Marianne Ophardt

Garden Tips: Awesome science behind fall leaf color

Fall has taken a firm hold across the Mid-Columbia as leaves continue to turn colors and birds take advantage of drying sunflowers.
Fall has taken a firm hold across the Mid-Columbia as leaves continue to turn colors and birds take advantage of drying sunflowers. Herald file

Fall is my favorite season of the year. Last week when I was in Spokane, I experienced exquisite tree and shrub fall color. This change of color from green to intense yellows, oranges, burgundy and bright red has always been amazing to me. I can remember as a young girl collecting leaves each fall and my mother helping me iron them between two sheets of waxed paper.

Of course, there is science behind this awesome transition. The green color in leaves is because of the pigment chlorophyll. Also produced in the leaves are yellow (xanthophyl) and orange (carotene) pigments that are usually masked by the green chlorophyll. In fall, as the days shorten and the weather cools, chlorophyll production slows, and the chlorophyll starts to break down, revealing the yellow and orange pigments.

What about red? Anthocyanins are the red to purple pigments in plant tissues. They are sometimes present during the growing season in plants with reddish to purple leaves, like red barberry or Crimson King maple. However, the red and purple pigments that show up in autumn are the result of anthocyanin production that starts as chlorophyll production slows and sugars in the leaf increase. Leaf sugar content and anthocyanin production is greater when sunny days and cool nights prevail, providing a more spectacular display of intense fall colors.

Why do some trees like gingko and birch only have yellow and gold fall colors and others like red maple and scarlet oak have orange and red fall colors? While the amount and intensity is related to growing conditions and weather, the type of colors a tree produces depends on its genetic makeup.

What about trees that turn brown or copper in the fall? As just noted, some trees are not genetically programmed for fall color. Many types of oaks do not have a colorful fall display. This is because their leaves contain plant compounds called tannins. They are present all season, but are also masked by chlorophyll. When the chlorophyll disappears, the brown tannins become visible.

Each fall, I long for the beautiful autumn color display put on by the sugar maple forests of the Northeast. Thankfully, that yearning has been assuaged as more homeowners and municipalities have planted tree species that provide marvelous color.

Sugar maples do not thrive in our climate, but red maples well and provide nice fall color. Two of my favorites are the red maples, especially Autumn Blaze with orange-red fall color, and October Glory, with orange to red color. You also cannot beat the bright golden yellow of gingko trees like Autumn Gold, another one of my favorites. Add to that list Tiger Eyes sumac, American sweetgum, flowering dogwood, scarlet oak and red oak.

If you want to plant a tree with great fall color, visit your favorite local nursery to pick a tree with the fall color that you like the best.

Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.