GARDENING: Fall colors — The science behind leaf-turning |

GARDENING: Fall colors — The science behind leaf-turning

Curt Swift
Free Press Gardening Columnist

Paul Jensen, an acoustic guitarist, recently returned from Vermont where he recorded 14 new songs for his next album. Paul, a singer/songwriter, has a number of prestigious awards under his belt and I’ll be adding his new album to my collection as soon as it is available. I happened to run across Paul and his brother, Dale, while picking up a latte at Roasted on Fifth, where we conversed about his recent trip to Vermont and the change in color of the foliage of hardwoods.

I grew up in northern New Hampshire where much of the forest is spruce. There were several ash and sugar maples around our country home but not in the number to create the fall colors much of Vermont and the southern part of New Hampshire experience.

During our talk I mentioned the change in color is due to stress, the stress of cooler nights and the reduction in length of day. These environmental changes result in the build up of anthocyanins, red pigments we sometimes see occurring in the shrub oak on the foothills of the Grand Mesa. Anthocyanins have important roles in plants and serve many multifaceted roles in human health maintenance as well. We take these flavonoids as nutrient supplements when we consume tablets of ginkgo biloba and grapeseed extract. These same flavanoids are produced by plants to protect their cells from excessive levels of sunlight and helping the plant recover the last bit of nutrients from the leaves.

When there is an abundance of sugars in the leaves in the fall, the amount of anthocyanins increase. This happens when we have cool nights to slow down respiration. Warm leaves burn up more carbohydrate than cool leaves so when our nights are warm, anthocyanin development is less and the color is less brilliant than when we have cool nights. Respiration takes place all the time burning carbohydrates, using oxygen, and giving off carbon dioxide. The only cells that don’t respire are dead cells.

The environmental changes responsible for the development of anthocyanins also create conditions that result in the degradation of chloroplasts, the organelles containing the green and blue pigments chlorophyll a and chlorophyll b. With the death of these green light-absorbing organelles, the yellow pigment is “unmasked.” In other words it shows through. The yellow pigment was always there, helping the chlorophyll molecules harvest light energy. Since they absorb a slightly different spectrum of light and reflect parts of the yellow and orange spectrum, we see them not as green pigment but as yellow and orange. The yellow and orange pigments (some appear red, but not as brilliantly as anthocyanins) are called carotenoids.

Different substances in the leaves absorb different wavelengths of light in differing amounts. The wavelengths which they do not absorb are reflected. The reflected light of a particular wavelength of an object is perceived by us as the object’s color.

We provide plants various colors to enhance the growth of plants. We know far-red light inhibits germination so we need to choose a light source with little if any far-red being produced. If we want to promote vegetative growth, we should be using a light source that produces mostly blue light. When we want to increase flower production, we need to shift to red light (not far-red). Each of these colors has a different wavelength and different amounts (packets) of energy. Many growers are going to light-emitting diodes (LEDs) to better control the spectrum (wavelength) of light their plants receive. It is not the intensity of the light that is important but the wavelength. There is no reason to provide plants the wavelengths they can’t use. While we see different colors, plants are very selective on what they need and can use.

Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at, 970-778-7866 or check out his blog at He owns Swift Horticultural Consulting and High Altitude Lavender.

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