A possumhaw viburnum in the fall offers the striking contrast of scarlet leaves with the plant’s blue or blue-purple berries. 

Autumn always seems to sneak up. Slowly, the heat and humidity of summer is replaced with cooler, drier days. Here and there, autumn colors peek out of the green landscape. Then, before you know it, nature’s festival of color is in full swing. And just as quickly, it seems, the brilliant fall hues are replaced by dismal browns, and leaves carpet our lawns and gardens.

Actually, this leaf shedding process, known as abscission, begins before the colors appear. As summer's heat fades, the cells where the leaf stem is attached to the tree toughen and begin to form a protective waterproof scar. The cells in the leaf stem swell, weaken and degenerate. This interferes with the flow of moisture and nutrients into the leaf, reducing the production of chlorophyll, which gives leaves their green color.

The leaf is the food factory for the tree. Chlorophyll in a leaf uses the sun’s energy to convert carbon dioxide and water into sugar, which is food for the tree. As the days shorten, there is less sunlight to manufacture food. Nutrients and minerals are withdrawn from leaves and transported to the permanent parts of trees, such as the trunk, stems and roots. Chlorophyll breaks down.

But leaves contain other pigments that give them their fiery fall colors. These colors are hidden in the spring and summer by the abundance of chlorophyll. Leaves reveal their autumn colors as chlorophyll breaks down and other pigments are unmasked.

The pigment called xanthophyll gives leaves a yellow color, and carotene produces yellow-orange. Leaves continue to produce sugar during the day, but colder night temperatures prevent trees from withdrawing the food from the leaves. Sunny days and cool nights can produce anthocyanin, a sugar-related pigment that turns leaves fiery red. Other chemicals and breakdown products give us bronze, purple and crimson hues.

The leaves of birches, beeches and tulip poplars turn golden. Sassafras leaves take on an orange tone. One of the more colorful trees, the sugar maple, may assume a yellow, orange, or red color — or any combination of these hues. The biochemistry that determines which of those colors it will be is not well understood, except that anthocyanin is abundant in red leaves and colder weather plays a role.

The red maple and staghorn sumac are two of the more vibrant red trees. Vines such as Virginia creeper and poison ivy also turn crimson. Oaks turn yellow, orange or bronze, or blends thereof. Leaves stay on oaks the longest and it is often their dry leaves that you hear rustling in the breeze. Some oak species, as well as beeches and hornbeams, hold on to all or some of their dead leaves throughout winter, in a phenomenon known as marcescence. There is no widely accepted theory as to why this happens, though some suggest it may be the tree’s way of capturing moisture by trapping snow or a strategy of waiting until spring to “fertilize” the ground below with decomposing leaves.

The final step of the abscission process is when a tree sheds its leaves. Gradually, the bond between the leaf and the branch weakens. The tiny veins that carried sap to the leaves all summer are sealed off. Wind and gravity finish the job, sending the leaves to the ground, where decomposition begins in earnest and turns them various shades of brown.

Dry, brown decaying leaves may not be beautiful, but they are valuable. Instead of bagging leaves, consider composting them and using the compost to enrich your garden soil. It’s an ecological and economical way to dispose of them and generally less labor intensive than raking and bagging.

Or you can simply mow the leaf-covered grass with a mulching mower, which chops the leaves into smaller pieces that decompose faster. Mulched leaves can be left on lawns to enhance the soil.

You can also spread whole leaves around vegetable gardens and flowerbeds or at the bases of bushes and trees. These leaves will form an insulating barrier around plants, reducing moisture loss and damage from severe winter weather. By putting whole and composted leaves on gardens and leaving mulched leaves on lawns, you reduce the need to fertilize. This cuts down on the amount of nutrients that run off the land into streams, rivers and, ultimately, the Chesapeake Bay.

Of course, leaves are good for other things too. Nothing beats jumping into a big pile of them on a crisp autumn day!

For information on leaf management and backyard composting, contact your state or local cooperative extension service.

Kathy Reshetiloff is with the U.S. Fish and Wildlife Service's Chesapeake Bay Field Office in Annapolis.

Kathryn Reshetiloff, a Bay Journal columnist, is with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office in Annapolis.

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