Extensive areas of potentially productive but depleted range land in the United States are occupied by a weed commonly known aa cluster tarweed (Madia glomerata). Moat attempts at seeding these areas to useful forage plants have resulted in failure. Many of these failures have been attributed to the vigorous competitive growth of tarweed plants. More recent observations of seedings in tarweed areas and laboratory work at the University of Utah indicate that tarweed is not only a strong competitor but may also produce substances which inhibit the germination and growth of other species of plants.