Planning for Plant Hardiness

As the world's climate changes generally, the climate of specific places is changing in more particular ways. Even the variability of temperature, precipitation, and the timing of each increases, a more detailed understanding of biogeography is a necessary tool for climate adaptation. 

Journalist Susan Philips at WHYY in Philadelphia provides an excellent example in her recent story Climate Fixers. This five-minute story provides a lot of important insights as it explores the efforts of researchers and fruit growers who are anticipating changes in the regional climate as they plan future crops. This is particularly important in any kind of food (or beverage) production that relies on trees, because the productive years of a tree -- be it apple, peach, coffee, or tea -- begins at least a few years after planting and may continue many years after that.

See interactive map at USDA

The preparations include reliance on the USDA Plant Hardiness Zone Map, which is familiar to gardeners and landscapers. As often as I have consulted these maps, I did not realize that they are mapping just one variable as a proxy for winter harshness: the average lowest low temperature. 

A ZIP Code search of the map returns a zone rating that can easily be used at nurseries and seed companies. It also provides the average temperature for 2013 and 2023; it is not clear which years go into a given average, but it is something like a trailing ten-year average. 

Philadelphia clearly is experiencing substantial change by this measure, and as the WHYY story indicates, it might soon be in a new category altogether. 

This map is akin to biome maps, though the latter rely on a more complex set of climate metrics. In both cases, the map is pointing to past experience rather than future patterns. Choosing an appropriate period of record is important: it must be both long enough to minimize random fluctuations and recent enough to capture relevant experience.