[Cal-IPC] Invasive: Elymus caput-medusae (medusahead) is a winter annual (family Poaceae) that typically invades disturbed sites, grasslands, openings in chaparral and oak woodlands. Medusahead out-competes native grasses and forbs and is found throughout northwestern California. After they set seed, medusahead plants persist as a dense litter layer that prevents germination and survival of native species, ties up nutrients, and contributes to fire danger in the summer. Cal-IPC Rating: High (link added by Mary Ann Machi)

[Wikipedia] Europe, North Africa, Asia native, Range: 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).[2][6][7][8][9] It is also naturalized in southern Australia, Chile, and parts of North America.[10][11][12][13] This aggressive winter annual grass is changing the ecology of western rangelands in North America.[14] It was first observed in the United States in Oregon in 1903 by Thomas Howell. Forty-eight percent of the total land area of the United States is rangeland, pastureland, national parks, nature preserves, and other wildlands. These lands are essential for agriculture and for protecting the integrity of ecological systems. 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.[16] (link added by Mary Ann Machi)