden than winter temperatures, such as summer temperatures, microclimates, and ecoregions.
What Are Plant Hardiness Zones?
Hardiness zones are, per the USDA’s website, “based on the average annual extreme minimum winter temperature.” That’s all they are—they’re nothing more, and nothing less. The zones consider how cold it gets in the winter.
Though it may seem simple, this single metric is especially helpful in determining when to plant seeds and which crops to grow. Many ornamental species have a rating on their labels or tags that tells you which hardiness zones they’ll likely survive in. Use this rating to help you choose hardy plants that’ll thrive in your garden.
These zones aren’t static, and the USDA has updated them over the years. The most recent change was in 2023 when they updated the zones to reflect warming trends of the last 30 years. Many cold zones got bumped into a higher one, showing that winters are, on average, warmer than they were in past decades.
What is a Bookend?
A “bookend,” as Epic Gardening founder Kevin Espiritu explains, is when your growing season begins and ends. The end and the beginning are bookends, meaning they mark the times you can and cannot grow plants outside.
Warm zones, like 10 and above, lack bookends. They have endless growing seasons, allowing gardeners to grow veggies, fruits, and wildflowers all 12 months of the year! I garden in zone 8b. My bookends are November and February, as most crops won’t grow during the frosty winter.
Colder zones, like 2a, have short growing periods. Gardeners in these areas have bookends right next to each other! Another way of finding the bookends is to consider the average frost dates for your local area. Most crops mature between the final and first frosts, when temperatures are warm, days are long, and sunlight is abundant.
What Hardiness Zones Don’t Tell You
There are many things hardiness regions don’t tell you. Some conditions will pop out right away, while others will take a year or more of studying the garden to notice. Paying close attention to your space and how the sun travels over it will give you a good idea of which plants will grow where.
The main thing these measurements don’t tell you is how hot it gets! Some regions, like Southern California and Florida, have intensely hot summers. Others, like the Pacific Northwest, have mild, cool seasons that favor a variety of species. Zone 8b in Oregon is much different from 8b in Florida!
Alongside summer average temperatures, gardens vary depending on the average frost dates, the altitude, the soil type, moisture and humidity levels, microclimates, and ecoregions. All of these metrics and considerations are tools to help you decide where to put your plants. There’s a plant for every space!
Microclimates in the Garden
Microclimates are like mini-ecosystems. There may be many in your yard! A fence provides a shady microclimate on one side and a sunny, hot one on the other. Trees, houses, hills, and buildings influence how air, water, and light reach the area.
Using microclimates to your advantage, you can convert a bare space into a blooming oasis! Put shade-loving species under fences and trees, and drought-tolerant ones in exposed areas. If a windy area prevents plants from thriving, plant a wind-resistant tree or hedge to redirect the wind.
The possibilities are endless! To notice all the variable differences in your yard, you’ll need to walk through it throughout the year to see how the seasons change the space. Using microclimate knowledge and zonal recommendations will allow you to make gardening decisions like a professional!
Consider Ecoregions Instead
Ecoregions are another way of looking at North America. Instead of dividing up the continent by the minimum winter temperatures, it uses the geography of the landscape as a marker. There are overarching categories and small, detailed ones that help inform gardeners and landscapers which plant species they should grow.
Ecoregions are also helpful in determining which plants are native and non-native to your garden. Native plants invite pollinators, beneficial predatory insects, and wildlife to your yard. They’re essential for natural-style plantings and pollinator gardens.
Some examples of ecoregions are the Northern Forests, the Great Plains, and Mediterranean California. Determine which ecoregion you live in, then consult resources to determine which wildflowers will flourish in your space.
Key Takeaways
- These zones are important tools for the gardener, but they aren’t the only tools.
- Consider ecoregions, microclimates, and average frosts in determining when to sow seeds and transplant seedlings.
- Summer temperatures are equally as important as winter temperatures.
- It may take a few years of gardening in the same space to fully understand the site.
Frequently Asked Questions
How are plant hardiness zones determined?
The USDA averages the lowest winter temperatures from the past 30 years to determine where each zone lies.
Are plant hardiness zones changing?
No, they are not currently changing. They were last updated in 2023 to accommodate our warming climates.
What does cold hardiness mean?
Cold hardiness is a term that describes how hardy a plant is to winter cold. Some species are frost-hardy, while others are frost-tender.
