Last week, I reviewed the history of USDA Plant Hardiness Zone Maps (PHZM) and their relative change over recent time.  These maps are intended to be a guide for gardeners attempting to predict the potential for winter injury to plants, which is a key factor in establishment and successful cultivation of woody and herbaceous perennial plants.  This week, I will look at how gardeners should interpret these maps now and in the future as the impacts of climate change accelerate.

Cold tolerance of plants is much more complicated than a mean annual low temperature value as expressed in the PHZM.  Remember, this is the average low and doesn’t represent the extreme low temperature, which is often a more critical factor in freeze injury. 

Damage to perennial plants from cold temperatures typically happens at three stages throughout the year.  First, it can occur in the fall, as the plant ceases growth and begins to harden off.  At this time, a rapid cold snap can harm plant tissues that have not sufficiently acclimated to the cold and hardened off for the winter. 

Secondly, damage can happen during the extreme lowest temperature of the winter when plant tissues may succumb to freeze injury as their adaptive mechanisms are overwhelmed.  The average low temperature does not illustrate the extreme winter lows we can experience.

Finally, damage may also occur in the late spring as plants deharden from exposure to warmer temperatures.  An out-of-the-ordinary cold snap at this time can be detrimental to sensitive plant tissues.  Thus, cold injury is much more complicated than average low temperature, and can sometimes be more dependent on timing of the cold rather than an average of the winter temperature.

Another major factor in the effectiveness of plant hardiness zone delineations is the underlying data.  As I discussed last week, recent updates to the PHZM have included more extensive datasets, with the most recent map using data from over 8,000 weather stations collect over a time period from 1976-2005.  However, this is only a snap shot in time and becomes less relevant as the current map becomes more out of date (it’s already 12 yrs old) and climate change accelerates.

It is important to note that the USDA PHZM is not intended to document climate change, but rather they are meant to be used as regional planting guides based on historical weather data.  In recent national climate assessments, a different historical dataset from 1981-2010, along with additional climate modeling data, was used to predict climate change. 

The most recent report, the Fourth National Climate Assessment (NCA4) was released earlier this year and indicates the effects of climate change in Illinois are already occurring and are happening at faster rate than previously understood.  

“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 NCA4. 

“As we move forward, we are expecting the overall warming pattern to continue and accelerate in the Midwest,” Angel added.  

This warming pattern will certainly change the plant hardiness zone maps.  Over the next 30 years, a significant shift is predicted to occur which will extend Zone 6 (currently just south of Champaign County) northward to include all of the Chicagoland area.

The NCA4 also predicts more extreme weather events, which means we may be more susceptible to out-of-the-ordinary cold snaps in spring a fall, making average low temperature a less dependable metric.  All of these findings lead me to believe that planting guides based on past weather data may become less reliable over time with accelerating climate change.

All gardeners know that microclimate can have a much greater impact on what works in your landscaping than any regional map of average low temperatures.  A well protected backyard with just the right combination of light, moisture and “urban heat island” effect can allow a southern Magnolia (Magnolia grandiflora, Zone 7 to 9) to flourish in the Zone 5b landscape of Champaign-Urbana. 

On the flipside, an overly exposed windy site in central Illinois is not suitable for a hemlock (Tsuga canadensis, Zone 3 to 7) and will severely stress the plant from desiccating winter winds, regardless of how mild winter low temperatures may be. 

Gardeners should always apply their first-hand experience with microclimate in combination with the PHZM when assessing plant suitability.  It will definitely be interesting to see how our less predictable and changing climate will impact these considerations in the future. Please join me next week as I continue with my discussion of the recent NCA4 findings and what Illinois gardeners should consider when planning for the future climate.