Efficacy of Four Preemergence Herbicides Declines Over Time

Lack of longevity of weed control using preemergence herbicides is a problem during containerized plant production, especially in situations where irrigation and rainfall amounts are high and weed seed germination extends over a long time period due to dormancy and/or reintroduction. Four preemergence granular herbicides containing the active ingredients flumioxazin (BroadStar), oxyfluorfen + oryzalin (Rout), trifluralin + isoxaben + oxyfluorfen (Snapshot) or trifluralin + dimethenamid-P (Freehand) were applied to containers filled with a pine bark:peat:sand growing medium. Untreated containers served as controls. Containers were held outdoors, fertilized with controlled-release fertilizer, and watered daily using overhead irrigation. One-third of the containers in each treatment were inoculated with 20 weed seeds of either Chamaesyce maculata, Commelina benghalensis, Eclipta prostrata or Parthenium hysterophorus, two containers per weed, one day before the herbicides were applied. At both 28 and 56 days after treatment (DAT), another third of the containers were inoculated in the same manner. Successful germination and percent coverage of the medium surface were visually evaluated bi-weekly. At 90 DAT for the first weed seed sowing, Chamaesyce control was good (80–100%) with FreeHand and Rout, and fair (60–79%) with BroadStar and Showcase. Control of Chamaesyce from the second sowing was only acceptable (~60% or greater) with BroadStar and FreeHand at 90 DAT. Control was unacceptable for all herbicides at 90 DAT when the weed seeds were sown 56 DAT. Control at 90 DAT for Commelina from the first sowing was fair using FreeHand and good with BroadStar.  BroadStar control was also good for seed from the second sowing. None of the other treatments provided acceptable control for 90 days, regardless of sowing date. For Eclipta from the first sowing, control was good with BroadStar and Rout at 90 DAT; however, control was unacceptable for Eclipta from the later sowings for both products and also for the other treatments for all three sowings. At 90 DAT, Parthenium control from the first sowing was still good for all herbicides except FreeHand. None of the herbicides provided effective control of Parthenium seeds from the second and third sowings. These results demonstrate the need for correct herbicide selection depending on weed species and the need to prevent subsequent weed seed contamination after herbicides are applied.