He smacked into the fence.
iNaturalist suggested blunt spikerush (Eleocharis obtusa), but this seems shorter, and the flowers seem to match slender clubrush (Isolepis cernua) more.
Could still potentially be blunt spikerush. According to https://www.illinoiswildflowers.info/grasses/plants/bl_spikerush.htm
"This plant is a summer annual about 2-10" tall."