You probably remember learning about fall color when you were in elementary school. You know that leaves turn colors before they fall, and it had something to do with chlorophyll. But when is the last time you really thought about fall foliage from a botanist’s point of view?
As gardeners, we want to know which plants turn which colors so we can use them effectively in the landscape. Here in Colorado, most of us know that aspens turn yellow golden, Gambel’s (scrub) oaks become a flaming reddish orange, and burning bushes (Euonymus alatus) shine in stunning shades of fluorescent pink, purple, and red. But why exactly do they do that? And how?
It makes sense for plants to drop their leaves when freezing temperatures arrive. The leaves can’t handle severe cold, and they’d die anyway. In the case of a sudden cold snap (such as the one we had last fall), that’s exactly what happens. The leaves turn directly from green to dead brown. But most years, deciduous plants manage to make an orderly retreat. Is there a benefit to doing so?
Green chlorophyll is essential, but it isn’t the only pigment found in plants. First of all, there is more than one type of chlorophyll. Some are found only in bacteria and algae, but the plants in your garden contain both chlorophyll a, which appears blue-green, and chlorophyll b, which is yellow-green. Then there are also greenish-yellow xanthophylls, orange-yellow carotenoids, and red anthocyanins. These chemicals absorb wavelengths of light that chlorophyll cannot, so they help the plant gather more energy from sunlight than it could with chlorophyll alone.
Making all these complex organic molecules takes energy. In the fall, with cold weather soon to arrive, the leaves stop making chlorophyll. Why waste the effort now, when that energy will be needed in the spring? As the chlorophyll breaks down over the next several days, the colors of the other pigments are revealed. Actually, they’ve been there all along, and we just couldn’t see them before. Carotenoids and anthocyanins persist a bit longer, so they become the primary coloring agents for the leaves. Eventually they too break down, and the dull brown leaves are left to rot. Different species of plants have different amounts of these pigments, which is why their leaves turn different colors.
There’s a simple, fun experiment that you can easily do in your kitchen that will reveal the fall color of the leaves. It’s called paper chromatography. Take a green leaf, mash it up in a little bit of alcohol or acetone (nail polish remover), then let it soak into a coffee filter. Since some pigments will travel up the paper further than others, the colors in the leaf are exposed. You can find detailed instructions online.
I appreciate fall foliage as much as the next person, and often take time to venture into the Rockies to see the golden aspen groves this time of year. But the more I learn about the science behind the beauty, the more I appreciate just how awe-inspiring life is. If you’ve read this far, I bet you do too.
Plants, from top: maple, burning bush, aspen, unknown tree at Denver Zoo, green ash.