As we move into fall, we are starting to see a wide variety of colors among the leaves. The changes in the length of daylight and temperature causes the leaves to stop their food-making process. The leaves stop making chlorophyll which results in the green color starting to fade while the reds, oranges, and yellows become more visible. There are four main groups of biochemicals that cause the variety of reds, oranges, and yellows: chlorophyll, carotenoids, anthocyanins, and tannins. Each of these has their own color and chemistry resulting in a variation of color from one leaf to the next.
Chlorophyll: A pigment that causes the green color we see in leaves throughout the spring and summer. It gathers energy from the sunlight in the process of photosynthesis. As the days get shorter and the temperatures decreases, chlorophyll is broken down faster than it is produced. As it breaks down, the carotenoids are revealed.
Carotenoids: The yellows and oranges we see are the result of the carotenoid compound. Carotenoids are present throughout the growing season but are only revealed when chlorophyll breaks down. You will see the brilliant colors when the fall weather has warm, sunny days with cool nights.
Anthocyanins: The red and purple colors you see in leaves are caused by anthocyanins. You get these colors when sugars combine with compounds called anthocyanidins. The pH of the cell sap in the leaves can affect the color produced by anthocyanidins. With an acidic pH, the anthocyanins are usually red. Alkaline pH turns them a purple color. Bright, sunny days produce the best color rather than cloudy days. Combinations of anthocyanins and carotenoids can result in yellow, orange, and red colored leaves at the same time on the same tree.
Tannins: Typically, oak trees do not have a variety of fall colors. They often will turn brown instead. This is because of the group of compounds called tannins. Tannins are revealed when chlorophyll and carotenoids break down in the leaves.
The type of tree can affect the colors that you will see present in the leaves. Below is a list of color information according to the tree.
- Ash: Green ash leaves turn yellow, but white ash turns a purplish color.
- Elms: Elm leaves turn a variety of yellow shades and some will even turn brown.
- Hickory: Hickory leaves start by turning yellow and then turn to brown before falling.
- Maple: Soft maples leaves turn yellow. Hard maple leaves turn bright red before falling.
- Bur Oak: These leaves turn a buff/yellow color while on the trees and then turn into a brown color before falling.
- Oak: Red oaks have bright red leaves. Although they are not as intense as the hard maple leaves. White oaks have a more dim, purple color. The leaves will turn brown, but will often times stay on the tree until the new leaves begin to grow in the spring.
When do colors peak?
Typically, peak fall colors in northern Iowa occur during the weekend closest to October 10th. Below is a map of fall colors for Iowa according to the Iowa Department of Natural Resources.