What Causes Plants to Shed Their Leaves in
the Fall

What causes plants to shed their leaves in the fall? It is a complex interaction among genetics, light and temperature. Starting in late summer, many species of deciduous plants, including trees and shrubs, turn brilliant colors and shed their leaves. To understand the mystery behind this annual fall show is to uncover the magical factories inside the plant's leaves.

Different trees turn different colors in fall.

Starting in late summer throughout the United States, you may observe the first hint of fall color. Tiny dots of scarlet, ochre and gold sprinkle the hillsides as trees and shrubs begin to don their fall glory. What causes plants to shed their leaves in the fall and turn brilliant colors? The answer lies in the plant's genetics and reaction to its environment.

Within each cell of a plant's leaves is a substance called chlorophyll. That's what gives leaves their green color. The chemical called chlorophyll interacts with water, carbon dioxide and sunlight to create the simple carbohydrates plants need to grow and thrive.

During the spring and summer when sunlight is abundant and temperatures are warm, the plants' leaves contain plenty of chlorophyll. It masks other colors or pigments found within the leaves. Depending upon the plant, the leaves may contain varying amounts of two other chemical pigments: carotenoids and anthocyanins.

As the summer days wane, the duration of daylight and the angle of the sun's rays change as the earth moves through space. Plants can sense these minute changes day by day. As the days grow shorter, the lack of sunlight starts to signal a slowdown of food production.

Along with less sunlight, temperatures begin to cool. As the night time temperatures grow cooler, this also signals the plants to stop or slow production of food. As chlorophyll production stops altogether, the carotenoids and anthocyanins inside the plant's leaves become visible.

This combination of stopped chlorophyll production, less sunlight, and cool temperatures acts like a switch within the plant's genetic system. It flips to the "off" position and signals the leaves to stop growing and manufacturing food. First, chlorophyll production stops. The masked anthocyanins and cartenoids are now visible, revealing the leaves' hidden coats of scarlet, crimson, ochre, and golden yellow. However, as time goes by and no energy is produced in the leaves, the plant releases them and leaves fall to the ground.

Deciduous trees and shrubs lose their leaves in the fall as a protective measure. Their leaves are tender, and cold temperatures would kill them. Water flowing through their tender leaves would freeze, stopping energy production. Evergreen trees and shrubs, or those that retain their green leaves through the winter, maintain a thick, waxy coating on each needle. This waxy coating protects the leaves against cold.

There's a difference inside the leaves, too. Special chemicals act as a sort of antifreeze within evergreen needles to keep the liquids flowing through the plant from freezing. Thus evergreens can maintain their leaves (needles) throughout the harsh winter months while deciduous trees must shed them.

Now that you understand what causes plants to shed their leaves in the fall, it's fun to note which trees and shrubs have the brightest fall color. The best weather to tease out bright fall color are mild, warm, sunny days followed by a succession of cool but not freezing nights.

Carotenoid produces orange and yellow colors, and trees usually have a set amount of carotenoids that doesn't vary much from year to year. Anthocynanins, on the other hands, can vary from year to year. The more sunlight in early fall, the deeper and richer the crimson and purple anthocyanin color.

Warm periods during the fall can actually dull the fall colors by jump-starting chlorophyll production again. Summer droughts can delay fall color by a few weeks, as trees will tend to hold onto their leaves as long as possible to make up for lost time making food.

Each tree is an individual, and some trees will provide better fall color than others. In general, the following trees product these fall colors:

• Oak: brown, red, or crimson
• Dogwood: purple-red to scarlet
• Maple: red maples turn bright red, sugar maples turn orange-red, and black maples turn gold
• Sourwood and black tupelo: crimson

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