By Andrew Bunting, PHS VP of Horticulture 

With global climate change creating extreme weather events, such as extreme summer temperatures, periods of drought, warmer than normal winters, periods of significant precipitation inundation, etc., we need to consider plants and plantings that will be more tolerant of these extremes.

Here are PHS's three tips to mitigate climate change in the garden.

Choosing Plants for the Changing Climate  

Certain types of plants or even specific species are already showing the effects of the changing climate. For example, the true cedars, Cedrus which are native to higher elevation mountainous regions of the world seem to be suffering from hotter and longer summers. The cedar-of-Lebanon, Cedrus libani is native to the mountains around Lebanon and the Atlas cedar, Cedrus atlantica occurs in the Atlas mountains in northern Africa. Both of these regions of the world are dry with low humidity climates. Cedrus atlantica and Cedrus libanii for decades were stately conifers that thrived in this region with beautiful specimens at both Philadelphia-area venues Tyler Arboretum and Scott Arboretum; however, with changing climates both species are suffering. 

Scientists have predicted that certain species like the native white oak, Quercus alba will succumb to the changing weather over time, and this species will retreat to the north where the summers are cooler. 

Top Native Trees for Hot and Humid Summers 

The Pennsylvania Department of Conservation and Natural Resources has published a helpful gardening resource, Selecting Trees for Pennsylvania’s Climate Change. This document focuses on native trees that will be tolerant of hotter and more humid summers, drought conditions, and wet soils due to episodes of large rainfall events. 

The Kendall Crosslands Arboretum at Kendall Crossland Communities in Kennett Square, PA has developed impressive curatorial tools for the campus arboretum to start to proactively plant to mitigate climate change. They have developed a list of trees exhibiting low to medium vulnerability to climate change in the Philadelphia area, including those existing plants in their collection. They have created a long list of “target species” for climate change to guide future plantings. The document states: “Climate change resilient trees for PA should be southern tree species that tolerate/prefer wetter soils and warmer temperatures."

This list includes many “southern” species including native species that currently exist in North Carolina, South Carolina, northern Florida, Georgia, Alabama, and Mississippi. These plants, in theory, will have genetics that make them better adapted to hotter, drier, and longer summers.  They have included Southern natives, such as mountain pepperbush, Clethra acuminata; Cornus florida subsp. urbiniana which is a selection of the flowering dogwood from Texas and feverfew, Pinckneya bracteata which is native to Georgia. 

Heat-Tolerant Plants from Around the World 

Other plants from across the globe from more southern latitudes than Philadelphia should prove to have heat and humidity tolerance. This might include plants from central or southern China, southern Japan, Taiwan, and possibly even northern Vietnam. On their list is the amazing Emmenopterys henryi which has large tropical-looking leaves and an amazing profusion of white bracts in the summer. Cornus wilsoniana is a Chinese dogwood that currently thrives in Raleigh, North Carolina. This dogwood species develops alabaster white bark over time. Pinus ayacahuite is a relative of the white pine, Pinus strobus which is native to Mexico. It exhibits many of the attributes of the ubiquitous white pine, but over time should be more tolerant of hot summers and relatively warmer winters. Quercus myrsinifolia is the Chinese evergreen oak. 20 years ago, this evergreen tree was considered marginally hardy in the Delaware Valley and New Jersey. Today, it is a small tree that reaches 30 feet tall at maturity and provides considerable winter interest with its evergreen leaves.  

Cultivar selection can also play a role in mitigating climate change. For years, Betula nigra Heritage® was the most popular cultivar of river birch. Heritage® was made from a population of river birches with climatic conditions similar to Philadelphia. In more recent years, Dura Heat® has been promoted because it was selected from a more southern native population which has heat tolerance embedded in its genetic make-up. 

Following these three steps for managing climate change in the garden and landscape will undoubtedly evolve over time as new information and research results are made available. The home gardener can play a role by observing which plants seem to be struggling and which are thriving using these changing conditions as a foundation for future decisions.