Dr. Roch Gaussoin from the University of Nebraska-Lincoln posted on the Turf iNfo blog:
Prostrate knotweed is a summer annual broadleaf weed that typically begins germinating in February in Nebraska. The weed is most common in highly trafficked, compacted soils, and areas fitting this description were hopefully cultivated and overseeded this fall to improve soil structure, increase turf density, and reduce prostrate knotweed establishment next year. If cultural management is not improved, this troublesome weed will return each year.
See pictures and read more here