Monterey County conservationists launched a project yesterday to get rid of an invasive weed in the Salinas River. Over 1,000 acres of arundo plants will be removed from the 50-mile stretch between Greenfield and the Monterey County-San Luis Obispo County line. The ‘giant reed’ was introduced in the 1940s to prevent erosion. The plant has since become a problem for farmers and environmentalists, wasting water and blocking the flow of the river.