As the weather starts to turn a little cooler, it's time to talk about fall foliage. Leaves are already starting to change color in the Rockies and northern Minnesota. You can use this map to see where colors should be spreading and peaking all through the season!
A few of our leaves are just starting to turn color and fall, like this one I found in our WDRB parking lot last week. We still have a while to wait until our peak fall coloring. The greatest influence to when the leaves change color and fall is the length of night. As the nights get longer in fall, the leaves stop producing the chlorophyll. This allows the true colors of the leaves to start to show through. The longer nights begin the process of the leaves changing colors and ultimately falling off the trees.
Both temperature and moisture can also affect the brilliance of the fall foliage, in particular during late summer and early fall. Ideally we would like warm afternoons and crisp nights for the most brilliant fall displays. In addition, too much or too little rain can lead to less brilliant displays and can speed up or delay the color change by a few weeks. With that said, our fall colors typically peak here in late October. Click here to see the Leaf Cam in Brown County, Indiana - our local gem of fall foliage.
Why do leaves change colors?
A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. When they are abundant in the leaf's cells, as they are during the growing season, the chlorophylls' green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.
In late summer, as daylight hours shorten and temperatures cool, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease.
Pigments That Contribute To The Colors
Carotenoids are present in leaves the whole year round, but their orange-yellow colors are usually masked by green chlorophyll. As autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf's life begin to show through. These are carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between.
The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences — both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced. The brown color of leaves is not the result of a pigment, but rather cell walls, which may be evident when no coloring pigment is visible.