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Weather has been ideal for fall colors in Inland Northwest

Thu., Oct. 31, 2013

Do the larches look especially golden this year?

The aspens, brighter than usual?

And the Japanese maples, a particularly brilliant shade of crimson?

It’s more than perception. This year’s Indian summer has produced ideal conditions for showy fall colors in the Inland Northwest.

“Everyone is talking about it – about what an amazing fall it is,” said Angel Spell, urban forester for the city of Spokane. “And everyone is posting their pictures.”

Weather has a significant influence on the brightness of leaf colors, said Katy Kavanagh, a University of Idaho professor of forest ecology.

Dry, sunny days followed by cool, dry nights are best for unmasking the yellows, oranges, browns and reds hidden by the chlorophyll in the leaves during the growing season.

Production of chlorophyll, the green pigment needed for photosynthesis, drops off as the days shorten. Chlorophyll gets broken down by bright sunlight, and when the plant can’t replace it, other pigments in the leaf emerge.

Red and purple hues are most sensitive to weather, and they are particularly vivid during autumns with lots of sunshine and cool, but not freezing, nights, Kavanagh said.

As deciduous trees prepare to go dormant, a membrane forms at the base of the leaf-stalk, disconnecting the tubes that carry nutrients to other parts of the tree. As a result, sugars get stored in the leaves instead of being transported elsewhere.

“Those sugars … really are the ones responsible for the red colors,” said Kavanagh, who grew up in Connecticut amid blazing fall foliage.

Too much rain dims displays of fall colors. And Kavanagh still remembers the year that temperatures in the Palouse plunged to 10 degrees in early October. The shock of the sudden hard freeze brought fall colors to an abrupt end. Leaves turned brown and dropped off overnight.

Spokane’s most colorful deciduous trees aren’t native to the region, said Spell, the urban forester. She’s partial to oaks, dogwoods and mountain ash, where single trees produce a spectrum of colors.

But Spell also appreciates the show that native trees put on this time of year. She recommends river corridors for displays of aspen and cottonwoods.

Alan Tower, owner of Tower Perennial Gardens, a Spokane nursery, encourages homeowners to think beyond trees when they’re planting for fall colors.

In small yards, shrubs, vines and perennials can be striking sources of color. One of his favorites is the humble strawberry plant, whose leaves turn scarlet.

Leaves are now dropping quickly, and Tower has been watching his black walnuts. In a phenomenon that intrigues him, the walnuts drop their yellow leaves together.

“They shed their leaves on the same day,” he said. “One morning I can look out, and they’re coming down like it’s raining.”

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