Posted: October 15, 2012
It’s a combination of several factors. First, the eastern seaboard is home to a wide variety of deciduous trees that naturally color well, such as maple, oak, birch, hickory, ash, black gum, and sweet gum. Second, at our latitude, as autumn progresses into winter, we get temperatures cooler than those of more southerly latitudes combined with days longer than those of more northerly latitudes. Third, our normal weather pattern is such that we often get long stretches of ideal conditions for the brightest colors: dry, relatively warm, sunny days and cool, but not freezing, nights.
Of course, “normal” weather varies from year to year, and we don’t always get those ideal conditions. Drought, excessive rain, cloudy days, warm nights, or an early freeze can all lessen the intensity of fall foliage; but there will always be some trees that live up to our brightest expectations.
That’s because some of that bright fall color is in the leaf throughout the growing season. We just don’t see it because it is masked by the green chlorophyll, the substance in the leaves that absorbs sunlight to transform carbon dioxide and water into oxygen and carbohydrates used by the plant.
In autumn, as the days begin to shorten, a special layer of cells develops at the point where the leaf stem attaches to the branch. This “abscission layer” gradually cuts off nutrients and water to the leaf. As a result, the green chlorophyll in the leaf begins to break down, exposing the more stable pigments present in the leaf. Orange colors result from carotenoids, the same pigments found in carrots, oranges, and marigolds, for example; yellow colors come from xanthophylls, the pigments that also color dandelions, egg yolks, and bananas, among others.
The reds and purples of autumn are the result of another class of pigments, called anthocyanins, that also give apples, grapes, cranberries, violets, and other fruits and flowers their color. In certain leaves, these pigments occur only in autumn as sugars are trapped in the leaves. Red maples, with more acidic sap, turn red; while ash, which is more alkaline, turns purplish. The amount of anthocyanins that develops depends on the weather; the ideal combination of dry sunny days and cool nights means the brightest reds and purples.
It’s a pleasure to take a hike in one of our nearby state parks to enjoy those wonderful colors of fall. But fall colors are not limited to red, orange, and yellow, nor found only in leaves. Flowers, berries, bark, and seedheads expand the range of autumn colors to include tans, lavenders, pinks, mauves, and soft rose. And we don’t all have room for a sugar maple in our backyard. Shrubs, perennials, ornamental grasses, bulbs, and annuals can bring just as much fall color as a sugar maple to your garden or even a container, but on a smaller, more manageable scale.
To find out more about prime fall foliage viewing locations and dates in Pennsylvania, visit this website: http://www.dcnr.state.pa.us/forestry/stateforests/fallfoliage/index.htm. Fall Foliage Reports by Pennsylvania Bureau of Forestry experts are posted weekly beginning at the end of September.