Autumn always seems to sneak up on us.
Finally, drier days and cooler nights replace the heat and humidity of summer. Here and there, we glimpse autumn colors peeking out of the green landscape. Then, just as we’re beginning to enjoy them, those warm hues are replaced by dismal browns as leaves carpet our lawns and gardens.
Actually this leaf shedding process, known as abscission, has already been occurring for several weeks. Cells located at the spot where the leaf stem attaches to the tree, toughen and begin to form a protective waterproof scar. The cells in the leaf stem then swell, weaken and degenerate. This interferes with the flow of moisture and nutrients into the leaf, reducing the production of a pigment, known as chlorophyll, which gives leaves their green color.
Why do the leaves turn colors?
The leaf is the food factory for the tree. Chlorophyll uses the sun's energy to convert carbon dioxide and water into sugar, food for the tree. As the days shorten, there is less sunlight energy 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 Nutrients and minerals are withdrawn from leaves and transported to the permanent parts of trees such as the trunk, stems and roots.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 also contain other pigments that give them their fiery fall colors. These other colors are hidden most of the year by the abundance of chlorophyll, making leaves appear green. Leaves reveal their autumn colors as chlorophyll breaks down and other pigments are unmasked.
Xanthophyll produces the color yellow and carotene, like that in carrots, produces yellow-orange. Leaves continue to produce sugar during the day, but cold night temperatures prevent trees from withdrawing the food out of the leaves. Sunny days and cool nights can produce a sugar-related pigment, anthocyanin, which produces fiery reds. Other chemicals and breakdown products produce bronze, purple, and crimson.
Birches, beeches, and tulip poplars turn golden. Sassafras trees 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 red maple and staghorn sumac are two of the more vibrant red trees. Vines such as Virginia creeper and poison ivy also turn crimson. The oaks turn a variation of yellow, orange or bronze. Leaves stay on oaks the longest and it is often their dry leaves that you hear rustling in the breeze.
Gradually, the bond between leaves and a tree weakens. The tiny veins that carried sap to the leaves all summer long are sealed off. Leaves then fall to the ground encouraged by wind or by the sheer weight of gravity. Now the dominant color is brown as the chemical reaction of decomposition starts.
Don’t bag those leaves!
Dry, brown decaying leaves may not be beautiful, but they are valuable. Instead of bagging leaves, trying composting them. It’s the most ecological and economical (and least labor intensive) way to dispose of them. As the leaves decompose, they release nutrients back into the soil. Mulched leaves can be left on lawns and will actually enhance the soil, reducing the need to fertilize your yard next spring.
Leaves also form an insulating barrier around plants, reducing moisture loss and damage from severe winter weather. By putting whole and composted leaves on yards and gardens, you can reduce the need to fertilize. This cuts down on the amount of nutrients that runoff the land into streams and rivers and, ultimately, the Chesapeake Bay.