Climate-Conscious Gardening
Join Master Gardeners in the Arlington/Alexandria unit of Virginia Cooperative Extension in a series of monthly articles in 2021 as we explore the topic of climate change and practical actions individuals can take in their home landscapes in response.
By Elaine Mills, Extension Master Gardener
Photos by Elaine Mills, unless otherwise credited
For the east coast of the United States, any plants that grew here prior to European settlement are said to be “native.” Historically, that has meant that gardeners in Northern Virginia have had a wide set of plants to choose from. Located in the midpoint of the east coast, we have been positioned at the southern range of Mid-Atlantic native species and the northern end of the range of Southeastern species. Those of us living along the fall line (located roughly along Glebe Road in Arlington) have had the option of choosing Coastal Plain species to the east and Piedmont species to the west.
As columnist Adrian Higgins observed in a recent article in The Washington Post, the new trend toward hotter summers and warmer winters is necessarily changing the way we garden. While the DC metropolitan area has previously been designated as Zone 7 on the USDA’s Plant Hardiness Zone Map, our current lowest average temperature of 14 degrees now would place our region in Zone 8, which is presently located around Virginia Beach, a little over 200 miles further south.
This means that some native plants may no longer be able to survive in portions of their historic ranges. Native trees, such as eastern or Canadian hemlock (Tsuga canadensis) known as the “redwood of the East,” are now retreating northward and into the mountains. Warming temperatures are enabling the hemlock woolly adelgid, a sap-feeding pest, to survive through the winter, damaging large populations of this keystone plant.
Other plant species may be able to evolve, permitting them to maintain their original natural ranges. For example, common milkweed (Asclepias syriaca) has been able to adapt and thrive despite the increased temperatures of cities like Washington, DC, and even to extend its range southward.
Photo © Elaine Mills
Photo © Elaine Mills
Opinions differ about the possibility and value of introducing plants native to the Southeast to our region and regarding the consequences for the animals that depend on them. In an article published in the newsletter of the Ecological Landscape Alliance, Dr. Jenica M. Allen reports that her research suggests that planting native species from other areas of the United States “may be a viable adaptation strategy not just for managed landscapes, but for natural areas as well.” Entomologist Doug Tallamy believes that climate-displaced plant species may be able to slowly move their ranges northward and up mountains in sync with the insect species they have co-evolved with, the insects providing pollination services and the plants offering nectar and pollen and serving as host plants.
Scientists at the University of Maryland point out, however, that for natural northward migration to occur, there must be extended, contiguous blocks of natural area. Plant species that are already located at the top of slopes in mountainous regions will have nowhere to migrate to. Plants in tidal habitats threatened by sea level rise and intense storms may be limited in their migration by development of coastal regions. In addition, they suggest that plants acquired from too far south will perform poorly due to different winter conditions and day-length patterns. They warn that “assisted migration” of plants should only be undertaken after careful study to prevent potential hybridizing with related species or the spread of pathogens or insect pests, such as southern pine beetle, that could have unpredictable effects in the new environment
A recent blog post on the website of Earth Sangha, a volunteer-based ecological restoration program for the greater Washington, DC region, comments that studies on possible climate shift patterns and responses of native plant communities have only begun, making it difficult to determine the proper course of action with regard to selecting species from further south. Staff advise that the best approach to planting for resiliency is to maintain good stewardship of our lands: supporting the conservation of existing natural areas, creating habitat corridors for flora and fauna, and purchasing native plants for our own gardens that are sourced from local plant populations that have already adapted well to current levels of climate change and whose genetic diversity will allow them to undertake further adaptation.
References
- Allen, Jenica M. “Using Gardens for Climate Change Adaptation and Conservation,” September 15, 2015. Ecological Landscape Alliance Newsletter.
- “Gardening for Climate Change,” National Wildlife Federation website.
- Higgins, Adrian. “How hotter summers and warmer winters are changing the garden,” Washington Post, June 2, 2021.
- “Planting for Resiliency in the Face of Climate Change,” Earth Sangha website.
- Sundberg, Maureen. “Local Ecotypes – What’s Your Interest?” November 15, 2019. Ecological Landscape Alliance Newsletter.
- Tangren, Sara. “Native Plants and Climate Change.” Updated April 20, 2021. University of Maryland Extension.