Soon the lush leaves of green that shaded backyard picnics and whispered in the breezes of summer will change to red, orange, yellow and purple as the season turns to autumn.
Marylanders are fortunate to witness this phenomenon. It doesn't happen everywhere.

Mike Kay, a forester with the Maryland Forest Service in Frederick County, recalled meeting with a group of farmers visiting from Guatemala when he started with the service 21 years ago. "One of them said, 'We've never seen such color on trees.' They thought the leaves were this color all the time. I had to explain to them it was autumn," said Kay.

Why leaves change color

According to an American Indian legend, the spectacular color change first came about when hunters in the sky slew the Great Bear in autumn. The bear's blood dripped on the leaves, changing them to red. The hunters cooked the meat, and fat splattering from the kettle turned some leaves yellow.

A Cherokee legend says that, in a time when only plants and animals lived on earth, the Great Spirit told the plants and animals they would receive great powers if they could stay awake for seven days and seven nights. The few creatures that did were granted the gifts of eternal color and the power of night. Trees such as pine and cedar were allowed to stay green. Other trees had to shed their leaves and sleep during the winter.

Now we know the science behind what causes leaves to change color and fall. The vibrant hues are there in spring and summer, but hidden by the green pigment of chlorophyll. Fewer hours of daylight and the cooler temperatures of fall trigger deciduous trees to develop an abscission -- a thin layer of pithy cells at the base of their stems. During the growing seasons, this stem is the pathway of nourishment. The pithy layer cuts off the flow of food and water between the leaf and tree. Gradually, it separates and the leaf falls.

Before that happens, the chlorophyll starts breaking down. "Once that happens, other pigments become more visible," said Kay.

What color they turn depends on which pigments are present, and that is specific to the tree. Black gum and scarlet oak turn red. Hickory, tulip poplar and sugar maple leaves become vibrant yellow. "Some trees like red maple can have a lot of colors," he said. Horticulturists have developed varieties with other characteristics, but in nature each species has its own coloration.
"There are some species that turn faster than others," said Marsha Williard, garden center manager at Mar-Lu View Nursery and Landscaping in Jefferson. "We do have some customers ask for (trees and shrubs for) a succession of colors, with different colors going on at the same time."

Ash trees, with leaves of yellow, are among the first to turn, said Dave Stadler, part of the family-owned Stadler Nurseries in Frederick. "Everything will follow suit with the oaks last to turn color."