You've almost certainly seen this grass before and maybe just didn't know it. Johnson grass is an introduced grass from the Mediterranean. It can be very invasive, making pure colonies of itself along the creeks and in disturbed areas and can be resistant to herbicides. It's fairly large and reed-like, growing up to 8 feet tall, but I've usually seen it at about chest height (5 feet). It can be identified by its broad leaves with white center vein, its large orange seed heads, and prominent ligule (a little growth located where the sheath of the leaf meets the leaf blade).