We tend to think of fall color in deciduous trees in clear terms. The fall leaves of Gingko are yellow. Those of Red Maple are red. Except, sometimes the Red Maple has yellow leaves.
Which asks a few questions.
- Why are plants not the same color year after year in the fall?
- What’s going on inside a leaf?
- Why do leaves turn color?
For the first question, the answer is not singular, but the easiest for us to manipulate is sun. Some plants only go yellow, others only go red, and some are switch-hitters (like maples). If you want more red fall color, put it in the sun. If more yellow is desired, more shade.
Red Maple at the Portland Japanese Garden, denying its namesake by being mostly yellow. It spent the summer under 50% shadecloth.
Same tree 20 years ago under the care of Anne Spencer, likely with more summer sun. In the above photo, south, and more sun, is to the right of the tree, which is where we see the blush of orange.
What then are these colors?
Green leaves sport their let’s-grow color because of chlorophyll. The molecule reflects green light.
In the fall many deciduous plants break down the chlorophyll—rich in magnesium and nitrogen—to bring back into the plant before winter. What is left behind is the interesting bit.
This Stewartia’s leaves are an intense red, which tells us they have lots of anthocyanin.
Anthocyanin is the pigment in spring shoots that give them the red blush at their tips, later turning green as chlorophyll increases. The red pigment protects the delicate young leaf from the sun. And again we see anthocyanin in the fall following the loss of chlorophyll.
A Hawthorn with reddish leaves on the outer canopy, which gets more sun, and yellow leaves in the interior, which gets less. Carotenoids makes leaves yellow. Photo courtesy Vince Smith.
Like anthocyanin, carotenoid is often left behind when the plant breaks down chlorophyll, so we see a yellow leaf. Plants produce anthocyanins and carotenoids at different levels, so we see species variations in which fall color appears.
As for intensity of color, it changes from year to year based on water availability and temperature. Sharp drops that are still above freezing improve color intensity. Too little water can dull colors. Too much stress can cause a plant to go from green to leaf drop and skip the fall fireworks altogether.
So if we were to intensify the color of our fall deciduous bonsai, we shouldn’t water stress them too much in the summer. And maybe put them in a bit more sun.
October 2025 Bulletin Board:
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Errata from the recent post Using Birds In Bonsai Display: Part I: the artist named was Iwazaki: not Iwaizaki. And unless you eat your myrtles, it’s Crape, not Crepe
- And, if you like nerdy tree info like fall color in leaves, Bonsai Heresy has reams of it. With the major gifting holidays fast approaching it’s an answer to all the money you failed to spend in September