So in driving around town this September, it seemed to my untrained eye that the autumn leaves were turning earlier than usual, with some trees already golden brown or red and half empty. Could it be a by-product of the hot summer?
I asked some experts at the University of Washington Botanical Gardens and it turns out it's yes and no.
First of all, according to Dr. Sarah Reichard, the director of the University of Washington Botanic Gardens, there are some tree species around here that are typical "early shedders" of their leaves in September, like Red Maples and Vine Maples -- probably a lot of what I was seeing.
What triggers the leaf color-changing process in those trees -- and eventually the other deciduous species -- is the increasing length of nighttime darkness in September and early October. The lack of sunlight messes with the photosynthesis process and it begins to breakdown, leaving the green colors to fade away and revealing the colors of other materials in the leaves. (If you want the really in-depth botanic explanation, check out this page by the United States National Arboretum.)
So that's why the time autumn leaves begin to change are pretty consistent for any region year-to-year, because the changes in length of day are the same every year, and why the seasons begin a little earlier in the north than the south because the length of days are getting shorter, quicker.
But it turns out weather can have a hand in both how brilliant a display is… and how long it lasts.
The perfect scenario for a lengthy, robust autumn leaves season is to have sunny, warm days, and cold nights -- but not too cold -- with little or no wind and ample soil moisture. According to the United States National Arboretum, cool nighttime temperatures combined with sunny days create more of the colorful materials inside the leaves, leading to a more brilliant display of color. But if it gets too cold and we get an early frost or freeze, it destroys the process, meaning an early end to the color generations and, in turn, an early end to the season.
Drought conditions like we've had around here tend to accelerate the decay process that keeps the leaves attached to the trees, making them fall earlier. Some at the UW think this may also be contributing to what seems like more leaves on the ground earlier this year around here -- the trees are feeling the stress of the drought and the leaves don't have as strong a bond to the trees. So while temperature-wise we've had good conditions to create colorful displays so far this September and early October, the dry weather will likely mean the trees will go bare sooner. (In other words, enjoy it while it lasts!)
Other factors that can spell an early end to the season would be stormy conditions. Usually the "stormy" season doesn't really get going in Western Washington until the third week in October -- which works well since that's past the typical peak of the fall colors anyway. But pelting raindrops and strong winds will knock the leaves off earlier too.
So summing it all up for what to expect in Western Washington for an autumn leaves season this October: With the generally sunny weather and cool overnight temperatures we've had this first week of October, we should have great colors, but the oveall dry weather means the season probably won't last as long as usual.
Stormier weather is in the forecast for the end of the week and weekend which could start knocking the leaves off -- more than it would in a normal rainfall year. However, extended forecasts are trending drier again for the middle of the month so whatever survives the weekend should look really good.