A question to ask when choosing plants in a heating climate.
Summers are getting hotter. As I’m writing this, BC and Alberta are experiencing a record-breaking heat wave, with actual temperature readings in the upper 40s. This spring in Ottawa, Ontario included temperatures in the mid 20s before the middle of April.
This is a trend that we’ll be living with for some time to come.
Trees planted now, if they can withstand the coming heating and extreme weather, will be experiencing this trend for far longer than you or I will.
How can we help improve the chances that the trees we plant will be around for decades to come?
Start by thinking about heat hardiness, as well as cold hardiness. The American Horticultural Society has developed a plant heat-zone map, much like the cold hardiness zone map that many of us are more familiar with.
Based on the official Climate Resiliency Report for Ottawa, by 2050, right around when trees planted now will really be maturing, summers will be 5 weeks longer than they are now, with an average of 43 days of heat warning (over 30c) weather, up from the current average of 11 days each summer. These are averages, so a warm summer will be far hotter.
This roughly coincides with the current summers in the state of Virginia.
Drought and flood cycles, as well as freezing rain and damaging winds, are also expected to become more frequent and more extreme.
Caring for soil health is one of the best things we can do to mitigate the worst of the stresses caused by both flood and drought cycles. Keep soil covered with plants and filled with roots all of the time and chop and drop all of the plant material that comes from the landscape, or just leave it in place. In moist temperate areas, water shouldn’t fall from the sky and hit the ground without its momentum being interrupted by foliage. Sunlight shouldn’t reach the soil without being captured by a leaf, or at least falling on a layer of organic matter (mulch). When soil has the support and protection of plants, it becomes a biologically active space, filled with voids that hold on to water, while allowing excess to flow down rather than away, and is bound together by bacteria, fungi and roots and resistant to erosion.
Trees do a lot to support soil health. Healthy soil does a lot to support tree health.
Once you have healthy soil, or a plan for fostering soil health, checking the current southern range of trees before planting them is one of the best things that we can do to ensure that they have the heat tolerance to grow and thrive in the coming summers, when we’ll need them the most.
While pulling together information for this post, I came across a new tool on the NRC website that allows you to input your location and get a list of what existing species are expected to still be present at various points in the future.
Searches for plants in Ottawa:
- From 1971 to 2000 come back with 1673 species
- From 2011 to 2040 come back with 1712 species
- From 2041 to 2070 come back with 1314 species
- From 2071 to 2100 come back with 197 species
They also have a resource that shows the projected range of a very long list of species, based on a variety of scenarios.
You can find all of these tools, as well as the official plant hardiness zone map, at planthardiness.gc.ca