Plant zones and changing climate
With the climate continuing to change, much of our focus is spent studying the adaptation of plants to the new climatic paradigm. We are concerned that too much time is spent on the hand wringing of those who wish climate change wasn’t happening, instead of learning to cope, and embracing the changing conditions.
At JLBG, we have 60 years of local weather data. In the 20 years between 1973 and 1992, we recorded 250 nights where the winter temperatures dropped below 20 degrees F. In the next 20 years, between 1993 and 2012, the temperature only dropped that cold on 125 nights…half of the total from the previous period. We are only 10 years into the current 20-year period (2023), but so far, we have recorded only 35 nights below 20 degrees F. Extrapolating that to 20 years, would have us only experiencing 70 nights below 20 degrees F. This would be yet another dramatic drop in cold winter weather.
Let’s put this in perspective; the winter of 1976/77 recorded 38 nights where the temperature dropped below 20 degrees F. During the last decade from 2013/2014 through 2022/2023, we only recorded 35 nights below 20 degrees F. In other words, a single year in the 1970s was colder than an entire decade that occurred 37 years later.
As we mentioned in a recent blog, the new USDA Hardiness Zone Map, shows our location outside Raleigh, NC, as moving from Zone 7b to Zone 8a. Since the purpose of the map is to show people what they can grow in each zone, this is troubling. Using a 30 year data set, our winter low temperatures average out to 13.03 degrees F. That temperature is certainly Zone 8a, but when you look at the low temperatures experienced during that 30 year stretch, we experience six years where the temperatures were well below Zone 8a.
During that stretch, we have recorded:
4 Zone 9a winters, with a low of 20-25 F
7 Zone 8b winters, with a low of 15-20 F
13 Zone 8a winters, with a low of 10-15 F
3 Zone 7b winters, with a low of 5-10 F
2 Zone 7a winters, with a low of 0-5 F
1 Zone 6b winter, with a low of -5-0 F
Therefore on 6 occasions during the last 30 years, all true Zone 8a plants would have died. Consequently, we are not able to grow more plants that are solely sensitive to the absolute lows, like Agave victoriae-reginae.
We are, however, able to grow more plants, like windmill palm, that were more sensitive to the often repeated low temperatures of decades past. In the 1980s, Windmill palm (Trachycarpus fortunei) was not reliably winter hardy for us. Today, it is one of the most reliable palms in our region.
Despite this dramatic warming, we still must be wary of climatic outliers. From 2020 to 2022, much of Texas experienced devastating winters that would be normal for a Zone 6b/7a climate, instead of the Zone 8b/9a climate in which they occurred.
Most plant species that currently exist, including those that we call “native”, evolved between 10,000 and 100,000 years ago, when the climate was far different than it is today. Some plants are very adaptable, while others aren’t. This is why in-situ conservation isn’t the best game plan for preserving species.
If we don’t study to find out where each species can survive, outside its current native range, we are at high risk of losing that species. To do that we must overcome the closed mindset of those who insist that the location of the worlds flora must remain static. It is this mindset that is causing plants to go extinct at an alarming rate. Even the current location of the continents on the Earth didn’t reach its current configuration until 66 million years ago. Prior to that point, there was no North America, and consequently, no North American native plants. We also didn’t have any artificial geopolitical boundaries to use for profiling whether a plant was good or bad. We must embrace the concept that native is not a place in location, but instead a place in time.
Many plants have relictual hardiness that belies their current natural range. Some were moved around by glaciers, while others lived atop tall extinct volcanoes, which now have sunk low enough to be tropical. Ex-situ trialing can then lead to re-introduction of the plants back into the wild in a completely different location than humans have previously known the plants to exist.
We feel that ex-situ conservation and climatic adaptability trialing is essential for the long term survival of currently existing plant species. We encourage other botanic gardens to join us in dramatically expanded trials, and then sharing of that information in a public forum.