Garden Variety: Many factors play a role in deciduous trees’ fall color

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Why do some trees display brilliant fall color while others drop their leaves without a show? Fall color in leaves of deciduous trees is determined by genetics, site conditions and weather. Here in Kansas, weather and site conditions can be especially limiting and make the extent of each year’s display difficult to predict.

Changes in leaf color on deciduous trees are triggered by the onset of shorter days and cool temperatures. Deciduous trees are ones that shed their leaves or needles each year, as opposed to evergreen trees, which remain green and retain their leaves or needles. During the growing season, leaves appear green because of the presence of chlorophyll, although they contain yellow, orange and gold pigments as well. The arrival of fall weather causes the trees to slow photosynthesis and chlorophyll production, allowing the other pigments to show through.

Red and purple pigments are not present throughout the year but develop in the leaves with increased sugar content. Chlorophyll production must still slow for the color to shine through.

Trees display the best fall color in years when there are many bright, sunny days with cool, clear nights around the same time that day length is shortening and generally cooler temperatures arrive. Ample rainfall (but not too much) is also important. A warm, dry autumn or one with too many cloudy days can delay the onset of fall color or stress trees. Then, sudden temperature changes may make leaves fall without much of a display or turn from green to brown.

Cool nights and sunny days are especially important for the development of red and purple pigments.

In the Lawrence area, mid-October is typically the peak of fall color.

Going back to genetics, maples are probably best known for their fall color displays. Different species and cultivars reliably display vivid reds, golds and oranges. Leaves tend to color early and remain on the tree for a few weeks to a month.

Blackgum and sweetgum (not related) also have good fall displays of reds, golds and oranges in most years and often have a mix of colors on the same tree. They may also show some purple hues.

Most ash trees turn yellow in fall, but a few varieties are known for their purple coloration. Unfortunately, much of the ash population in the U.S. has been lost to the destructive emerald ash borer, an exotic invasive insect pest. There are still ash trees in Lawrence currently, but the insect is taking its toll here also.

For yellow and gold coloration, look at birch, cottonwood, elm, ginkgo, hickory, honeylocust, redbud and tulip poplar.

If trees that are known for their fall color display less-than-stunning coloration from year to year and weather is ideal for color development, the problem is probably the site. Soil pH, nutrient levels and drainage also play a role in color development. This is most evident when two trees of the same species are growing a short distance from each other and display differences in fall coloration. Have soil tested and work to remedy the issues if this is a concern.

There is one downside to bright displays of fall color, especially when they arrive early in the season: They can signal stress. Young or recently transplanted trees are notorious for turning bright red, orange or yellow in August or September as a result of stress. Often, these trees are too far gone at that point and they are unlikely to leaf back out the following spring. Follow good planting practices to reduce the chances of this happening in young trees. With older trees, decline and death are more likely to be a result of many years of stressful conditions.

— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.

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