Biology and Management of Horseweed and Hairy Fleabane in California

In recent years, increasing populations of horseweed, or mare’s tail, (Conyza canadensis) and hairy, or flax-leaved, fleabane (Conyza bonariensis) have been observed in vineyards, orchards, canal banks, and roadsides in California, especially in the Central Valley. Numerous growers, pest control consultants, and managers have complained that the recommended rates of some postemergent herbicides, such as glyphosate, are no longer effective on these weeds. Since glyphosate-resistant biotypes of these species have now been confirmed (Shrestha et al., 2007), alternative integrated techniques need to be employed to effectively manage resistant and nonresistant biotype populations and to prevent the further development of herbicide resistance. A basic understanding of the biology of these weeds is essential to develop an integrated management approach.