Last fall, an infestation of Japanese stiltgrass (Microstegium vimineum) was found in Johnson County Iowa. This represents that first time this species has been found in that state and the furthest Northwest it has been found.
Even though stiltgrass is an annual plant that ‘comes on’ primarily in the summer, it can still be identified this time of year by the thatch. New germinates will start in late spring / early summer.
Below is a link to a document on the identification of stiltgrass that you might find useful –
Lastly, if you do know of or find infestations of stiltgrass in areas that are not indicated on this map, please enter the record into EDDMapS at www.eddmaps.org. Just from looking at the map, I would think the Mississippi River corridor between Illinois and Missouri is a prime spot to find new infestations.