The Chemistry of Fall Colors

Sometimes we as agronomic professionals become a bit anesthetized to the amazing and complex natural systems that we work with every day. The myriad of intricate chemical, physical, and biological interactions that make our soils function to support plant growth, clean our water, recycle nutrients, and overall sustain life on this precious planet are awe inspiring. And the plant, that "simple" form of life that has existed in different forms over millions of years, and that we as professionals have harnessed to feed  the world, is a delicate and complex machine, capable of utilizing light from the sun and elements in our atmosphere and the soil to feed almost every other form of life on the planet.

While most of the time these lowly systems work without much pomp or fanfare, every fall they put on a show for all of us to enjoy.

The tourism industry is gearing up for 6-8 weeks of high traffic, full restaurants and hotels and increased incomes as travelers head to their favorite locations to view the fall colors. As days become shorter, temperatures become cooler the chemistry of the changing tree leaves begin to reveal the spectacular fall scenery.  The National Park Service fall foliage map published September 21, 2020 show areas across northern MN, WI and MI at or near peak fall color and the prime viewing will move southward over the coming weeks.

Arborists have discovered some of the key environmental conditions needed to bring out the best colors are adequate summer rainfall with good growing conditions followed by a dry fall with cool nights, warm daytime temperatures and good sunlight.

Chlorophyll (green), carotenoids (orange), flavonoids (yellow), Anthocyanins (red/purple) and tannins (brown) are present in the leaves during the summer growing season but chlorophyll dominates the light spectrum absorbing reds, yellows and blues while reflecting the green color that we see.  Chlorophyll production slows during the early fall and anthocyanin production increases which allows the vibrant colors to come into view. In late fall tannin content increases giving many of the leaves a dark brown color and the bright reds and yellows slowly disappear.

These chemical processes involve many of the same components that we see in crop production, ornamentals and foods. Anthocyanin is produced and stored in corn leaves early in the season when nights are cool and plant growth is slow and it reveals the same purple colors in corn leaves that are apparent in the trees during the fall. Carotenoids give carrots their orange color and the yellow flavonoids give egg yolks their bright color.  Many combinations and variations of these same chemical components make up the bright flowers and unusual leaf colors that we see in ornamental plants. Tannins that give leaves their brown colors are responsible for the signature flavor of a green persimmon and they give tea leaves a variety of flavors.

The staff at A&L Great Lakes Laboratories would like to wish you a safe a prosperous harvest season and we hope you take some time to enjoy the beautiful fall colors.