New USDA Plant Hardiness Zone Map and Adapting to the Changes

By Leslie Cameron, Extension Master Gardener, and Kirsten Conrad, Extension Agent, Agriculture and Natural Resources & Environmental Horticulture

The 2023 USDA Plant Hardiness Zone Map, released in November 2023, was the first update since 2012. Not surprisingly, the new map reflects warming temperatures.

Hardiness Zones Indicate Extremes of Cold

The USDA bases its hardiness zones on average lowest winter temperatures for a given area over the previous 30-year span (not lowest temperature experienced, but averages over the period). The 2023 map is based on 1991–2020 weather data and incorporates information from 13,412 weather stations compared with the 7,983 used for the 2012 map. The map is searchable by zip code. Hardiness zones are divided into 10-degree Fahrenheit ranges and further divided into 5-degree Fahrenheit half zones. For example, for zone 7a, average lowest winter temperatures from 1991 to 2020 ranged from 0 to 5 degrees F. For zone 7b, average lowest winter temperatures in that period ranged from 5 to 10 degrees F. The new map shows that about half of the country shifted to the next warmer half zone, compared with the 2012 map, and the other half of the country remained in the same half zone.

Heat Zone Maps Indicate Extremes of Heat

While the USDA hardiness zone map is based on the average coldest minimum temperatures for each zone, high heat can also affect plant growth and survival, and information on heat and high temperatures is also useful for gardeners. The US Commerce Department’s National Oceanic and Atmospheric Administration (NOAA) website, NOAA.gov, has among its resources a climate resilience toolkit, Climate Change Pressures in the 21^st Century, which includes information on heat zones and growing degree days, in addition to plant hardiness zones.

Heat zones are based on the number of days per year with a maximum daily temperature greater than or equal to 86 degrees. Growing degree days are another measure of heat and are calculated as heat units over calendar time. They can be used to predict developmental stage for plants and pests. More information on growing degree days is available at Pest-Predictive Calendar from the University of Maryland Extension.

Growing Degree Days Map
USDA Forest Service Office of Sustainability and Climate.

Growing Degree Days Map 1980-2009
USDA Forest Service Office of Sustainab

Growing Degree Days Map 2070_2099
USDA Forest Service Office of Sustainab

The interactive Climate Change Pressures maps compare recent conditions (1980-2009) to projected conditions by end of the century (2070-2099). The Climate Explorer map at HEAT.gov shows how climate conditions in the U.S. are projected to change in the coming decades, assuming either higher emissions or lower emissions. Sustained, high temperatures can slow chemical activity and growth in plants and cause heat stress. Heat wave in the garden: How to identify and prevent heat stress in plants from Oregon State University Extension reviews the signs of heat stress in plants and ways to mitigate heat damage.

Stretching the Zones Using Microclimates

On the new hardiness zone map, some Alexandria and Arlington zip codes to the east and south are in warmer half zone 8a, while other zip codes to the west and north remain in zone 7b. To identify the hardiness zone in your neighborhood, search the map by zip code, keeping in mind that the zones are based on average minimum temperatures and that past weather records cannot provide a guaranteed forecast for future weather.

Gardeners know that many factors contribute to survival of plants, including soil type, amount of precipitation, humidity, pollution, and wind, as well as high and low temperatures. Microclimates are smaller localized areas with site conditions that differ from surrounding areas in terms of sun or shade, warmth (e.g., near a building), wet or dry soil, exposure to wind, and other factors. Some areas may be more frost prone. Elevation and slope may affect microclimates. Careful observation of local microclimates may allow gardeners to choose plants that stretch the hardiness zone. Gardeners may also avoid losing tender plants by planting to a half zone lower.

Choosing Locally Adapted Native Plants

Not all plants of the same species grow equally well in all locations. Trees and plants grown from the seeds of plants harvested in one region tend to have more similar genetic characteristics than those of the same species propagated from seeds grown farther away. Local genotype refers to genetic makeup of plants specific to a given region. Local ecotype native plants are a subset of the species that has adapted to local climate and conditions and is genetically distinct from members of the same species grown from seed secured elsewhere.

Local genotype or local ecotype native plants usually do better in local conditions and are better adapted to the needs of local pollinators. Maryland Grows has more on the advantages of choosing local genotype or local ecotype native plants. Some native plant vendors grow local ecotype native plants. The Virginia Native Plant Society and Plant NOVA Natives list native plant nurseries in this area. Check with vendors on availability of local ecotype plants.

Building Sustainability and Resilience Into Landscapes

As the climate warms and precipitation patterns change, we will continue to learn about the impact on plants, plant communities, and the ecosystem they inhabit. At the Earth Sangha blog, Planting for Resiliency in the Face of Climate Change suggests proceeding with caution:

Moving plants around comes with risks. Some are obvious, like the spread of exotic pests like Spotted Lanternfly on nursery stock. Other risks are less obvious. Research into common insect herbivores suggests that even common generalist insect herbivores may specialize in a particular species in any given part of their range. This is likely from long-established coevolutionary lineages within these populations. So, for example, a black cherry from a few states away may not be as appealing to a Luna moth here. The risk is that by introducing plants from other regions — even of species already present here — we may not be supporting ecological relationships in the ways we anticipate. So how then should we build resiliency into our landscapes? The best solution may not be novel and exciting. It may just mean doing more of what we already know works: good stewardship of our lands.