This looks exactly like what we have been trying to id that has come up by the hundreds in a truckload of screened topsoil mix we got in the fall for new beds at the house we just built in north GA. It definitely came up from seeds, and is not in a damp location, and doesn’t look that much like polygonum photos on the USDA site. Baffled!!

The polygonum odoratum apparently would not be likely in Michigan. Its recommended USDA temperate zone is 8. A second look at lady’s thumb indicates that it can grow to 24 inches and would be found in Michigan. As Metrosideros indicated a flower would be the best option for correct ID. That should be occuring soon.

Yes, a Persicaria / Polygonum. There are many species. Need to see a flower to determine a species. http://plants.usda.gov/java/imageGallery?category=sciname&txtparm=Polygonum&familycategory=all&growthhabit=all&duration=all&origin=all&wetland=all&imagetype=all&artist=all&copyright=all&location=all&stateSelect=all&cite=all&viewsort=25&sort=sciname

It is a Smartweed variety. Lady’s thumb is a smaller growing cousin I am familar with. This is possibly polygonum odoratum. I have found most sites referring to this as Vietnamese coriander. http://www.herbsociety.org/herbs/documents/Persicariaodorata_000.pdf