This year and more specifically this fall is the first I noticed this plant around the edge of some of my ponds. What is it and can it become a problem?
It looks kind of neat so if it is not an issue I wouldn't mind keeping it around. Looks like it produces a lot of seeds. I suspect the birds have brought it in.
This first picture is actually the last one I took. The plant looked healthy and growing. The rest of the pictures were on the opposite side of the pond and looked more mature like they were near the end of their life. They could have been hit harder by the last couple frosts we have had. But they do show the seed pods better and have my fingers for size reference.
What you have looks very much like Purple Ammannia.
I got that ID from the following link, under the "Midwestern Wetland Flora Field Office Guide to Plant Species (PDF; 29.6 MB)" link. It's a PDF for Midwestern plant ID, very useful if you can take some time to learn the vocabulary that it defines at the beginning...
Looks like it is just an annual wetland plant. Not a submergence perennial so unlikely to be any more problem than any other shoreline weed. It is not going to be out in the pond to any degree.
Kind of pretty when it turns red in the fall. I like it.
