Effect of Excessive Rainfall on Preemergent Herbicides

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A preemergent herbicide application is an effective, commonly performed practice for annual weed control in many established turfgrass systems. As the name suggests, a preemergent application should be made prior to germination, which in North Carolina is no later than mid-September for winter annual weed control. While preemergent herbicide labels typically state irrigation/rainfall is required for herbicide activation, they probably did not mean quite as much as our state received in late September through early October. The State Climate Office of North Carolina reported September 28, 2015 to October 4, 2015 was the seventh wettest (i.e. most accumulated rain) seven-day period in North Carolina dating back to 1895. So what effect, if any, did the cumulative rain have on a preemergent application made prior to the rain setting in? That depends on numerous factors that are covered in the following paragraph, but ultimately scenarios may unfold where preemergent herbicides move laterally across or downward in the soil which may compromise weed control later during the season.

The potential for adverse impacts on your preemergent application prior to prolonged periods of saturated soils caused by heavy rainfalls can be broken into three camps:

  1. 1. Site specific information
  • Preemergent herbicide movement potential increases as:
  • Soil texture coarseness increases (i.e. increasing sand content)
  • Soil organic matter decreases
  • Slope increases
  • Turfgrass cover decreases
  1. 2. Herbicide applied
    • Most notable properties pertain to soil/organic matter binding affinity. Herbicides with lower binding affinities are more prone to move from the intended site.
    • General binding affinity ranking for common PREs is (from least to highest):

Simazine = Atrazine < Indaziflam < Dithiopyr < Oxadiazon < Pendimethalin = Prodiamine

  1. 3. Time between application and excessive rainfall onset
    • This is not so much from a calendar day-count perspective, but the number of soil wet-dry cycles. The more cycles that occur between
      preemergent application and excessive rainfall, the more likely the herbicide is to be bound to the soil, which should reduce its potential to move laterally or downward via water flow.

So you think you may have a problem, but not sure? Unfortunately, only time will tell as the season progresses but if breaks occur, additional weed control measures can be taken to compensate. Monitoring historically weedy areas, as well as areas with conditions conducive for preemergence herbicide movement and weed encroachment (ex. upper portion of a south facing slope with marginal turfgrass coverage) should provide an early indicate for an herbicide break; however, a postemergent herbicide will be required at this point. Further, depending on when the break occurs, tank-mixing a preemergent herbicide (at a reduced rate) with your postemergent herbicide may be needed to maintain adequate control until spring preemergence applications are made. Another option to assess preemergence remaining and being able to potentially avoid applying a postemergent herbicide is pulling a few soil cores over time from both areas you expect your preemergent moved from, and moved to (i.e. upslope areas where movement may have occurred and downslope areas where products may have concentrated) and complete a bioassay. Once pulled, pot the plugs up and seed a sensitive plant species such as perennial ryegrass (Lolium perenne L.) on the surface. If germination varies between areas cores were pulled, this MAY be an indication to expect reduced preemergent efficacy and one should consider an additional application at a reduced rate in the areas with anticipated weed control concerns. Always read and follow product label directions.