Using preemergent herbicides correctly to kill weeds before or soon after they emerge is often more complicated and difficult than using postemergent herbicides. Most preemergent herbicides do not kill weed seeds. They only work on weed seeds that have germinated and they must be in the right place at the right time and at high enough concentrations or failure or partial control can result. Weed seeds are killed by some products such as the fumigants chloropicrin, metam sodium and Telone and by soil solarization and flooding. Most preemergent herbicides only work once they have been absorbed by roots and shoots of germinated seeds. Herbicides like Goal or Chateau are absorbed at the soil surface as the shoot emerges. Others are absorbed only by at the root tips as they grow into the treated soil. The location of the herbicide is critical. Roots or shoots that are already well established before they contact the herbicide often survive. Roots that have grown past the herbicide and still have vigor, will also often recover. Some herbicides such as Kerb(pronamide), are absorbed and translocated throughout the plant while others like trifluralin(treflan) or bensulide(Prefar) do not move into the plant and only work at the root tips.

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To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.