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While fishing on Saturday 9/29/07 in the headwaters of the Mattawoman River (Charles & Prince George County)
I photographed wildflowers that I've never seen before.  The leaves looked like Arthraxon hispidus, an alien,
miniature deertongue-looking grass but even more so like Asiatic dayflower or Virginia dayflower.

Using a few wildflower books from home, nothing keyed-out, except that the plant is in the dayflower/spiderwort family. 

At work today, using my good books, I keyed the plant as marsh dewflower, Murdannia keisak (Gleason & Cronquist, Manual of Vascular Plants and the more important Illustrated Companion to Gleason and Cronquist’s Manual).  Once I know I've got the plant right, I always do a Google image search to further qualify my identification. 

It turns out that marsh dewflower (a.k.a. Asian spiderwort and marsh dayflower) is a member of the spiderwort family and is a native to eastern Asia (Japan, Korea and China).  It was first documented in literature in 1935, occurring in cultivated rice paddies in South Carolina.  It has escaped and become established in the wild in 18 southern states, just beginning to reach into Maryland. 

Marsh dewflower (an OBL, obligate wetland species) prefers damp soil at the edge of freshwater tidal marshes, around ponds, and along slow-moving streams.  Its aggressive growth enables it to out-compete native plants by forming dense monocultures.  The flowers create several seeds each and are primarily dispersed by wildlife and/or moving water.  Flowering occurs from late August through September.

A 2004 document produced by MD DNR (Classification of Vegetation Communities of Maryland) makes the first Maryland note that “the marsh dewflower is an aggressive alien weed that can negatively affect Maryland wetlands.” 
It goes on to state that “farther south, in the south Atlantic coastal plain, the plant can be found on flats adjacent
to tidal reaches of rivers.” 

I found the plant along stream and pond margins, south and downstream of Cedarville State Park.  I was first attracted to photographing aggregates of purple gerardia, Agalinis purpurea and then began to notice more and more of the dewflower. 

I fear that the plant may be getting a foothold in the upper Mattawoman watershed.  For the most part we do not have too many invasive, non-native wetland plants, but the list is growing.  This year is the first time I've seen creeping jenny Lysimachia nummularia (OBL) at several job sites on both the coastal plain and piedmont, and now this. Move over Microstegium, mile-a-minute, Ampelopsis, Asiatic bittersweet and Arthraxon, here comes marsh dewflower!?