Once again, almost like a fall ritual, everyone wants you to predict if it will be a good year for leaf colors and when will leaf colors change. As always, I offer solid scientific responses to their questions…“Yes, it could be a good year for colors” and “Leaf colors will change when they are good and ready!” So, let’s review the science as to why leaves change colors and then maybe you can forecast your own spectrum of “fall color” chances.
Intriguingly enough, the “fall” colors are always present in the leaves, but they remain masked or overshadowed by the overwhelming numbers of chlorophyll molecules. Now don’t disparage the green loveliness of plants because they all appear to be “just the same boring color”. After all, what other entities on this Earth can create their own endless energy source with sunlight, carbon dioxide and water? That single fact about leaves alone is SO VERY COOL!!
The chlorophyll molecule is not stable and must be synthesized continually by plants, requiring both heat and sunlight. With the decreasing daylength, currently down to 10 hours 57 minutes and decreasing, accompanied by diminishing outdoor temperatures, deciduous plant leaves cease their chlorophyll production. As the chlorophyll already present in the leaf begins to degrade, the varying shades of green subtly disappear to reveal other colors. Suddenly, there are myriads of hues with orange pigments called carotenes, emerging yellow pigments called xanthophylls and reddish-purple pigments called anthocyanins.
For some plants in late summer, the formation of anthocyanins is initiated by the progressive accumulation of sugars in cells of the leaves. Anthocyanins are the result of complex chemical reactions involving sugars, acids and proteins in leaf vacuoles, resulting in the creation of those stunning and exquisite red tints and hues in leaves.
Any combination of anthocyanins, carotenes and xanthophylls in the same leaf will be manifested as the infinite hues, shades or tints of oranges, like peach, apricot, salmon, etc. The color production in certain plants can be quite uniform, as in viburnums or blueberries, which by the way, can be stunning!!! In other plants, individual trees, like sugar maples, can vary quite widely between their expression of leaf colors, from stunning arrays to simply yellow. Some plants will vary in their intense leaf color expressed just on specific branches on the same tree; meanwhile other plants will show intense leaf color variations on single leaves within the same branch! Bring on the array of colors I say!
Not every plant will always exhibit the full spectrum or range of colors, even if they have the genetic potential to achieve it. Let’s look at a tree from the red oak group and use it for an example, like the Scarlet oak or Quercus coccinea. One would think that being in the red oak group and with the name of scarlet oak, that the fall color should be a glorious scarlet-red! While the potential to have incredible scarlet fall leaf colors is there, I have seen many scarlet oaks exhibiting a ho-hum, deep reddish-brown leaf color. Most oaks seem to naturally produce a burnt sienna fall “color”, due to the commingling of anthocyanins and chlorophyll pigments in their leaves.
The fall weather also exerts its considerable influence on the colors of plant leaves. Sunlight intensity, or the lack thereof, air temperatures and available water to the plant, will all impact both magnitude and persistence of fall leaf colors. Days with cool temperatures above freezing, accompanied by partly cloudy skies, rapidly degrades the chlorophyll in the leaf. Those same low, cool temperatures also enhance anthocyanin formation, creating those stunning red to purple colors.
On the other hand, early frost events weaken the intensity of anthocyanin colors. Stormy, overcast days tend to increase the intensity of most fall colors. The best days would be after a few rainy, overcast days with nice cool nights but no frosts, and cool, clear skies to see the color intensities. Well, that should happen in the next few days, therefore go outside to enjoy and marvel at the array of INCREDIBLE fall leaf colors… before winter hits and the leaves fall off!