In 2023, the USDA released a new hardiness zone map for the U.S. What does this mean for gardeners? First, we must examine why these zones are important. They measure the lowest average temperature in winter, which helps us decide if a plant will survive in a specific area. But that’s only part of the puzzle since other factors play a role in the success of a plant. Let’s dive deeper!
It’s easy to think of these growing zones as telling us the climate of a given location. However, they only focus on winter temperatures, and not the whole climate. Did you know parts of Alaska and Florida are in the same growing zone (a small portion of both states are zone 8)? Despite being in the same zone, those places have much different climates. The zones are a good general guide for selecting plants, but you should focus on the specific conditions of your garden.
When Your Gardening Zone Changes, Now What?
Has your garden shifted to a new zone on the map? My garden used to be zone 4, but now it’s zone 5. This leads us to wonder if we can add new plants that reflect the zone change, like using a zone 5 plant in zone 4. The short answer is yes, but use caution. Think of those plants as an experiment and only grow a small amount. You may notice that some plants survive but don’t thrive. When plant breeders test a new variety, they evaluate how much cold a plant can tolerate while still thriving in the garden. Their trials last several years to understand how well a plant will perform in each area. Even with the best trialing, it only takes a few record-cold days or extreme weather to damage a plant, even when it’s hardy for our zone. Keep this in mind when growing any plant.
How Microclimates Affect Plant Survival
Almost 20 years ago, my mom planted Gladiolus in our zone 5 garden. Since then, they’ve come back and bloomed almost every year without us digging them up (Gladiolus should be stored indoors during winter if you live in zones 3-7). The bulbs are only hardy to zone 7, but were planted in a south-facing corner that stays warm in winter. Microclimates can make a huge difference for some plants. Another example is Boxwood. Many varieties are hardy to zone 5 but will suffer winter burn if given full sun in winter. As you’re adding new plants, look for those microclimates on your property and see if they match the needs of each plant.
Some of these differences are specific to a local area. When I lived by Lake Michigan, our winter temperatures stayed slightly warmer than inland areas. This allowed gardeners to grow plants that would struggle in other nearby areas.
Do you live in an urban or rural area? Large cities have a heat island effect that can raise temperatures throughout the year. That can impact the length of your growing season and the winter temperatures. The extra heat can also cause soil to dry out quicker and cause stress. In general, rural areas will remain cooler.
Some plants, like fruit trees, are impacted by freezing temperatures in spring. In winter, woody plants are fully acclimated to cold temperatures. As spring arrives, the trees come out of dormancy, which makes the new buds prone to freezing temperatures. Those cold spring temperatures can damage trees that bloom early, so the right microclimate is helpful. Along the Great Lakes, fruit orchards benefit from cooler spring weather, which delays the bloom period and reduces the risk of frost. Orchards also place their fruit trees to avoid low areas where the air gets colder in spring. If you’re growing fruit trees, consider the microclimates on your property before planting.
Plant varieties also play a role. One example is magnolias, which bloom early in the Midwest. Their flowers are beautiful but can be damaged in subfreezing temperatures. Some varieties of magnolia bloom earlier than others, so choosing a later-blooming variety can reduce the risk of freeze damage. This concept also works with fruit trees, especially apples.
How Soil Affects Plant Survival
My mom loves growing lavender, which is hardy in our zone 5 garden. However, we have clay soil that remains wet in winter. Lavender is native to the Mediterranean and doesn’t like cold, wet soil. With those soil conditions, the plants do well some years, but don’t come back other years (Nepeta is a good alternative for lavender if you have the same problem). Soil plays a big role in a plant’s survival, so use plants that can work in your existing soil. You can also amend the soil with organic matter (like compost) to retain water and nutrients in the future.
Why Snow Matters More Than You Think in the Garden
Have you ever seen a forsythia with flowers on the bottom branches, but none on top? This is a common problem for some varieties of forsythia, especially in zones 4 and 5 (Meadowlark is a great choice for gardeners in these zones, since the flower buds are hardy to -35ºF). During the coldest parts of winter, snow covers the bottom branches and insulates the flower buds, while the other flowers are damaged by the cold.
Snow is a great insulator for plants, but many of us don’t have reliable snow cover. This means our gardens have deeper frost in the ground during winter, which can damage plant roots and cause them to dry out. Dry winters will also affect plants that aren’t fully hardy in your area. If you’re planting trees or shrubs, keep them watered until the ground freezes.
How Drought Stress Affects Plant Hardiness
Drought is a major source of stress for plants. And a stressed plant will be less hardy. No matter what you’re growing, keep it watered during the first year after planting. Even one deep watering each week can make a big difference. If you have soil that dries out easily, use native plants and other species that grow well in dry conditions. Less stress will also help your plants fight against pests and disease issues. The zones don’t tell us everything, but they’re a good starting point when choosing plants for the garden.
Other Reading Recommendations
- 5 Fruit Trees You Can Easily Grow
- Raised Beds 101: Planning, Building, & Planting
- Add Dimension To Your Garden With Summer-Blooming Bulbs
- 7 Environmental Factors To Consider With Gardening
- The Ultimate Guide to Choosing Apple Trees & Growing Them Successfully
At Jung Seed Co, we strive to be your go-to guide for all your gardening needs. Our YouTube channel, The Garden Doctor, by Dick Zondag is where he provides gardening tips for all levels of gardeners. When you need reliable gardening advice, turn to the trusted experts at Jung.
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About the Author: Matthew Olson is a professional horticulturist and garden writer. He has a bachelor’s degree in horticulture from UW-River Falls and is a certified professional with the Minnesota Nursery and Landscape Association. His enthusiasm for plants and the outdoors brought him to the green industry. He regularly writes articles about gardening for both gardeners and industry professionals. He can be reached at matt@mattolsonhorticulture.com.