When I used to commute, flying up and down route 495, day after day, year after year, I always wondered what all that pretty purple stuff in the lower and damper sections off the highway was. I wonder no more! :-) Purple Loosestrife, Lythrum salicaria.
From Wild Flowers:
"The Long Purples, as country people call the Loosestrife, are among the most conspicuous as well as the most richly- tinted of all the flowers which grow among the rushes and sedges of our streams. Far away, and long ere any other blossom can be discerned, the tall rich spikes may be seen bowing slightly to the breeze which ripples the current, and colouring the landscape for miles along the margin of the waters. The square stem is two, three, or sometimes even four feet high, and often more than a foot of its upper portion is during July and August crowded with the whorls of blossoms."
Pratt, Anne. Wild Flowers. London, 1853. 189. Web. Google Book Search. 15 Jul 2009.