Fall foliage is a bittersweet phenomenon. The reds, oranges and yellows that tourists peep at from their car windows are broken remnants going out in a blaze of decay.
The leaves of a tree are like little solar panels. Though they’re useful in the sunny summer, when the days get shorter, the energy cost of keeping those leaves in shape becomes less and less justified. So, most deciduous, or leaf-shedding, trees, such as maples and oaks, have a system for getting rid of the leaves. The internal chemistry of the dying leaves can result in vivid colors, the plant’s swan song for the year.
Before a leaf falls off, the tree makes a seam between it and the branch called an abscission layer. The abscission layer dries up, and forms a cork-like barrier that prevents nutrients and other substances from being shuttled to the leaf and wasted on it. Chlorophyll, the green pigment that’s essential for photosynthesis, starts to break down in the sun with no fresh supplies coming in. The decay of the green reveals other pigments in the leaf: orange carotenoids, yellow xanthophylls, and red and purple anthocyanins.
But for the trees to really put on a show, the weather has to cooperate, explains Tom Kines, a senior meteorologist at Accuweather.
“During the month of September, maybe early October, you want adequate rainfall,” Kines said in a phone interview. “When it gets [too] dry, the trees get stressed out a little bit.”
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And if the mercury dips too low, that can stress the trees out even more. An occasional frost, on the level of say, 32 or 29 degrees Fahrenheit, probably won’t dull the colors; but go too low, and the leaves won’t be as enticing.
“A hard frost early in the season tends to turn the leaves brown,” Kines says. “On the other hand, [you don’t want it to be] too warm; you want a bit of frost that tends to bring out the brighter colors.”
The Mid-Atlantic area of the East Coast, including states like Pennsylvania and West Virginia, currently looks like a place ripe for a good leaf-peeping season. September’s weather will be the primary factor now -- one wrinkle is that many areas of the East Coast are experiencing temperatures slightly above average in the first half of the month. New England in particular has been seeing some higher temperatures, which could mean a slightly less spectacular display.
Still, “the trees are very full and healthy with lots of green foliage, so that’s a good thing,” Western Carolina University botany professor Kathy Matthews told The Weather Channel recently. “They haven’t started dropping their leaves early."
About the only thing that could really dampen the autumnal colors this year would be a late-season tropical storm or hurricane, or any other violent weather. A barrage of wind and rain would strip the leaves from the trees before they had a chance to mature into their full palette. Luckily, according to Kines, it doesn’t look like there are any big storms headed towards the Northeast – for now.