Up here in sugar maple land, everyone wants to know when the leaf colors will change and if it will be a good year for colors. Like always, I give them the solid scientific response to their questions of “When they are ready”… and “Yep, it could be a good year!” So, I thought I’d review the science as to why leaves change colors and then maybe you can forecast your own “fall color spectrum” chances.
Intriguingly enough, the “fall” colors are always present in the leaves, but they remain masked or overshadowed by the overwhelming numbers of the chlorophyll molecules. Now don’t disparage the green loveliness of plants because they appear to be “just the same color”. After all, what other entities on this Earth can create their own endless energy source with sunlight, carbon dioxide and water? That fact alone is 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 47 minutes and decreasing, along with 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 a 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 of 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, as well as the lack thereof, 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, will rapidly degrade chlorophyll in the leaf. The low temperatures will also enhance anthocyanin formation, creating those 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 few days, so get yourself outside to enjoy the autumn colors at their finest.