Fall Color Report: August 27, 2023

Fall Color Report for Week of August 27th, 2023

I am asked each year whether I expect this year’s fall colors to be “on time”, or whether they will be late. I admit that I often forget when they occurred in previous years, so I have a difficult time answering this question.

Last year I went back over all my fall color reports from 2008 to 2021 (14 years’ worth of reporting) to determine what week I told people would be peak color time for each of those years. This year, I have added in 2022 and revised the statistics. The numbers I report are for the Boone-Blowing Rock drive along the Blue Ridge Parkway, which ranges between 3,000’ and 4,000’ elevation. 

As we found last year, there were some surprising findings. 

Below is a graph labeled “Trend in Peak Fall Color in October by Year”. This is simply a plot of the day in October when colors were at their peak. More specifically, it is the day in the middle of the week that I specified in my reports. Note I have added in last year’s peak color time.


Here’s the surprising finding: despite what it looks like on the graph, there is NO statistical relationship between year and the week when peak fall colors occur. Although the slope of the line appears to be going up, indicating that peak color is coming later each month as the years go by, it is variable enough that this could just as likely be due to chance. That means we can’t conclude that there is a significant relationship between year and peak color time. At least for now.

But one senses that something is going on there. Let’s look at some other statistics to see if we can find out what else is happening.

In the other graph, labeled “Week of Peak Fall Color in NC Mountains”, I have plotted the week that my reports said would be peak fall color along the Parkway. It’ s just another way of plotting the data shown in the other figure, but this time I highlight the entire week, not just the midpoint of that week.


Note that in 2017 I have two lines. That’s because this was a very unusual year. Here’s what happened: as we got into October, it cooled down and leaves began to turn color, only to have a heat wave move in and stop further color development. Then, after a bit, it began to cool down again. But by this time, the trees that had turned color earlier had dropped most of their leaves, and without them, the forest turned back to green again, as if fall colors hadn’t even started. I had never seen that happen before! But, shortly thereafter, due to this second cool period, the remaining trees started turning color, and we had a second, albeit late, display of color, hence the two lines.

Now back to the second figure. You can see that in most years, the week of peak fall color fell within a narrow window between October 9th and October 20th. For example, from 2008 to 2016 (a 9-year period), 8 out of those 9 years had peak colors essentially “on time”, that is, between October 9 and October 18. Only one year, 2010, had a delayed color season, and it started around October 18 and extended to nearly October 24. Up to 2016, there doesn’t appear to be any trend with time.

But look at the years after 2016. Now the weeks of peak color are all over the place, and four of those six years, which includes the second line in 2017, have their peak week at least one or more weeks later than those in the preceding 9 years. But on occasion, we can have years when the colors are right on time, such as in 2020 and 2023. In fact, last year was not only on time, but also perhaps the most vibrant in terms of color for any of the past 15 years. And that vibrancy was seen all up and down the east coast, from Maine to Maryland to North Carolina. Something was going on that led trees across a wide swath of North America to produce more pigments in their leaves than in past years. Wish I knew what that recipe was!

Now, regarding this increase in the variability of peak color timing. How can we quantify this? One way is to calculate a measure of variability known as the standard deviation. I won’t go into the heavy statistics on this, but all you need to know is that if this number is high, there is more variability in your sample.

For the years 2008-2016, the standard deviation is 2.93 while for the years 2017-2022 it is 7.93. If I calculate the ratio of the standard deviation to the mean for each of these periods, which gives me a statistic known as the coefficient of variation, I find that in the earlier years the CV is 20, while for the more recent years it is 37. This means that the variability in peak color timing is almost twice as high as the earlier years. Or to put it another way, it is now nearly twice as hard for me to predict the timing of peak fall color!!

If climate change begins to extend warming farther into the fall, as scientists predict it will, then I’m pretty sure that in a few more years, the trend for later peak color will become statistically significant (the new normal!) and I’ll be able to answer the question poised at the beginning of this report by saying “Yes, fall colors are coming later in the season than they did in past years.” 

As for this year, stay tuned. If I have to guess now, I’d say we might be 3-5 days late this year, because the long-range NOAA forecast calls for slightly elevated temperatures through November. Otherwise, all other conditions point to a good fall color season (no drought, no major diseases, no new insect infestations that I am aware of). The only other cause for concern would be hurricanes, since this might be an active year. But the chances of one hitting right during peak week, which is when the leaves are the easiest to dislodge, is small. When the trees have green leaves, they are quite resistant to high winds and rains, but once they start changing color, they can be blown off quite readily.

That’s the report for this week. Forests are still green, while the dogwoods and urban maples are coloring up quite well right now. And all those brown trees along the roadsides are the black locust trees whose leaves are being eaten by the locust leaf miner (a native insect) and not a cause for concern.

Next Saturday is my son’s birthday and I’ll be going down to the Atlanta burbs to see him, his wife and my first grandson, Oliver, born just over 4 months ago. I’ll check out the trees up and back for you (I travel through Franklin, NC to Clayton, GA down US 441 to Gainesville, GA, then get on I-85 and finally the Beltway (5 hrs). A nice trip until you hit that beltway, which is always under construction. But it will be good to see everyone. Hope you all have a good week!

Photos Week of August 27th