Fall color: Why the leaves change and where to see them

This time of year, hardwood trees on the East Coast explode into a kaleidoscope of fall color.

Sweet gum trees burst into a flaming crimson. Sugar maples become red, orange, yellow. Ash trees turn red, yellow, even purple.

Some East Coast natives like to brag about their fall color. They say that here in the Northwest, by comparison, all we get is yellow.

They might want to look again.

Yes, big-leaf maples, alders, poplars and cottonwoods create smatterings of amber amidst a dark-green conifer sea. Every now and then, though, out pops a vine maple, its small, pointy leaves as brilliant red as anything in the East. Many shrubs native to the Northwest, such as huckleberry, sumac and ninebark, turn a bright red or purple in the fall.

And whenever we feel a need to call in the color cavalry, we have nonnative ornamentals to save the day. Japanese maples can turn anything from a bright gold to a deep red. Other ornamentals, such as parrotia, katsura, raywood ash and sourwood, turn a shade of red in the fall.

Hap Wertheimer, who runs Hap’s Garden Design out of her Everett home, moved here 20 years ago from Vermont.

“I miss the drama of the sugar maples,” she said. “But they have long since been replaced by the variety of things we have here.”

The bright days and cool nights we’re having this year could be setting us up for great leaf viewing in a couple of weeks, experts say.

The Northwest’s relatively mild winter climate allows nonnative trees and shrubs to grow here that generally don’t survive the cold back East, said Linda Chalker-Scott, a horticulturist with the Washington State Cooperative Extension in Puyallup.

Plus, some of the trees that grow in the East, such as sugar maples, also do fine here, Chalker-Scott said.

Many of these trees and shrubs — planted in yards, parks and commercial areas — can make an autumn drive through a residential neighborhood just as enjoyable as a miles-long quest for color into the mountains or elsewhere.

“You’ll find them integrated into landscapes here in the city areas,” said Steve Smith, owner of the Sunnyside Nursery.

Ornamentals in nurseries tend to turn color sooner than those outdoors, Smith said, “just from the stress of being in a black, plastic pot.”

The process that makes leaves turn color is well-established science. During the summer, photosynthesis produces chlorophyll, which makes the leaves green. As the days get shorter and the temperature cooler, photosynthesis shuts down, causing the green to fade. As a result, carotene and anthocyanins in the leaves, which create yellows-oranges and reds, respectively, show through.

The question of why it all happens, however — what it does for the trees — is subject to some debate, according to Chalker-Scott.

There are three theories, she said. One is that trees stop photosynthesis to protect themselves from too much light on bright fall days. The second is that trees manufacture anthocyanins as antioxidants in the late summer and early fall to offset the stress of less heat and light.

The third theory — the one Chalker-Scott put forth in her two articles on the subject — is that anthocyanins help a tree to draw sugars from the leaves back into the trunk to store them for the winter. Otherwise, the leaves would freeze and the sugars would be lost.

“It’s a natural antifreeze,” she said.

Still, this applies only to the trees with leaves that turn red. It doesn’t explain the trees with leaves that turn yellow or orange.

It’s a mystery, Chalker-Scott said.

“We might know 10 percent of the chemicals that plants make,” she said. “We don’t even come close to knowing them all.”

Regardless, it seems that bright early fall days and cool, crisp nights produce the most colorful leaves, experts say.

“It stresses the trees, which makes them have better color,” Wertheimer said.

A warm summer is good but if it’s too dry it can cause trees to turn prematurely, Chalker-Scott said. A rainy early fall is probably the worst-case scenario.

“If it rains too much, the leaves just sort of rot before they turn,” said Smith of the Sunnyside Nursery.

So far, our weather this year could be perfect for good fall leaves, experts say. Some trees have turned or are turning, but the heart of the autumn-leaf season has yet to kick in.

The trees in the mountains tend to turn earlier than at sea level. Leavenworth had its Autumn Leaf Festival the last weekend in September.

Many of the bushes that grow in the mountains, such as blueberries and mountain ash, turn a bright red, Wertheimer said.

Another advantage of mountain viewing is leaves in different places turn at different times because of the elevation changes.

Last year, “every time I went over (the passes) there was different spot that was gorgeous,” said Nancy Trucano, executive director of the Cascade Loop Highway Association in Wenatchee.

The leaves seem to be turning a little later this year than usual, she said. This week, however, nighttime temperatures have dipped into the low 40s and even the 30s.

“We’re just starting to get that cold weather that’s going to start setting things off,” Wertheimer said.

A lot of fall color can be seen in parks.

The Everett Arboretum at Legion Park has a grove of Japanese maples that are just starting to turn.

Other good places around the county include Kayak Point Park near Warm Beach, Olympic View Drive in Edmonds, places along the Centennial Trail and Squire Creek near Darrington, local parks directors say.

“I think fall is pretty neat most everywhere,” Wertheimer said.

Bill Sheets: 425-339-3439, sheets@heraldnet.com.

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