Autumn arrived a short time back — on Sept. 22 at 8:44 p.m. to be exact. The exact time that the earth’s rotation positions the equator exactly across the center of the sun and sets the stage for fall, dances around a bit from year to year, but it is very consistently either Sept. 22 or 23. In fact that moment is even predictable if you have either a) the skills in math and astronomy to figure it or b) a ready-made table put together by someone else who has the skills in math and astronomy to figure it.
If you are a clock watcher, you probably may also recognize that the length of time between sunrise and sunset on the days of the Spring and Fall Equinoxes are exactly the same. Other than those two very precise measurements, the entrance of fall takes a while for those using the usual signs of the season as markers.
The air gradually cools and the length of time available for sunny outdoor activities gradually diminishes. Leaves on the trees gradually turn from green to yellows, oranges, purples or browns and then they separate from the tree branches. They flutter to the ground or a passing wind captures them and whips them along until they drop away from the breeze into ditches and indentations or catch at the base of fences or plants. They crunch and rustle as people walk through them or the breezes lift them.
Not every tree responds the same to the advent of fall. Some hasten to don their fall hues and doff their leaves, while others linger on, sometimes until after the snow flies. Each has its own set of instructions and the varieties follow them oblivious to the changes of other plants around them. Some trees seem to take the word “fall” personally and drop their leaves seemingly all at once. Others loiter along with colors changing and leaves flying at a much slower pace.
Watching and knowing which trees and shrubs turn which colors and how long those colors last can be helpful in planning your landscape. Why not take advantage of the variety of added vivid colors of fall as the summer colors begin to decline?
Changing colors is natural for the life cycle of these plants. As winter approaches, the trees must become dormant to weather the storms and cold of winter. Plant tissues harden as days shorten and become cooler. Leaves no longer grow larger. Instead, a corky layer of cells known as the abscission layer forms at the base of the petiole and the processes of photosynthesis halt. The abscission layer clogs the tissues that conduct food and nutrients to and from the leaves, so food production in the leaf slows down. This means chlorophyll production ceases and the green disappears.
The leaves don’t drop immediately. The brittle, corky cells hold the leaf in place until the wind or frosts force it to break allowing the leaves to fall.
The colorful pigments that we see in the fall have been there all through the summer but the green pigment created by chlorophyll production is the key feature for the summer months. It effectively masks other colors in the leaf tissue until production stops in the fall. As photosynthesis ends, the green coloring declines and the other colors begin to show up. Yellow and orange pigments are carotenoids — the type found in carrots. They form in the protoplasm of the leaf cells. Purple and red pigments form in sap in cells rich in sugar, such as are available in maples, sumac, some oaks and pears.
Dropping temperatures are important to the process of coloring leaves, but they are not as important as daylight changes. Most plants never develop good fall color unless they have plenty of light. Cloudy or rainy weather or hot dry conditions prevent pigments from developing into brilliant colors. Insufficient water during the summer months reduces the brilliance of the fall color hues.
When the weather is just right, the colors in our mountains and valleys can be spectacular.
The most stunning colors develop when days are bright and sunny and nights are cool. These same conditions produce the most vibrant fall colors in our Utah mountains. Similar conditions are responsible for the showy colors in New England in the fall. Although the same trees grow in other areas of the country, they do not develop the striking colors that appear with sunny days and cool nights. Warmer areas do not produce the same show of autumn color — at least not from the same trees — so enjoy and capitalize on this unique phenomenon of our temperate zone climate.
Frost is often responsible for leaf drop. Ice crystals form in the abscission layer of the petiole of the leaf where it attaches to the stem. This breaks the woody fibers that hold the leaf in place and as the ice melts when the sun rises the following day, the leaves flutter to the ground in a shower of color.
Some oaks and several other plants never form strong abscission layers, and their leaves remain throughout the winter. Leaves that die prematurely from diseases or other causes are also never able to form an abscission layer and remain on the tree. They do not develop the beautiful vibrant colors of fall either. Dead branches are easily recognized because they hold their leaves after others have fallen.
The scientific reasons for fall color are interesting and show the wondrous environmental changes that take place in the world around us. Learn to take advantage of these to create beauty by selecting woody plants to create and enhance spectacular colors in your landscape.
Knowing that people even plan trips around the appearance of autumnal beauty, the U.S. Forest Service has set up a hotline to help you find the places where the color of the season is peaking. To find out where these color changes are taking place, call the Fall Color hotline at 800-354-4595 or go to their website at www.fs.fed.us/fallcolors/2013. The service has also started some smartphone apps. Find more information on their website.
The hotline was originally designed to begin on Sept. 22, the first day of fall, but to accommodate northerly states where the changes begin earlier (such as Alaska and Maine) the hotline information opened on Sept. 6. Fall is a great time to visit our forested land. There are fewer crowds, fewer bugs and gorgeous fall changes taking place.