It always seems as though fall arrives secretly over the course of a night. And the next morning, coffee in one hand and phone in the other, daily checklist already forming a scroll in your mind, you stop your clip at the sight of that first maple, blazing orange in the morning light. This is one of my favorite moments of the year, but for those of you who both live and work downtown, you may still be waiting in anticipation for a similar occasion.

One factor contributing to this annual color show is the changing temperature. As nights grow colder and longer and trees prepare to go dormant for the winter, chlorophyll (the molecule that absorbs sunlight and uses that energy to create plant tissue, thus making the leaves big and green) breaks down, allowing for other pigments to show. Yellow comes from pigments called xanthophylls, reds and purples from anthocyanins, and orange from carotenoids. But these natural processes are complicated by our urban ecosystem, where temperatures remain slightly warmer due to residual heat released from sidewalks, roads, and buildings. This is phenomenon is referred to as the “Urban Heat Island” and is part of the reason for a delay in color change on the Greenway and other urban trees. Additionally in the Greenway’s case as a rooftop garden, soil temperatures are also often higher than those found in more natural settings. Soil depth can be as shallow as 4ft in some areas of the park and can absorb a tremendous amount of heat generated from the kinetic energy of the I-93 tunnel below.

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Drought can also play a role in autumn leaf changes. With our previous dry spell, many Greenway trees were stressed and dropped their leaves early. The perfect storm for peak foliage leaf peeping? Mild temps and moderate rainfall.

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As you wait just a few more weeks for your own inspired moment from one of nature’s most beautiful transformations, you can treat yourself to the other joys still blooming in full color out on the Greenway. A new bed of Asters are out in the Wharf District Park near the Harbor Fog sculpture and Itea virginica ‘Henry’s Garnet’ is abundant in Ft. Point Channel. So get those items checked off your to-do list and get outside!

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See you on the Greenway,

Meredith