Unlocking the mysteries of autumn leaves

October 30, 2007|By ANNETTE IPSAN

Kicking up a rainbow of fall leaves is one of the great joys of autumn. Red, orange, yellow, purple and brown salute the season in leaves that trickle from trees and crunch under our feet. But why do they fall? And why do they change color?

Leaves fall because trees don't need them anymore. Leaves make food for trees from water, sunshine and carbon dioxide. It's called photosynthesis. The chemical, chlorophyll, makes it happen and gives leaves their usual green color.

As temperatures drop, trees realize they have enough food stores to last the winter. So, they tell the leaves to leave (Sorry, I couldn't resist a little wordplay) .

A thin layer of cells called the separation layer connects leaves to branches. Food and water pass back and forth in this layer. In autumn, shorter days and less sunlight trigger a change in the separation layer, making it brittle. Leaves' hold becomes tenuous and they drop.


Now, where do those spectacular fall colors come from?

When the separation layer gets weak, water and food can't pass between leaves and the tree. The food factory that is photosynthesis shuts down. Without water to renew it, chlorophyll disappears and you can see other pigments that have been there all along.

Oranges and yellows come from two common pigments found in foods like carrots, corn, bananas and egg yolks. Reds and purples come from sugars trapped in the leaves. Some reds come from an antioxidant common in plants like strawberries, grapes and apples. And browns come from tannins, waste products left in the leaves.

Why do oak trees hold onto their leaves?

The separation layer in oak leaves never fully disintegrates. So, oaks stubbornly hold onto their leaves all winter.

Are fallen leaves beneficial?

Fallen leaves have great benefits to the environment. Once they fall, they are broken down by bacteria, fungi, microbes and worms. Decomposing leaves enrich the soil with organic matter and nutrients. And they help us stay healthy, thanks to the exercise we get from raking them!

Should you keep some leaves on your lawn and gardens?

Allowing some leaves to collect in part of your yard is a big help to beneficial insects and butterflies, which lay eggs and pupate in leaf cover. I allow leaves to pile up along my stone wall to create a good habitat and give ground-feeding birds a place to hunt for worms.

I also use some of my leaves in my compost pile, layering them with grass clippings and fruit and veggie peels. The "black gold" that results is great food for plants. Some of my leaves also get chipped up and used as mulch on my garden beds. It's a natural alternative that blocks weeds, holds moisture and feeds the soil.

However, a heavy blanket of whole leaves is not good for plants. When the leaves get wet, they mat and encourage rot and disease. So, rake most of the leaves out of your garden and off your lawn. It's okay to chip a few with your mulching mower and let them lie, but most need to go.

Why don't evergreens lose their needles in the fall?

Evergreens' needles and leaves are covered in a waxy substance that protects them from cold and water loss in the winter. Fluids in their cells also act like a natural antifreeze. So, whether your evergreens have skinny needles like a spruce or flat leaves like a holly, they tough out the winter.

A few trees like white pines shed some needles in the fall, (that's the yellowing inside pines you're seeing now), but most hang onto their leaves all winter.

Have you ever noticed the leaves of rhododendrons curling up on very cold days? That's the plant's way of protecting itself from temperature extremes. When it warms up again, the leaves unfurl. I use my "rhodies" as unofficial thermometers. When it's cold, the leaves are standard drinking straws. When it's bitter, they are tight little cocktail straws.

Why are fall colors brighter in some years?

Water and temperature influence the intensity of our fall show. You'll see the brightest colors in years when our summer is not too hot or dry and our fall has plenty of warm, sunny days and cool nights. Our droughty year has nixed a spectacular fall display, but some colors are still quite lovely.

The bottom line is that fall leaf colors are a gift. We are blessed in the eastern part of the U.S. with some superb autumnal colors. Go out there and enjoy them!

Annette Ipsan is the Extension Educator for Horticulture and the Master Gardener program in Washington County for the University of Maryland Cooperative Extension. She can be reached weekdays by telephone at 301-791-1604 or by e-mail at

The Herald-Mail Articles