By Jay Kratz
Perhaps you noticed that many of our native flowers bloomed earlier this year. We spotted sharp-lobed liver leaf (Anemone acutiloba) blooming in February! This trend of early blooming has been occurring a lot lately as our region continues to warm. In response to the changing climate, the United States Department of Agriculture (USDA) recently updated its Hardiness Zone Map. These hardiness zones reflect the average annual extreme minimum winter temperatures of a region. You’re probably already familiar with this tool and might have seen the hardiness ratings on plant tags, which tell you whether a perennial plant will likely thrive in your location.
Image 1: Updated USDA plant hardiness zone map.
Here in northern Delaware, we have shifted a full hardiness zone warmer compared to the 1990 USDA map, and nearly a half zone warmer compared to the 2012 map. Mt. Cuba Center is now right on the cusp of zones 7a & 7b, with the majority of Delaware now a solid 7b. A 7b designation translates to a minimum winter temperature range of 5-10° F. That’s similar to the type of winter we would have historically associated with Washington, D.C.
Image 2: Changes in the USDA plant hardiness zones from 2012 to 2023.
The changes in our climate reflected in the USDA’s new hardiness map have a lot of implications, most directly for the plants that will thrive in our gardens in the future. What can we do for our own spaces? With warmer temperatures on the horizon, selecting native plants tolerant of heat and drought will help to ready your garden for these potentially adverse conditions. Taking it a step further, consider the native range of a plant that you would like to add to your garden by consulting the Biota of North America Program (BONAP) website, and avoid species at their southernmost range in your area. For instance, scrub pine (Pinus virginiana) and long-leaf pine (Pinus palustris) have historically grown in the mid-Atlantic region, but this area is at the northernmost edge of their range. As their range moves north with warmer temperatures, these species will be best adapted to our warmer conditions. Conversely, eastern white pine (Pinus strobus) and American larch (Larix laricina) have historically grown in our region, though we are at the southern edge of their native range. As their range moves north, our area will likely become too hot for them to thrive here.
Image 3: Long-leaf pine.
If you’re looking for a tree, you might also explore the USDA Forest Service’s Climate Change Tree Atlas. Since trees live for hundreds of years, and our climate will continue to change, choosing a tree that can adapt is especially important. The tree atlas provides traits, habitat suitability, and adaptation potential for 125 tree species. The trees are given an “adaptability rating” to indicate how likely it is they will adjust to climate conditions in the year 2100 (considering both low and high greenhouse gas emissions scenarios). In the Lower Chesapeake area, for example, oaks (Quercus species), red maple (Acer rubrum), sour gum (Nyssa sylvatica), and loblolly pine (Pinus taeda) are among the species that are expected to be more adaptable in the future.
Image 4: Red maple with vibrant fall color.
In fact, one of the many considerations Mt. Cuba’s arborists factored when selecting plants for the South Terrace replacement project was data from climate models. According to arborist Nicole DeLizzio, “Nyssa sylvatica had an average rating of “Good” to “Very Good” for both the mild and severe climate predictions for our area in the future.” The arborists planted Nyssa sylvatica ‘David Odom’ Afterburner with the aspiration that these trees will delight guests at Mt. Cuba for years to come.
Wherever you are in the U.S., make sure to check the latest hardiness zone map before adding plants to your garden. As always, ensuring that you are planting the right plant in the right place is a good recipe for success. Happy gardening!