Originally native to the Mediterranean, Johnson Grass now occurs all over the world in warm-temperate regions. It entered the U.S. southeast early in the 19th century and by the 1890s was becoming a problem in Arizona's Salt River Valley. It is now common throughout the state below 6,000 feet.
(Click on the image at right to enlarge it.)
A tall, coarse grass (may reach more than 6 feet) with stout and deep rhizomes, it adapts to a wide range of conditions in the subtropics. It readily invades disturbed bottomlands, and has competitive advantage due to its rhizome system, its prolific seed production, its production of substances which inhibit growth of other types of plant, and its early seasonal growth. This is widely considered one of the 10 worst weeds in the world, and while it has not been reported on our lands, it is intruding at various locations along the San Pedro River. (These images come from photos taken along the Cascabel Road just north of Pomerene.) we should be familiar with this invasive grass and keep our eyes out for it..
As you can see from the image at left, the florescence of Johnson Grass has a distinctoively reddish cast.
(Click on the image at left to enlarge it.)
The following links provide more details and images:
http://tncweeds.ucdavis.edu/esadocs/sorghale.html (for images and click on MS Word file for text)
http://usgssrv1.usgs.nau.edu/swepic/asp/swemp/iframe.asp?Symbol=SOHA (see images and click on "Plant characteristics" for more data)