How do trees get their fall color?

It’s autumn and there is a crackle in the air. Aside from the time for soccer, school activities, and upcoming holidays, it’s time to soak up some fall color from our trees before full leaf management gets going and I still need to remind homeowners and businesses to mulch the leaves instead of mulching them send to landfill.

What causes and controls the fall color anyway? Temperature? Soil moisture? Shorter days? Sunny days? It’s actually all of these factors, but technically this is what happens.

Green chlorophyll is found in abundance in leaves during the growing season. As the days get shorter, chlorophyll production slows down.

Orangeola is one of Howard Garrett’s most popular Japanese maples and has an impressive fall color.(Howard Garrett / special article)

When chlorophyll breaks down in the leaves, two compounds called anthocyanins and carotenoids take over. Carotenoids are leaf pigments that are responsible for yellow and orange colors. They are present in leaves during the growing season but are masked by the green chlorophyll, except for plants that are stressed or have naturally yellow leaves. Carotenoids are helpful because they absorb wavelengths of light that chlorophyll won’t accept – mainly blue-green and green. They also use up excess energy that is produced in the leaves, as is the case with bright lighting. In autumn they can serve as sunscreen to protect the leaves without chlorophyll.

Anthocyanins are pigments that are responsible for red and purple autumn leaf colors. These compounds are only produced in the fall, when sugar is trapped in the leaves. They work much like carotenoids to help the leaves use up any remaining energy when the chlorophyll goes away. Dry weather and sunlight lead to more sugar in the leaves, resulting in lighter autumn reds. More red in the leaves and earlier shades of red can sometimes result from plant stress from poor nutrient intake, temperatures near freezing but not freezing, and root flickering covered in plants, soil, and mulch.

Hard freezing stops the process of pigment formation, and when colder weather sets in, even these color pigments decompose, leaving behind brown tannins. Some trees hold the brown leaves all winter until the buds swell before the leaves emerge. This condition is known as marcescence.

Soil moisture, cold nighttime temperatures, and nutrient availability all affect the amount of anthocyanins and carotenoids leaves can produce, but cool to cold nights, soil on the dry side, and bright sunny days produce the most brilliant fall color. Some people mistakenly believe that more rain and soil moisture will help the color fall, but just the opposite is true. Trees that are easily stressed produce the most vibrant fall colors.

The fall colors of North Texas can’t match the great show trees put on in colder parts of the country, but there are some tree species that are fairly reliable in color. My favorites are Texas Red Oak, Ginkgo, Texas Ash, Bigtooth and Caddo Maple, Flameleaf Sumach, Persian Ironwood and especially the Japanese maple.

The fall color of Texas ash ranges from bright yellow to deep red, all on the tree at the same time.(Howard Garrett / special article)

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