Couch Grass

Agropyron repens
Originally native to Europe and now naturalized all over North America, this is one of the worlds worst weeds. A single plant may reach up to 4 feet in height and spread up to an inch per day, forming a mat up to 10 feet across in a single growing season.
The creeping underground shoots are even known to penetrate blacktop. A 2 year plant may have as many as 300 miles or more of total root length. This plant is very difficult to exterminate, with any fragment of root that breaks off being capable of producing a new plant.
Couch Grass growing around desirable trees or shrubs should be sprayed with Roundup and killed. Couch Grass not only competes with nutrients and water but the roots also release a natural herbicide that inhibits the growth of neighboring plants. This chemical may even persist after the Couch Grass has been removed.