[3][4][5] The only recognized species is medusahead (Taeniatherum caput-medusae) which is native to southern and central Europe (from Portugal to European Russia), North Africa (Algeria, Morocco, Tunisia), and Asia (from Turkey and Saudi Arabia to Pakistan and Kazakhstan).

[10][11][12][13] This aggressive winter annual grass is changing the ecology of western rangelands in North America.

Forty-eight percent of the total land area of the United States is rangeland, pastureland, national parks, nature preserves, and other wildlands.

Natural areas contain many nonnative plant species that occur as self-sustaining populations in the continental United States, including medusahead.

As of 2005, medusahead infested approximately 972,700 acres (3,936 km2) in the 17 western states (from North Dakota south to Texas and west to the Pacific coast), and spreads at an average rate of 12% per year.

[15] As medusahead spreads, it can outcompete native vegetation in overgrazed rangelands, reduces land value, and creates a wildfire hazard.

In the 1960s, it was suggested by Jack Major of the University of California that there are three geographic and morphologically distinct taxa: T. caput-medusae, T. asperum, and T. crinitum.

After traveling in Russia, Major thought the proper classification for the plant introduced to North America was Taeniatherum asperum.

[17] The subspecies caput-medusae is a native species to Europe, and is mostly restricted to Spain, Portugal, southern France, Algeria, and Morocco.

[17] In Asia, medusahead is widespread in Turkmenistan, Iran, Syria, and in the northern portion of Israel, inhabiting low mountains and plateau areas.

This litter suppresses native plant growth while encouraging the germination of its own seed, and after a few years it creates an enormous load of dry fuel that can lead to wildfires.

A recently developed method of assessing greenness in aerial color infrared (CIR) imagery using Normalized Difference Vegetation Index (NDVI) values to differentiate between medusahead and other more desirable species may help land managers determine where control methods are necessary.

The relatively long period of medusahead seed dispersal from July to October may be an adaptation to increase the likelihood of adhesion to animals.

It can exacerbate the decline of sage-grouse (genus Centrocercus) as it replaces plant communities that provide critical habitat for the bird.

[23] Other species, such as mule deer and chukar partridges, tend to avoid areas overrun with medusahead because it is not a good food source.

This decrease in feeding was related back to the dominance of medusahead, which deer do not eat, and the subsequent lack of forbs.

This prevents the fire from advancing too rapidly and ensures that the current year's herbage is burned and periods of maximum temperature are long enough to kill medusahead caryopses.

[14] In the late 1960s, a study was conducted that determined the effectiveness of paraquat on medusahead control in different areas of the United States.

This study found that paraquat was effective in controlling medusahead in California, but did not suppress the grass in Reno, Nevada.

In contrast, crown rot had the greatest impact on water-stressed plants and therefore may be an effective biological control of grassy weeds in the arid regions of the western U.S.

In areas where desirable grasses have completed their life cycle by the winter or early spring, grazing during this time can help reduce medusahead.

Combining a tillage treatment followed by herbicide is most effective in controlling the weed and promoting desirable plant growth.

Without an active prevention program, this weed will continue to spread and increase its negative ecological and economical impacts.