For the first time in a decade, the U.S. Department of Agriculture (USDA) has updated its Plant Hardiness Zone map.
The new map, revealed in November 2023, is more accurate and includes deeper details to help homeowners and gardeners choose plants that will thrive in their locations. It also reflects an average 2.5 degree increase in winter low temperatures.
The new map is based on 30-year average low winter temperatures and includes a few new enhancements:
Data from almost 70% more weather stations;Weather patterns from 1991 to 2020 (the previous map was 1976 to 2006);Elevation data;A much more detailed resolution for Alaska;A “Tips for Growers” section on the website;The update shifts about half of the country into the next warmer half zone, with some areas warming up to five degrees.
Some of the zonal changes are due to the use of more sophisticated data and map resources, and some are likely due to climate change, according to the Plant Hardiness Zone Mapping (PHZM) team.
“We know for a fact that average temperatures are rising due to climate change,” the PHZM team wrote in a statement to Family Handyman. “Over the long term, this should cause Plant Hardiness Zones to gradually move northward.
“However, the Plant Hardiness statistic is the annual extreme minimum temperature, the coldest temperature of the year. As such, it is highly volatile from year to year, and subject to the magnitude and timing of the weather events that produce this coldest temperature.”
Here’s how the new zoning map could affect you, depending on where you live.
About the Experts
The Plant Hardiness Zone Mapping team (PHZM) is comprised of researchers from the USDA/Agricultural Research Service and Oregon State University collaborators.Kathy Glassey is an International Society of Arboriculture (ISA) certified arborist, healthy soil advocate and plant care expert at Inspire Green. She previously worked at numerous landscaping companies, including Monster Tree service.Mary Phillips is head of native plant habitat strategy and certification programs at the National Wildlife Federation. She also leads Garden For Wildlife and Certified Wildlife Habitat, helping wildlife, ecosystems and the people who live in them.
Where Did the Zones Change?
Some of the biggest shifts are in the Northern states.
“If you’re in Michigan and Wisconsin, you may have the ability to plant species that may not have been good options before, like red maples,” says Kathy Glassey, arborist and senior consultant at Inspire Green.
Another big change happened in mountainous regions. The added elevation data and new nuanced zone designations should help gardeners more accurately find their microclimate (more on that below). And in Alaska, the resolution detail was enhanced from 6.25 square miles to .25 square miles.
Regardless of where you are, there could still be microclimates too small to show up on the map.
“Microclimates, which are fine-scale climate variations, can be small heat islands such as those caused by blacktop and concrete or cool spots [aka frost pockets] caused by small hills and valleys,” wrote the PHZM team. “Individual gardens also may have very localized microclimates.”
How Will Temperature Changes Effect Your Garden?
If your Plant Hardiness Zone has changed, it might just be because of the enhanced data used in the new map. If that’s the case, the biggest effect for you will be pushing the boundaries by trying new, slightly warmer or colder-weather species.
“It does not mean you should start removing plants from your garden or change what you are growing,” the PHZM team wrote. “What has thrived in your yard will most likely continue to thrive. No Hardiness Zone map can take the place of the detailed knowledge that gardeners learn about their own gardens through hands-on experience.”
If the temperature is actively changing where you live, you might have already noticed a subtle difference over the years in what plants are thriving in your yard. And as climate change progresses, it will increasingly bring with it a disruption to traditional seasons in many places.
“Spring arrives earlier in some areas, causing flowers to bloom weeks before their usual schedule,” says Mary Phillips, head of native plant habitat strategy and certification programs at the National Wildlife Federation. “This not only throws off the pollination cycle but can also expose tender buds to the risk of late frosts.”
Depending on where you live, other possible changes include:
More intense and longer lasting summer heat;Decreased vegetable yields;Increased water needs of plants;Less predictable rainfall patterns;Longer droughts;More torrential downpours and floods, which disrupt topsoil;New pests, diseases and invasive species;More strain on native and beneficial species.
In dry regions, water scarcity and heat stress will make everything more challenging. “Adapting smart garden practices for water conservation will be crucial,” says Phillips.
How To Make Your Garden a Success in a Shifting Climate
The easiest path to success? Plant native, since native plants are more likely to adapt to changing conditions. They also require less water and maintenance and are less prone to pests and diseases. Most importantly, native plants provide vital resources to insects, birds and other wildlife.
“The choices we make in our gardens, from the plants we select to how we manage water, can cause a ripple effect, contributing to a more resilient and adaptable landscape in the face of a changing climate,” says Phillips.
“Native plants, with their deep root systems, help to bind and stabilize soil, preventing erosion and protecting waterways from sediment pollution, especially in areas prone to flooding.”
In drier regions, Phillips suggests incorporating drought-tolerant plants like goldenrod, mountain mint and sunflowers. In areas with more intense summers, try heat-tolerant natives like salvia, little bluestem and coreosis.
To alleviate the stress of changing conditions on plants:
Plant species tolerant to change;Don’t plant too deeply or too shallow;Don’t overwater, which can cause disease and root rot;Don’t underwater, which can lead to shallow rooting;Consult your local university extension office for guidance, along with the National Wildlife Federation’s Native Plant Finder and Ecological Landscape guide;Don’t plant in compacted soil;Keep your soil healthy.
“Everything we do requires building a strong foundation and giving our plants, shrubs and trees the best soil foundation will help make them more resilient, too,” says Glassey. “And all of the hard work you put into planting and the care of your plants will also save you money over time.”
While the Plant Hardiness Zones help gardeners choose plants based on the coldness of their location, it doesn’t address average high temperatures, which can sometimes be even more impactful on plants. This map from the American Horticultural Society can help with that.