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The Garden Scoop

Climate Change and USDA Plant Hardiness Zones Part Three

The Fourth National Climate Assessment was released last fall in two volumes, containing information about how climate change is affecting the physical earth system across the US and a detailed assessment of how those changes now, and in the future, will impact our country.  The report documents aspects of climate change already measured across the US and paints a dismal outlook, with climate change occurring faster than projected by past assessments. 

The assumption that current and future climate in Illinois will resemble climatic conditions of the recent past is no longer practical.  As gardeners and managers of the Illinois landscape, we need to have good understanding of what changes are occurring now and what is predicted in our future to ensure healthy plant populations in a changing climate.

 “The biggest impacts that we have seen so far in the Midwest are the increases in overall precipitation, more heavy rainfall events, and higher humidity levels,” says Jim Angel, State Climatologist with the Illinois State Water Survey and lead author of the Midwest Chapter of the Fourth National Climate Assessment. 

So, we are already seeing a warmer and wetter climate in Illinois.  This trend is predicted to continue, with warming accelerating as time goes on.

“I am a big fan of native species since they have a built-in resilience to climate change and variability in Illinois,” says Angel

Plants in our landscape need to be adaptable and ready to handle this change.  Native plants have excellent potential to fill this niche as they have evolved with our climate and offer some of the best chance to weather change.

As opposed to planting a new set of species in Illinois that can handle a warmer climate, one recommendation that really makes sense is to simply plant a more southern genotype of our native species.   To do this, the same native species is planted, but the individual plants used are from plant communities that developed south of Illinois, in a warmer latitude, but still within their natural range.   Those individuals will have developed genetics specific to a more southern climate and when planted here in Illinois the “more southern” genes will be beneficial as climate change accelerates.

Jim Angle also recommends, “When it comes to trees and shrubs, select those species that tolerate hotter summers.”

One great tool for gardeners to use in assessing relative heat tolerance of plants are the American Horticulture Society’s Heat Zone Maps.  These Heat Zones were developed for all of the US based on historical climate data and depict the average number of “heat days” each year for a given zone.  Heat days are expressed as any day a given region experiences temperatures over 86⁰F, which is the threshold at which most plants begin to experience physiological damage from heat.  Nowadays, most plant tags at nurseries contain AHS heat zone designations and many plant guide books communicate the values as well.

Perhaps the biggest hurdle we need to address in our gardens and landscapes is the overall increase in annual precipitation and more heavy or severe rain events.  We are already experiencing higher annual rainfall as a result of climate change and it is predicted to increase in the future.

With respect to the increasing rainfall, Angel notes that, “One consideration is to use rain barrels and rain gardens to help reduced neighborhood flooding by holding that water onsite just a little longer.” 

Our urban areas are highly susceptible to flooding due to the greater abundance of impervious surfaces, such as concrete.  In natural systems, water infiltrates into the soil profile at a much greater rate and dense networks of roots do an excellent job absorbing it.  With less infiltration and less overall vegetation to mitigate storm flows, urban and developed areas will need a greater capacity to handle storm water. 

This also translates into the microclimates that exist within our yards and gardens.  Our landscapes will need to convey storm water more differently to avoid flooding.  Rain gardens are one example of a gardening system designed to delay the delivery of storm water by holding and absorbing more of the runoff prior to sending it downstream.  Native plants are a great fit in raingardens as well, making rain gardening a great solution that we can begin to implement now. 

The most recent national climate assessment states, “Earth’s climate is now changing fast­er than at any point in the history of modern civilization, primarily as a result of human activities. The impacts of global climate change are already being felt in the United States and are pro­jected to intensify in the future—but the severity of future impacts will depend largely on actions taken to reduce green­house gas emissions and to adapt to the changes that will occur.”

Although it is important to plan resilient landscapes in the face of climate change, I think a more significant message is that we are to blame.  It is really important for all of us to take a close look at our individual carbon footprint.  We should consider the implications of every item we purchase, watt of energy we consume and mile we drive.  There is still time to reverse, or at least mitigate, future impacts of climate change, but the coming years and decades will be critical.