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NCSU Extension Publication:AG-763W
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Crabgrass is germinating

Smooth and large crabgrass germinate when 24 hour mean soil temperatures (four inch depth) reach 55 degrees F.

As can be seen from the temperature figure the soil has warmed to that temperature rapidly over the last several days.

Since these herbicides control susceptible species as they grow through the herbicide treated zone, the herbicide barrier should have been established prior to weed seed germination.

Applying preemergent herbicides after this week may not prevent crabgrass and other summer annual weeds from being a problem!! If you have not applied the herbicide yet...

DO NOT DELAY!

Unlike the name “preemergent” suggests, preemergent herbicides do not actually prevent weed seed germination.  Instead, as weed seed germinate and grow through the herbicide-treated barrier or zone, the herbicide is absorbed by emerging shoots and/or roots resulting in death of susceptible species.  Because weed species absorb the herbicide as it grows through the treated zone, uniform application coverage is imperative for acceptable weed control.

Most preemergent herbicides MUST receive rainfall or irrigation to be activated. 

In summary, preemergent herbicides offer a great option for weed control in turfgrass environments.  However, turfgrass managers must make timely herbicide applications, select the proper herbicide, and always read and follow product label instructions.

We would like to offer the following suggestions to improve annual weed control with premergent herbicides:

  1. Apply herbicides in a timely manner.
  2. Make certain preemergent herbicides are irrigated or receive rainfall soon after application.
  3. Consider splitting your application based on the product you are using.
  • dithiopyr - 1/2 of active ingredient rate initially followed by 1/2 in 6 to 8 weeks
  • indaziflam - 1/2 of active ingredient rate intially followed by 1/2 in 6 to 8 weeks (Warm-season grasses only!!)
  • pendimethalin - 1/2 of active ingredient rate initially followed by 1/2 in 8 weeks
  • prodiamine - 2/3 of active ingredient rate initially followed by 1/3 in 8 weeks

Source:

Drs. Travis Gannon, Fred Yelverton and Charles Peacock

Turf and Weed Management Specialists

NC State University