An age-old question: Why do plants have fall color?

Over the last few days I have had several people ask me, “Why do plants have fall color?” The why is tough; the how is pretty well known.

Light that we can see is picked up by our eyes and is called visible light. When all of the visible light is received by our eyes, it looks white. However, if the light is bent or refracted, visible light is spread out into many different colors of light ranging from violet, blue, green, yellow, orange and red. A rainbow is caused by the sunlight being refracted by the raindrops. These colors of visible light are what we call the visible light spectrum.

Plants do not have eyes, and cannot see the colors, but they have various pigments that react with the different colors of light. These pigments absorb the energy to start the process of photosynthesis (the process of making sugar in the plant). The most common pigment is chlorophyll, which is green.

Plants use mainly blue and red light. These colors are absorbed, and the colors not used are reflected out of the plant. Since green is not absorbed, the leaves look green, or light green.

Plants have other pigments in the leaves besides chlorophyll. These pigments are orange, red and yellow. These pigments are often called accessory pigments. They help the plant absorb different colors of light in order to collect more of the sun’s energy.

In fall, the chlorophyll starts to dissolve. Shorter day lengths and temperatures cause the chlorophyll to break down and fall colors to begin appearing. This is how the “why” may be harder. I believe it is not about fall colors, but summer growth.

During the summer, the green chlorophyll, which is more abundant, masks the other pigments. The green chlorophyll does not completely block the other pigments. For example, Modesto ash with their bright yellow fall color has pale green leaves in the summer. The green chlorophyll and the yellow pigments combined makes the leaves pale to yellow green. But raywood ash, which has red pigments combined with the green chlorophyll, makes the leaves dark green during the summer.

Plants with red fall color tend to have darker green leaves than plants that have yellow fall color. Many plants do not have a showy fall color, they may or may not have other pigments, and all of the pigments may decompose at the same time causing brown or slightly yellow leaves.

These same pigments color the flowers and fruit. Grapes that produce green grapes have little to no fall color, and grapes that produce red or purple grapes have pretty red fall color. So foliage color can give you a hint to fruit or flower color.

That was pretty technical, so maybe we should just enjoy fall color on plants.